[v4,07/12] aarch64: enable BTI at runtime

Message ID 54457ff0e2755acadc87dbf154b78a6f21cc19c8.1591899092.git.szabolcs.nagy@arm.com
State New
Headers show
Series
  • aarch64: branch protection support
Related show

Commit Message

Szabolcs Nagy June 12, 2020, 1:29 p.m.
From: Sudakshina Das <sudi.das@arm.com>


Binaries can opt-in to using BTI via an ELF object file marking.
The dynamic linker has to then mprotect the executable segments
with PROT_BTI. In case of static linked executables or in case
of the dynamic linker itself, PROT_BTI protection is done by the
operating system.

On AArch64 glibc uses PT_GNU_PROPERTY instead of PT_NOTE to check
the properties of a binary because PT_NOTE can be unreliable with
old linkers (old linkers just append the notes of input objects
together and add them to the output without checking them for
consistency which means multiple incompatible GNU property notes
can be present in PT_NOTE). A new _dl_process_pt_gnu_property
hook is introduced in dl-prop.h and to keep it maintainable the
rtld and dlopen code paths use the same function (if the main
map needs special treatment, that should be inferred by the hook
from the link map). Unlike the _dt_process_pt_note hook this one
is called after segments are mapped to avoid unbounded allocation
and additional read syscall. Otherwise the AArch64 logic follows
the x86 logic for handling GNU properties (but the code is not
shared because x86 needs to manage internal CET state and look
out for multiple property notes).

BTI property is handled in the loader even if glibc is not built
with BTI support, so in theory user code can be BTI protected
independently of glibc. In practice though user binaries are not
marked with the BTI property if glibc has no support because the
static linked libc objects (crt files, libc_nonshared.a) are
unmarked.

This patch relies on Linux userspace API that is scheduled to be
merged in Linux 5.8 and now it is in the for-next/bti-user branch
of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git.

Co-authored-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

---
 elf/dl-load.c                                 |  14 ++
 elf/rtld.c                                    |   6 +
 sysdeps/aarch64/Makefile                      |   4 +
 sysdeps/aarch64/dl-bti.c                      |  54 +++++++
 sysdeps/aarch64/dl-prop.h                     | 145 ++++++++++++++++++
 sysdeps/aarch64/linkmap.h                     |   3 +
 sysdeps/generic/dl-prop.h                     |  16 +-
 sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h  |   1 +
 sysdeps/unix/sysv/linux/aarch64/bits/mman.h   |  31 ++++
 .../unix/sysv/linux/aarch64/cpu-features.c    |   3 +
 .../unix/sysv/linux/aarch64/cpu-features.h    |   2 +
 sysdeps/x86/dl-prop.h                         |   6 +
 12 files changed, 280 insertions(+), 5 deletions(-)
 create mode 100644 sysdeps/aarch64/dl-bti.c
 create mode 100644 sysdeps/aarch64/dl-prop.h
 create mode 100644 sysdeps/unix/sysv/linux/aarch64/bits/mman.h

-- 
2.17.1

Comments

Carlos O'Donell via Libc-alpha June 12, 2020, 1:43 p.m. | #1
On Fri, Jun 12, 2020 at 6:34 AM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>

> From: Sudakshina Das <sudi.das@arm.com>

>

> Binaries can opt-in to using BTI via an ELF object file marking.

> The dynamic linker has to then mprotect the executable segments

> with PROT_BTI. In case of static linked executables or in case

> of the dynamic linker itself, PROT_BTI protection is done by the

> operating system.

>

> On AArch64 glibc uses PT_GNU_PROPERTY instead of PT_NOTE to check

> the properties of a binary because PT_NOTE can be unreliable with

> old linkers (old linkers just append the notes of input objects

> together and add them to the output without checking them for

> consistency which means multiple incompatible GNU property notes

> can be present in PT_NOTE). A new _dl_process_pt_gnu_property

> hook is introduced in dl-prop.h and to keep it maintainable the

> rtld and dlopen code paths use the same function (if the main

> map needs special treatment, that should be inferred by the hook

> from the link map). Unlike the _dt_process_pt_note hook this one

> is called after segments are mapped to avoid unbounded allocation

> and additional read syscall. Otherwise the AArch64 logic follows

> the x86 logic for handling GNU properties (but the code is not

> shared because x86 needs to manage internal CET state and look

> out for multiple property notes).


Can you make _dl_process_pt_gnu_property more generic so that it
can be shared with x86?  If there is PT_GNU_PROPERTY, we don't
need to check for multiple property notes.  Thanks.

> BTI property is handled in the loader even if glibc is not built

> with BTI support, so in theory user code can be BTI protected

> independently of glibc. In practice though user binaries are not

> marked with the BTI property if glibc has no support because the

> static linked libc objects (crt files, libc_nonshared.a) are

> unmarked.

>

> This patch relies on Linux userspace API that is scheduled to be

> merged in Linux 5.8 and now it is in the for-next/bti-user branch

> of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git.

>


-- 
H.J.
Szabolcs Nagy June 12, 2020, 3:08 p.m. | #2
The 06/12/2020 06:43, H.J. Lu wrote:
> On Fri, Jun 12, 2020 at 6:34 AM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:

> >

> > From: Sudakshina Das <sudi.das@arm.com>

> >

> > Binaries can opt-in to using BTI via an ELF object file marking.

> > The dynamic linker has to then mprotect the executable segments

> > with PROT_BTI. In case of static linked executables or in case

> > of the dynamic linker itself, PROT_BTI protection is done by the

> > operating system.

> >

> > On AArch64 glibc uses PT_GNU_PROPERTY instead of PT_NOTE to check

> > the properties of a binary because PT_NOTE can be unreliable with

> > old linkers (old linkers just append the notes of input objects

> > together and add them to the output without checking them for

> > consistency which means multiple incompatible GNU property notes

> > can be present in PT_NOTE). A new _dl_process_pt_gnu_property

> > hook is introduced in dl-prop.h and to keep it maintainable the

> > rtld and dlopen code paths use the same function (if the main

> > map needs special treatment, that should be inferred by the hook

> > from the link map). Unlike the _dt_process_pt_note hook this one

> > is called after segments are mapped to avoid unbounded allocation

> > and additional read syscall. Otherwise the AArch64 logic follows

> > the x86 logic for handling GNU properties (but the code is not

> > shared because x86 needs to manage internal CET state and look

> > out for multiple property notes).

> 

> Can you make _dl_process_pt_gnu_property more generic so that it

> can be shared with x86?  If there is PT_GNU_PROPERTY, we don't

> need to check for multiple property notes.  Thanks.


it can use a hook for NT_GNU_PROPERTY_TYPE_0 notes
that is called for each property, e.g.

int
_dl_process_gnu_property_note0(uint32_t type, uint32_t datasz, const void *data);

and then a target only needs to override that. i
assume the rest of the PT_GNU_PROPERTY handling
can be generic. (the int return value can control
if processing should stop or continue)

however this only helps on x86 if PT_GNU_PROPERTY
processing is before PT_NOTE processing otherwise
you cannot skip handling the notes.

i moved the _dl_process_pt_gnu_property into a
second pass in dl-load.c so load segments are
mapped by then, the note processing is currently
before that (but even if that's moved into the
second pass, PT_NOTE i think will appear before
PT_GNU_PROPERTY in the program headers), so it's
not clear if refactoring helps: you would need
separate target hooks for first and second passes
over pt headers i think.

> 

> > BTI property is handled in the loader even if glibc is not built

> > with BTI support, so in theory user code can be BTI protected

> > independently of glibc. In practice though user binaries are not

> > marked with the BTI property if glibc has no support because the

> > static linked libc objects (crt files, libc_nonshared.a) are

> > unmarked.

> >

> > This patch relies on Linux userspace API that is scheduled to be

> > merged in Linux 5.8 and now it is in the for-next/bti-user branch

> > of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git.

> >

> 

> -- 

> H.J.


--
Carlos O'Donell via Libc-alpha June 12, 2020, 3:18 p.m. | #3
On Fri, Jun 12, 2020 at 8:08 AM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
>

> The 06/12/2020 06:43, H.J. Lu wrote:

> > On Fri, Jun 12, 2020 at 6:34 AM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:

> > >

> > > From: Sudakshina Das <sudi.das@arm.com>

> > >

> > > Binaries can opt-in to using BTI via an ELF object file marking.

> > > The dynamic linker has to then mprotect the executable segments

> > > with PROT_BTI. In case of static linked executables or in case

> > > of the dynamic linker itself, PROT_BTI protection is done by the

> > > operating system.

> > >

> > > On AArch64 glibc uses PT_GNU_PROPERTY instead of PT_NOTE to check

> > > the properties of a binary because PT_NOTE can be unreliable with

> > > old linkers (old linkers just append the notes of input objects

> > > together and add them to the output without checking them for

> > > consistency which means multiple incompatible GNU property notes

> > > can be present in PT_NOTE). A new _dl_process_pt_gnu_property

> > > hook is introduced in dl-prop.h and to keep it maintainable the

> > > rtld and dlopen code paths use the same function (if the main

> > > map needs special treatment, that should be inferred by the hook

> > > from the link map). Unlike the _dt_process_pt_note hook this one

> > > is called after segments are mapped to avoid unbounded allocation

> > > and additional read syscall. Otherwise the AArch64 logic follows

> > > the x86 logic for handling GNU properties (but the code is not

> > > shared because x86 needs to manage internal CET state and look

> > > out for multiple property notes).

> >

> > Can you make _dl_process_pt_gnu_property more generic so that it

> > can be shared with x86?  If there is PT_GNU_PROPERTY, we don't

> > need to check for multiple property notes.  Thanks.

>

> it can use a hook for NT_GNU_PROPERTY_TYPE_0 notes

> that is called for each property, e.g.

>

> int

> _dl_process_gnu_property_note0(uint32_t type, uint32_t datasz, const void *data);

>

> and then a target only needs to override that. i

> assume the rest of the PT_GNU_PROPERTY handling

> can be generic. (the int return value can control

> if processing should stop or continue)

>

> however this only helps on x86 if PT_GNU_PROPERTY

> processing is before PT_NOTE processing otherwise

> you cannot skip handling the notes.

>

> i moved the _dl_process_pt_gnu_property into a

> second pass in dl-load.c so load segments are

> mapped by then, the note processing is currently

> before that (but even if that's moved into the

> second pass, PT_NOTE i think will appear before

> PT_GNU_PROPERTY in the program headers), so it's

> not clear if refactoring helps: you would need

> separate target hooks for first and second passes

> over pt headers i think.

>


On x86-64, there are

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000000040 0x0000000000000040
                 0x00000000000002d8 0x00000000000002d8  R      0x8
  INTERP         0x0000000000000318 0x0000000000000318 0x0000000000000318
                 0x000000000000001c 0x000000000000001c  R      0x1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000003628 0x0000000000003628  R      0x1000
  LOAD           0x0000000000004000 0x0000000000004000 0x0000000000004000
                 0x0000000000014541 0x0000000000014541  R E    0x1000
  LOAD           0x0000000000019000 0x0000000000019000 0x0000000000019000
                 0x0000000000008e18 0x0000000000008e18  R      0x1000
  LOAD           0x0000000000021f50 0x0000000000022f50 0x0000000000022f50
                 0x00000000000012f8 0x00000000000025a8  RW     0x1000
  DYNAMIC        0x00000000000229f8 0x00000000000239f8 0x00000000000239f8
                 0x0000000000000210 0x0000000000000210  RW     0x8
  NOTE           0x0000000000000338 0x0000000000000338 0x0000000000000338
                 0x0000000000000020 0x0000000000000020  R      0x8
  NOTE           0x0000000000000358 0x0000000000000358 0x0000000000000358
                 0x0000000000000044 0x0000000000000044  R      0x4
  GNU_PROPERTY   0x0000000000000338 0x0000000000000338 0x0000000000000338
                 0x0000000000000020 0x0000000000000020  R      0x8
  GNU_EH_FRAME   0x000000000001e32c 0x000000000001e32c 0x000000000001e32c
                 0x00000000000008fc 0x00000000000008fc  R      0x4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     0x10
  GNU_RELRO      0x0000000000021f50 0x0000000000022f50 0x0000000000022f50
                 0x00000000000010b0 0x00000000000010b0  R      0x1

Only the PT_NOTE segment with the correct alignment may contain property.
In the first pass, we record such PT_NOTE segment, but don't process it.
After all segments have been processed, we process the saved PT_NOTE segment
if there is no GNU_PROPERTY segment.

-- 
H.J.

Patch

diff --git a/elf/dl-load.c b/elf/dl-load.c
index 06f2ba7264..ab8237ac55 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -1188,6 +1188,20 @@  _dl_map_object_from_fd (const char *name, const char *origname, int fd,
 				  maplength, has_holes, loader);
     if (__glibc_unlikely (errstring != NULL))
       goto call_lose;
+
+    /* Process program headers again after load segments are mapped in
+       case processing requires accessing those segments.  */
+    for (ph = phdr; ph < &phdr[l->l_phnum]; ++ph)
+      switch (ph->p_type)
+	{
+	case PT_GNU_PROPERTY:
+	  if (_dl_process_pt_gnu_property (l, ph))
+	    {
+	      errstring = N_("cannot process GNU property segment");
+	      goto call_lose;
+	    }
+	  break;
+	}
   }
 
   if (l->l_ld == 0)
diff --git a/elf/rtld.c b/elf/rtld.c
index 882b070cc0..5aec2efaa5 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -1508,6 +1508,12 @@  of this helper program; chances are you did not intend to run this program.\n\
 	main_map->l_relro_size = ph->p_memsz;
 	break;
 
+      case PT_GNU_PROPERTY:
+	if (_dl_process_pt_gnu_property (main_map, ph))
+	  _dl_error_printf (
+"ERROR: '%s': cannot process GNU property segment.\n", _dl_argv[0]);
+	break;
+
       case PT_NOTE:
 	if (_rtld_process_pt_note (main_map, ph))
 	  _dl_error_printf ("\
diff --git a/sysdeps/aarch64/Makefile b/sysdeps/aarch64/Makefile
index 9cb141004d..5ae8b082b0 100644
--- a/sysdeps/aarch64/Makefile
+++ b/sysdeps/aarch64/Makefile
@@ -1,5 +1,9 @@ 
 long-double-fcts = yes
 
+ifeq ($(subdir),elf)
+sysdep-dl-routines += dl-bti
+endif
+
 ifeq ($(subdir),elf)
 sysdep-dl-routines += tlsdesc dl-tlsdesc
 gen-as-const-headers += dl-link.sym
diff --git a/sysdeps/aarch64/dl-bti.c b/sysdeps/aarch64/dl-bti.c
new file mode 100644
index 0000000000..3c92377cc8
--- /dev/null
+++ b/sysdeps/aarch64/dl-bti.c
@@ -0,0 +1,54 @@ 
+/* AArch64 BTI functions.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <unistd.h>
+#include <errno.h>
+#include <libintl.h>
+#include <ldsodefs.h>
+
+static int
+enable_bti (struct link_map *map, const char *program)
+{
+  const ElfW(Phdr) *phdr;
+  unsigned prot = PROT_READ | PROT_EXEC | PROT_BTI;
+
+  for (phdr = map->l_phdr; phdr < &map->l_phdr[map->l_phnum]; ++phdr)
+    if (phdr->p_type == PT_LOAD && (phdr->p_flags & PF_X))
+      {
+	ElfW(Addr) start = phdr->p_vaddr + map->l_addr;
+	ElfW(Addr) len = phdr->p_memsz;
+	if (__mprotect ((void *) start, len, prot) < 0)
+	  {
+	    if (program)
+	      _dl_fatal_printf ("%s: mprotect failed to turn on BTI\n",
+				map->l_name);
+	    else
+	      _dl_signal_error (errno, map->l_name, "dlopen",
+				N_("mprotect failed to turn on BTI"));
+	  }
+      }
+  return 0;
+}
+
+/* Enable BTI for L if required.  */
+
+void
+_dl_bti_check (struct link_map *l, const char *program)
+{
+  if (GLRO(dl_aarch64_cpu_features).bti && l->l_mach.bti)
+    enable_bti (l, program);
+}
diff --git a/sysdeps/aarch64/dl-prop.h b/sysdeps/aarch64/dl-prop.h
new file mode 100644
index 0000000000..b6f8a88667
--- /dev/null
+++ b/sysdeps/aarch64/dl-prop.h
@@ -0,0 +1,145 @@ 
+/* Support for GNU properties.  AArch64 version.
+   Copyright (C) 2018-2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _DL_PROP_H
+#define _DL_PROP_H
+
+#include <not-cancel.h>
+
+extern void _dl_bti_check (struct link_map *, const char *)
+    attribute_hidden;
+
+static inline void __attribute__ ((always_inline))
+_rtld_main_check (struct link_map *m, const char *program)
+{
+  _dl_bti_check (m, program);
+}
+
+static inline void __attribute__ ((always_inline))
+_dl_open_check (struct link_map *m)
+{
+  _dl_bti_check (m, NULL);
+}
+
+static inline void __attribute__ ((unused))
+_dl_process_aarch64_property (struct link_map *l,
+			      const ElfW(Nhdr) *note,
+			      const ElfW(Addr) size,
+			      const ElfW(Addr) align)
+{
+  /* The NT_GNU_PROPERTY_TYPE_0 note must be aligned to 4 bytes in
+     32-bit objects and to 8 bytes in 64-bit objects.  Skip notes
+     with incorrect alignment.  */
+  if (align != (__ELF_NATIVE_CLASS / 8))
+    return;
+
+  const ElfW(Addr) start = (ElfW(Addr)) note;
+
+  unsigned int feature_1 = 0;
+  unsigned int last_type = 0;
+
+  while ((ElfW(Addr)) (note + 1) - start < size)
+    {
+      /* Find the NT_GNU_PROPERTY_TYPE_0 note.  */
+      if (note->n_namesz == 4
+	  && note->n_type == NT_GNU_PROPERTY_TYPE_0
+	  && memcmp (note + 1, "GNU", 4) == 0)
+	{
+	  /* Check for invalid property.  */
+	  if (note->n_descsz < 8
+	      || (note->n_descsz % sizeof (ElfW(Addr))) != 0)
+	    return;
+
+	  /* Start and end of property array.  */
+	  unsigned char *ptr = (unsigned char *) (note + 1) + 4;
+	  unsigned char *ptr_end = ptr + note->n_descsz;
+
+	  do
+	    {
+	      unsigned int type = *(unsigned int *) ptr;
+	      unsigned int datasz = *(unsigned int *) (ptr + 4);
+
+	      /* Property type must be in ascending order.  */
+	      if (type < last_type)
+		return;
+
+	      ptr += 8;
+	      if ((ptr + datasz) > ptr_end)
+		return;
+
+	      last_type = type;
+
+	      if (type == GNU_PROPERTY_AARCH64_FEATURE_1_AND)
+		{
+		  /* The size of GNU_PROPERTY_AARCH64_FEATURE_1_AND is 4
+		     bytes.  When seeing GNU_PROPERTY_AARCH64_FEATURE_1_AND,
+		     we stop the search regardless if its size is correct
+		     or not.  There is no point to continue if this note
+		     is ill-formed.  */
+		  if (datasz != 4)
+		    return;
+
+		  feature_1 = *(unsigned int *) ptr;
+		  if ((feature_1 & GNU_PROPERTY_AARCH64_FEATURE_1_BTI))
+		    l->l_mach.bti = true;
+
+		  /* Stop if we found the property note.  */
+		  return;
+		}
+	      else if (type > GNU_PROPERTY_AARCH64_FEATURE_1_AND)
+		{
+		  /* Stop since property type is in ascending order.  */
+		  return;
+		}
+
+	      /* Check the next property item.  */
+	      ptr += ALIGN_UP (datasz, sizeof (ElfW(Addr)));
+	    }
+	  while ((ptr_end - ptr) >= 8);
+	}
+
+      note = ((const void *) note
+	      + ELF_NOTE_NEXT_OFFSET (note->n_namesz, note->n_descsz,
+				      align));
+    }
+}
+
+#ifdef FILEBUF_SIZE
+static inline int __attribute__ ((always_inline))
+_dl_process_pt_note (struct link_map *l, const ElfW(Phdr) *ph,
+		     int fd, struct filebuf *fbp)
+{
+  return 0;
+}
+#endif
+
+static inline int __attribute__ ((always_inline))
+_rtld_process_pt_note (struct link_map *l, const ElfW(Phdr) *ph)
+{
+  return 0;
+}
+
+static inline int
+_dl_process_pt_gnu_property (struct link_map *l, const ElfW(Phdr) *ph)
+{
+  const ElfW(Nhdr) *note = (const void *) (ph->p_vaddr + l->l_addr);
+  _dl_process_aarch64_property (l, note, ph->p_memsz, ph->p_align);
+  return 0;
+}
+
+#endif /* _DL_PROP_H */
diff --git a/sysdeps/aarch64/linkmap.h b/sysdeps/aarch64/linkmap.h
index 943a9ee9e4..847a03ace2 100644
--- a/sysdeps/aarch64/linkmap.h
+++ b/sysdeps/aarch64/linkmap.h
@@ -16,8 +16,11 @@ 
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <stdbool.h>
+
 struct link_map_machine
 {
   ElfW(Addr) plt;	  /* Address of .plt */
   void *tlsdesc_table;	  /* Address of TLS descriptor hash table.  */
+  bool bti;		  /* Branch Target Identification is enabled.  */
 };
diff --git a/sysdeps/generic/dl-prop.h b/sysdeps/generic/dl-prop.h
index 6b0f2aa95a..4192049739 100644
--- a/sysdeps/generic/dl-prop.h
+++ b/sysdeps/generic/dl-prop.h
@@ -20,11 +20,11 @@ 
 #define _DL_PROP_H
 
 /* The following functions are used by the dynamic loader and the
-   dlopen machinery to process PT_NOTE entries in the binary or
-   shared object.  The notes can be used to change the behaviour of
-   the loader, and as such offer a flexible mechanism for hooking in
-   various checks related to ABI tags or implementing "flag day" ABI
-   transitions.  */
+   dlopen machinery to process PT_NOTE and PT_GNU_PROPERTY entries in
+   the binary or shared object.  The notes can be used to change the
+   behaviour of the loader, and as such offer a flexible mechanism
+   for hooking in various checks related to ABI tags or implementing
+   "flag day" ABI transitions.  */
 
 static inline void __attribute__ ((always_inline))
 _rtld_main_check (struct link_map *m, const char *program)
@@ -51,4 +51,10 @@  _rtld_process_pt_note (struct link_map *l, const ElfW(Phdr) *ph)
   return 0;
 }
 
+static inline int __attribute__ ((always_inline))
+_dl_process_pt_gnu_property (struct link_map *l, const ElfW(Phdr) *ph)
+{
+  return 0;
+}
+
 #endif /* _DL_PROP_H */
diff --git a/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h b/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h
index 4ee14b4208..af90d8a626 100644
--- a/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h
+++ b/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h
@@ -72,3 +72,4 @@ 
 #define HWCAP2_BF16		(1 << 14)
 #define HWCAP2_DGH		(1 << 15)
 #define HWCAP2_RNG		(1 << 16)
+#define HWCAP2_BTI		(1 << 17)
diff --git a/sysdeps/unix/sysv/linux/aarch64/bits/mman.h b/sysdeps/unix/sysv/linux/aarch64/bits/mman.h
new file mode 100644
index 0000000000..ecae046344
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/aarch64/bits/mman.h
@@ -0,0 +1,31 @@ 
+/* Definitions for POSIX memory map interface.  Linux/AArch64 version.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _SYS_MMAN_H
+# error "Never use <bits/mman.h> directly; include <sys/mman.h> instead."
+#endif
+
+/* AArch64 specific definitions, should be in sync with
+   arch/arm64/include/uapi/asm/mman.h.  */
+
+#define PROT_BTI	0x10
+
+#include <bits/mman-map-flags-generic.h>
+
+/* Include generic Linux declarations.  */
+#include <bits/mman-linux.h>
diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
index 896c588fee..b9ab827aca 100644
--- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
+++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
@@ -83,4 +83,7 @@  init_cpu_features (struct cpu_features *cpu_features)
 
   if ((dczid & DCZID_DZP_MASK) == 0)
     cpu_features->zva_size = 4 << (dczid & DCZID_BS_MASK);
+
+  /* Check if BTI is supported.  */
+  cpu_features->bti = GLRO (dl_hwcap2) & HWCAP2_BTI;
 }
diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.h b/sysdeps/unix/sysv/linux/aarch64/cpu-features.h
index 1389cea1b3..a81f186ec2 100644
--- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.h
+++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.h
@@ -20,6 +20,7 @@ 
 #define _CPU_FEATURES_AARCH64_H
 
 #include <stdint.h>
+#include <stdbool.h>
 
 #define MIDR_PARTNUM_SHIFT	4
 #define MIDR_PARTNUM_MASK	(0xfff << MIDR_PARTNUM_SHIFT)
@@ -64,6 +65,7 @@  struct cpu_features
 {
   uint64_t midr_el1;
   unsigned zva_size;
+  bool bti;
 };
 
 #endif /* _CPU_FEATURES_AARCH64_H  */
diff --git a/sysdeps/x86/dl-prop.h b/sysdeps/x86/dl-prop.h
index 516f88ea80..8649314f9d 100644
--- a/sysdeps/x86/dl-prop.h
+++ b/sysdeps/x86/dl-prop.h
@@ -191,4 +191,10 @@  _rtld_process_pt_note (struct link_map *l, const ElfW(Phdr) *ph)
   return 0;
 }
 
+static inline int __attribute__ ((always_inline))
+_dl_process_pt_gnu_property (struct link_map *l, const ElfW(Phdr) *ph)
+{
+  return 0;
+}
+
 #endif /* _DL_PROP_H */