[v11,00/14] arm64: untag user pointers passed to the kernel

Message

Andrey Konovalov March 15, 2019, 7:51 p.m. UTC

=== Overview

arm64 has a feature called Top Byte Ignore, which allows to embed pointer
tags into the top byte of each pointer. Userspace programs (such as
HWASan, a memory debugging tool [1]) might use this feature and pass
tagged user pointers to the kernel through syscalls or other interfaces.

Right now the kernel is already able to handle user faults with tagged
pointers, due to these patches:

1. 81cddd65 ("arm64: traps: fix userspace cache maintenance emulation on a
             tagged pointer")
2. 7dcd9dd8 ("arm64: hw_breakpoint: fix watchpoint matching for tagged
	      pointers")
3. 276e9327 ("arm64: entry: improve data abort handling of tagged
	      pointers")

This patchset extends tagged pointer support to syscall arguments.

As per the proposed ABI change [3], tagged pointers are only allowed to be
passed to syscalls when they point to memory ranges obtained by anonymous
mmap() or brk().

For non-memory syscalls this is done by untaging user pointers when the
kernel performs pointer checking to find out whether the pointer comes
from userspace (most notably in access_ok). The untagging is done only
when the pointer is being checked, the tag is preserved as the pointer
makes its way through the kernel and stays tagged when the kernel
dereferences the pointer when perfoming user memory accesses.

Memory syscalls (mmap, mprotect, etc.) don't do user memory accesses but
rather deal with memory ranges, and untagged pointers are better suited to
describe memory ranges internally. Thus for memory syscalls we untag
pointers completely when they enter the kernel.

=== Other approaches

One of the alternative approaches to untagging that was considered is to
completely strip the pointer tag as the pointer enters the kernel with
some kind of a syscall wrapper, but that won't work with the countless
number of different ioctl calls. With this approach we would need a custom
wrapper for each ioctl variation, which doesn't seem practical.

An alternative approach to untagging pointers in memory syscalls prologues
is to inspead allow tagged pointers to be passed to find_vma() (and other
vma related functions) and untag them there. Unfortunately, a lot of
find_vma() callers then compare or subtract the returned vma start and end
fields against the pointer that was being searched. Thus this approach
would still require changing all find_vma() callers.

=== Testing

The following testing approaches has been taken to find potential issues
with user pointer untagging:

1. Static testing (with sparse [2] and separately with a custom static
   analyzer based on Clang) to track casts of __user pointers to integer
   types to find places where untagging needs to be done.

2. Static testing with grep to find parts of the kernel that call
   find_vma() (and other similar functions) or directly compare against
   vm_start/vm_end fields of vma.

3. Static testing with grep to find parts of the kernel that compare
   user pointers with TASK_SIZE or other similar consts and macros.

4. Dynamic testing: adding BUG_ON(has_tag(addr)) to find_vma() and running
   a modified syzkaller version that passes tagged pointers to the kernel.

Based on the results of the testing the requried patches have been added
to the patchset.

=== Notes

This patchset is meant to be merged together with "arm64 relaxed ABI" [3].

This patchset is a prerequisite for ARM's memory tagging hardware feature
support [4].

This patchset has been merged into the Pixel 2 kernel tree and is now
being used to enable testing of Pixel 2 phones with HWASan.

Thanks!

[1] http://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html

[2] https://github.com/lucvoo/sparse-dev/commit/5f960cb10f56ec2017c128ef9d16060e0145f292

[3] https://lkml.org/lkml/2018/12/10/402

[4] https://community.arm.com/processors/b/blog/posts/arm-a-profile-architecture-2018-developments-armv85a

Changes in v11:
- Added "uprobes, arm64: untag user pointers in find_active_uprobe" patch.
- Added "bpf, arm64: untag user pointers in stack_map_get_build_id_offset"
  patch.
- Fixed "tracing, arm64: untag user pointers in seq_print_user_ip" to
  correctly perform subtration with a tagged addr.
- Moved untagged_addr() from SYSCALL_DEFINE3(mprotect) and
  SYSCALL_DEFINE4(pkey_mprotect) to do_mprotect_pkey().
- Moved untagged_addr() definition for other arches from
  include/linux/memory.h to include/linux/mm.h.
- Changed untagging in strn*_user() to perform userspace accesses through
  tagged pointers.
- Updated the documentation to mention that passing tagged pointers to
  memory syscalls is allowed.
- Updated the test to use malloc'ed memory instead of stack memory.

Changes in v10:
- Added "mm, arm64: untag user pointers passed to memory syscalls" back.
- New patch "fs, arm64: untag user pointers in fs/userfaultfd.c".
- New patch "net, arm64: untag user pointers in tcp_zerocopy_receive".
- New patch "kernel, arm64: untag user pointers in prctl_set_mm*".
- New patch "tracing, arm64: untag user pointers in seq_print_user_ip".

Changes in v9:
- Rebased onto 4.20-rc6.
- Used u64 instead of __u64 in type casts in the untagged_addr macro for
  arm64.
- Added braces around (addr) in the untagged_addr macro for other arches.

Changes in v8:
- Rebased onto 65102238 (4.20-rc1).
- Added a note to the cover letter on why syscall wrappers/shims that untag
  user pointers won't work.
- Added a note to the cover letter that this patchset has been merged into
  the Pixel 2 kernel tree.
- Documentation fixes, in particular added a list of syscalls that don't
  support tagged user pointers.

Changes in v7:
- Rebased onto 17b57b18 (4.19-rc6).
- Dropped the "arm64: untag user address in __do_user_fault" patch, since
  the existing patches already handle user faults properly.
- Dropped the "usb, arm64: untag user addresses in devio" patch, since the
  passed pointer must come from a vma and therefore be untagged.
- Dropped the "arm64: annotate user pointers casts detected by sparse"
  patch (see the discussion to the replies of the v6 of this patchset).
- Added more context to the cover letter.
- Updated Documentation/arm64/tagged-pointers.txt.

Changes in v6:
- Added annotations for user pointer casts found by sparse.
- Rebased onto 050cdc6c (4.19-rc1+).

Changes in v5:
- Added 3 new patches that add untagging to places found with static
  analysis.
- Rebased onto 44c929e1 (4.18-rc8).

Changes in v4:
- Added a selftest for checking that passing tagged pointers to the
  kernel succeeds.
- Rebased onto 81e97f013 (4.18-rc1+).

Changes in v3:
- Rebased onto e5c51f30 (4.17-rc6+).
- Added linux-arch@ to the list of recipients.

Changes in v2:
- Rebased onto 2d618bdf (4.17-rc3+).
- Removed excessive untagging in gup.c.
- Removed untagging pointers returned from __uaccess_mask_ptr.

Changes in v1:
- Rebased onto 4.17-rc1.

Changes in RFC v2:
- Added "#ifndef untagged_addr..." fallback in linux/uaccess.h instead of
  defining it for each arch individually.
- Updated Documentation/arm64/tagged-pointers.txt.
- Dropped "mm, arm64: untag user addresses in memory syscalls".
- Rebased onto 3eb2ce82 (4.16-rc7).

Andrey Konovalov (14):
  uaccess: add untagged_addr definition for other arches
  arm64: untag user pointers in access_ok and __uaccess_mask_ptr
  lib, arm64: untag user pointers in strn*_user
  mm, arm64: untag user pointers passed to memory syscalls
  mm, arm64: untag user pointers in mm/gup.c
  fs, arm64: untag user pointers in copy_mount_options
  fs, arm64: untag user pointers in fs/userfaultfd.c
  net, arm64: untag user pointers in tcp_zerocopy_receive
  kernel, arm64: untag user pointers in prctl_set_mm*
  tracing, arm64: untag user pointers in seq_print_user_ip
  uprobes, arm64: untag user pointers in find_active_uprobe
  bpf, arm64: untag user pointers in stack_map_get_build_id_offset
  arm64: update Documentation/arm64/tagged-pointers.txt
  selftests, arm64: add a selftest for passing tagged pointers to kernel

 Documentation/arm64/tagged-pointers.txt       | 18 +++++++---------
 arch/arm64/include/asm/uaccess.h              | 10 +++++----
 fs/namespace.c                                |  2 +-
 fs/userfaultfd.c                              |  5 +++++
 include/linux/mm.h                            |  4 ++++
 ipc/shm.c                                     |  2 ++
 kernel/bpf/stackmap.c                         |  6 ++++--
 kernel/events/uprobes.c                       |  2 ++
 kernel/sys.c                                  | 14 +++++++++++++
 kernel/trace/trace_output.c                   |  5 +++--
 lib/strncpy_from_user.c                       |  3 ++-
 lib/strnlen_user.c                            |  3 ++-
 mm/gup.c                                      |  4 ++++
 mm/madvise.c                                  |  2 ++
 mm/mempolicy.c                                |  5 +++++
 mm/migrate.c                                  |  1 +
 mm/mincore.c                                  |  2 ++
 mm/mlock.c                                    |  5 +++++
 mm/mmap.c                                     |  7 +++++++
 mm/mprotect.c                                 |  1 +
 mm/mremap.c                                   |  2 ++
 mm/msync.c                                    |  2 ++
 net/ipv4/tcp.c                                |  2 ++
 tools/testing/selftests/arm64/.gitignore      |  1 +
 tools/testing/selftests/arm64/Makefile        | 11 ++++++++++
 .../testing/selftests/arm64/run_tags_test.sh  | 12 +++++++++++
 tools/testing/selftests/arm64/tags_test.c     | 21 +++++++++++++++++++
 27 files changed, 131 insertions(+), 21 deletions(-)
 create mode 100644 tools/testing/selftests/arm64/.gitignore
 create mode 100644 tools/testing/selftests/arm64/Makefile
 create mode 100755 tools/testing/selftests/arm64/run_tags_test.sh
 create mode 100644 tools/testing/selftests/arm64/tags_test.c

Comments

Vincenzo Frascino March 18, 2019, 4:35 p.m. UTC | #1

On arm64 the TCR_EL1.TBI0 bit has been always enabled in the Linux
kernel hence the userspace (EL0) is allowed to set a non-zero value
in the top byte but the resulting pointers are not allowed at the
user-kernel syscall ABI boundary.

This patchset proposes a relaxation of the ABI and a mechanism to
advertise it to the userspace via an AT_FLAGS.

The rationale behind the choice of AT_FLAGS is that the Unix System V
ABI defines AT_FLAGS as "flags", leaving some degree of freedom in
interpretation.
There are two previous attempts of using AT_FLAGS in the Linux Kernel
for different reasons: the first was more generic and was used to expose
the support for the GNU STACK NX feature [1] and the second was done for
the MIPS architecture and was used to expose the support of "MIPS ABI
Extension for IEEE Std 754 Non-Compliant Interlinking" [2].
Both the changes are currently _not_ merged in mainline.
The only architecture that reserves some of the bits in AT_FLAGS is
currently MIPS, which introduced the concept of platform specific ABI
(psABI) reserving the top-byte [3].

When ARM64_AT_FLAGS_SYSCALL_TBI is set the kernel is advertising
to the userspace that a relaxed ABI is supported hence this type
of pointers are now allowed to be passed to the syscalls when they are
in memory ranges obtained by anonymous mmap() or brk().

The userspace _must_ verify that the flag is set before passing tagged
pointers to the syscalls allowed by this relaxation.

More in general, exposing the ARM64_AT_FLAGS_SYSCALL_TBI flag and mandating
to the software to check that the feature is present, before using the
associated functionality, it provides a degree of control on the decision
of disabling such a feature in future without consequently breaking the
userspace.

The change required a modification of the elf common code, because in Linux
the AT_FLAGS are currently set to zero by default by the kernel.

The newly added flag has been verified on arm64 using the code below.
#include <stdio.h>
#include <stdbool.h>
#include <sys/auxv.h>

#define ARM64_AT_FLAGS_SYSCALL_TBI     (1 << 0)

bool arm64_syscall_tbi_is_present(void)
{
        unsigned long at_flags = getauxval(AT_FLAGS);
        if (at_flags & ARM64_AT_FLAGS_SYSCALL_TBI)
                return true;

        return false;
}

void main()
{
        if (arm64_syscall_tbi_is_present())
                printf("ARM64_AT_FLAGS_SYSCALL_TBI is present\n");
}

This patchset should be merged together with [4].

[1] https://patchwork.ozlabs.org/patch/579578/
[2] https://lore.kernel.org/patchwork/cover/618280/
[3] ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/psABI_mips3.0.pdf
[4] https://patchwork.kernel.org/cover/10674351/

ABI References:
---------------
Sco SysV ABI: http://www.sco.com/developers/gabi/2003-12-17/contents.html
PowerPC AUXV: http://openpowerfoundation.org/wp-content/uploads/resources/leabi/content/dbdoclet.50655242_98651.html
AMD64 ABI: https://www.cs.tufts.edu/comp/40-2012f/readings/amd64-abi.pdf
x86 ABI: https://www.uclibc.org/docs/psABI-i386.pdf
MIPS ABI: ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/psABI_mips3.0.pdf
ARM ABI: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0044f/IHI0044F_aaelf.pdf
SPARC ABI: http://math-atlas.sourceforge.net/devel/assembly/abi_sysV_sparc.pdf

CC: Alexander Viro <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Andrey Konovalov <[e-mail]>
Cc: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Branislav Rankov <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Chintan Pandya <[e-mail]>
Cc: Daniel Borkmann <[e-mail]>
Cc: Dave Martin <[e-mail]>
Cc: "David S. Miller" <[e-mail]>
Cc: Dmitry Vyukov <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Cc: Evgeniy Stepanov <[e-mail]>
Cc: Graeme Barnes <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jacob Bramley <[e-mail]>
Cc: Kate Stewart <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Kevin Brodsky <[e-mail]>
Cc: "Kirill A . Shutemov" <[e-mail]>
Cc: Kostya Serebryany <[e-mail]>
Cc: Lee Smith <[e-mail]>
Cc: Luc Van Oostenryck <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Ramana Radhakrishnan <[e-mail]>
Cc: Robin Murphy <[e-mail]>
Cc: Ruben Ayrapetyan <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Szabolcs Nagy <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Vincenzo Frascino <[e-mail]>

Changes:
--------
v2:
  - Rebased on 5.1-rc1
  - Addressed review comments
  - Modified tagged-pointers.txt to be compliant with the
    new ABI relaxation

Vincenzo Frascino (4):
  elf: Make AT_FLAGS arch configurable
  arm64: Define Documentation/arm64/elf_at_flags.txt
  arm64: Relax Documentation/arm64/tagged-pointers.txt
  arm64: elf: Advertise relaxed ABI

 Documentation/arm64/elf_at_flags.txt    | 133 ++++++++++++++++++++++++
 Documentation/arm64/tagged-pointers.txt |  23 ++--
 arch/arm64/include/asm/atflags.h        |   7 ++
 arch/arm64/include/asm/elf.h            |   5 +
 arch/arm64/include/uapi/asm/atflags.h   |   8 ++
 fs/binfmt_elf.c                         |   6 +-
 fs/binfmt_elf_fdpic.c                   |   6 +-
 fs/compat_binfmt_elf.c                  |   5 +
 8 files changed, 184 insertions(+), 9 deletions(-)
 create mode 100644 Documentation/arm64/elf_at_flags.txt
 create mode 100644 arch/arm64/include/asm/atflags.h
 create mode 100644 arch/arm64/include/uapi/asm/atflags.h