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Subject[PATCH v19 00/15] arm64: untag user pointers passed to the kernel
=== 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
3. 276e9327 ("arm64: entry: improve data abort handling of tagged

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 sbrk() (see the patchset [3] for more details).

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.

The mmap and mremap (only new_addr) syscalls do not currently accept
tagged addresses. Architectures may interpret the tag as a background
colour for the corresponding vma.

Other memory syscalls (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 & 3 kernel trees and is
now being used to enable testing of Pixel phones with HWASan.






=== History

Changes in v19:
- Rebased onto 7b5cf701 (5.3-rc1+).

Changes in v18:
- Reverted the selftest back to not using the LD_PRELOAD approach.
- Added prctl(PR_SET_TAGGED_ADDR_CTRL) call to the selftest.
- Reworded the patch descriptions to make them less oriented on arm64
- Catalin's patch: "I added a Kconfig option and dropped the prctl args
zero check. There is some minor clean-up as well".

Changes in v17:
- The "uaccess: add noop untagged_addr definition" patch is dropped, as it
was merged into upstream named as "uaccess: add noop untagged_addr
- Merged "mm, arm64: untag user pointers in do_pages_move" into
"mm, arm64: untag user pointers passed to memory syscalls".
- Added "arm64: Introduce prctl() options to control the tagged user
addresses ABI" patch from Catalin.
- Add to tools/testing/selftests/arm64/.gitignore.
- Added a comment clarifying untagged in mremap.
- Moved untagging back into mlx4_get_umem_mr() for the IB patch.

Changes in v16:
- Moved untagging for memory syscalls from arm64 wrappers back to generic
- Dropped untagging for the following memory syscalls: brk, mmap, munmap;
mremap (only dropped for new_address); mmap_pgoff (not used on arm64);
remap_file_pages (deprecated); shmat, shmdt (work on shared memory).
- Changed kselftest to LD_PRELOAD a shared library that overrides malloc
to return tagged pointers.
- Rebased onto 5.2-rc3.

Changes in v15:
- Removed unnecessary untagging from radeon_ttm_tt_set_userptr().
- Removed unnecessary untagging from amdgpu_ttm_tt_set_userptr().
- Moved untagging to validate_range() in userfaultfd code.
- Moved untagging to ib_uverbs_(re)reg_mr() from mlx4_get_umem_mr().
- Rebased onto 5.1.

Changes in v14:
- Moved untagging for most memory syscalls to an arm64 specific
implementation, instead of doing that in the common code.
- Dropped "net, arm64: untag user pointers in tcp_zerocopy_receive", since
the provided user pointers don't come from an anonymous map and thus are
not covered by this ABI relaxation.
- Dropped "kernel, arm64: untag user pointers in prctl_set_mm*".
- Moved untagging from __check_mem_type() to tee_shm_register().
- Updated untagging for the amdgpu and radeon drivers to cover the MMU
notifier, as suggested by Felix.
- Since this ABI relaxation doesn't actually allow tagged instruction
pointers, dropped the following patches:
- Dropped "tracing, arm64: untag user pointers in seq_print_user_ip".
- Dropped "uprobes, arm64: untag user pointers in find_active_uprobe".
- Dropped "bpf, arm64: untag user pointers in stack_map_get_build_id_offset".
- Rebased onto 5.1-rc7 (37624b58).

Changes in v13:
- Simplified untagging in tcp_zerocopy_receive().
- Looked at find_vma() callers in drivers/, which allowed to identify a
few other places where untagging is needed.
- Added patch "mm, arm64: untag user pointers in get_vaddr_frames".
- Added patch "drm/amdgpu, arm64: untag user pointers in
- Added patch "drm/radeon, arm64: untag user pointers in
- Added patch "IB/mlx4, arm64: untag user pointers in mlx4_get_umem_mr".
- Added patch "media/v4l2-core, arm64: untag user pointers in
- Added patch "tee/optee, arm64: untag user pointers in check_mem_type".
- Added patch "vfio/type1, arm64: untag user pointers".

Changes in v12:
- Changed untagging in tcp_zerocopy_receive() to also untag zc->address.
- Fixed untagging in prctl_set_mm* to only untag pointers for vma lookups
and validity checks, but leave them as is for actual user space accesses.
- Updated the link to the v2 of the "arm64 relaxed ABI" patchset [3].
- Dropped the documentation patch, as the "arm64 relaxed ABI" patchset [3]
handles that.

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"
- 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
- 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
- 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).

Signed-off-by: Andrey Konovalov <>

Andrey Konovalov (14):
arm64: untag user pointers in access_ok and __uaccess_mask_ptr
lib: untag user pointers in strn*_user
mm: untag user pointers passed to memory syscalls
mm: untag user pointers in mm/gup.c
mm: untag user pointers in get_vaddr_frames
fs/namespace: untag user pointers in copy_mount_options
userfaultfd: untag user pointers
drm/amdgpu: untag user pointers
drm/radeon: untag user pointers in radeon_gem_userptr_ioctl
IB/mlx4: untag user pointers in mlx4_get_umem_mr
media/v4l2-core: untag user pointers in videobuf_dma_contig_user_get
tee/shm: untag user pointers in tee_shm_register
vfio/type1: untag user pointers in vaddr_get_pfn
selftests, arm64: add a selftest for passing tagged pointers to kernel

Catalin Marinas (1):
arm64: Introduce prctl() options to control the tagged user addresses

arch/arm64/Kconfig | 9 +++
arch/arm64/include/asm/processor.h | 8 ++
arch/arm64/include/asm/thread_info.h | 1 +
arch/arm64/include/asm/uaccess.h | 12 ++-
arch/arm64/kernel/process.c | 73 +++++++++++++++++++
.../gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 +-
drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c | 2 +
drivers/gpu/drm/radeon/radeon_gem.c | 2 +
drivers/infiniband/hw/mlx4/mr.c | 7 +-
drivers/media/v4l2-core/videobuf-dma-contig.c | 9 ++-
drivers/tee/tee_shm.c | 1 +
drivers/vfio/vfio_iommu_type1.c | 2 +
fs/namespace.c | 2 +-
fs/userfaultfd.c | 22 +++---
include/uapi/linux/prctl.h | 5 ++
kernel/sys.c | 12 +++
lib/strncpy_from_user.c | 3 +-
lib/strnlen_user.c | 3 +-
mm/frame_vector.c | 2 +
mm/gup.c | 4 +
mm/madvise.c | 2 +
mm/mempolicy.c | 3 +
mm/migrate.c | 2 +-
mm/mincore.c | 2 +
mm/mlock.c | 4 +
mm/mprotect.c | 2 +
mm/mremap.c | 7 ++
mm/msync.c | 2 +
tools/testing/selftests/arm64/.gitignore | 1 +
tools/testing/selftests/arm64/Makefile | 11 +++
.../testing/selftests/arm64/ | 12 +++
tools/testing/selftests/arm64/tags_test.c | 29 ++++++++
32 files changed, 233 insertions(+), 25 deletions(-)
create mode 100644 tools/testing/selftests/arm64/.gitignore
create mode 100644 tools/testing/selftests/arm64/Makefile
create mode 100755 tools/testing/selftests/arm64/
create mode 100644 tools/testing/selftests/arm64/tags_test.c


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