Re: [PATCH v10 00/12] arm64: untag user pointers passed to the kernel

[Szabolcs Nagy]

On 22/02/2019 12:53, Andrey Konovalov wrote:
> This patchset is meant to be merged together with "arm64 relaxed ABI" [1].
> 
> 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 [2]) 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.
> 
> 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.
> 
> Since memory syscalls (mmap, mprotect, etc.) don't do memory accesses but
> rather deal with memory ranges, untagged pointers are better suited to
> describe memory ranges internally. Thus for memory syscalls we untag
> pointers completely when they enter the kernel.

i think the same is true when user pointers are compared.

e.g. i suspect there may be issues with tagged robust mutex
list pointers because the kernel does

futex.c:3541:	while (entry != &head->list) {

where entry is a user pointer that may be tagged, and
&head->list is probably not tagged.

> 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.
> 
> The following testing approaches has been taken to find potential issues
> with user pointer untagging:
> 
> 1. Static testing (with sparse [3] 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.
> 
> 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.
> 
> This patchset is a prerequisite for ARM's memory tagging hardware feature
> support [4].
> 
> Thanks!
> 
> [1] https://lkml.org/lkml/2018/12/10/402
> 
> [2] http://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html
> 
> [3] https://github.com/lucvoo/sparse-dev/commit/5f960cb10f56ec2017c128ef9d16060e0145f292
> 
> [4] https://community.arm.com/processors/b/blog/posts/arm-a-profile-architecture-2018-developments-armv85a
> 
> 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).
> 
> Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx>
> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> 
> Andrey Konovalov (12):
>   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
>   arm64: update Documentation/arm64/tagged-pointers.txt
>   selftests, arm64: add a selftest for passing tagged pointers to kernel
> 
>  Documentation/arm64/tagged-pointers.txt       | 25 +++++++++++--------
>  arch/arm64/include/asm/uaccess.h              | 10 +++++---
>  fs/namespace.c                                |  2 +-
>  fs/userfaultfd.c                              |  5 ++++
>  include/linux/memory.h                        |  4 +++
>  ipc/shm.c                                     |  2 ++
>  kernel/sys.c                                  | 14 +++++++++++
>  kernel/trace/trace_output.c                   |  2 +-
>  lib/strncpy_from_user.c                       |  2 ++
>  lib/strnlen_user.c                            |  2 ++
>  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                                 |  2 ++
>  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     | 19 ++++++++++++++
>  25 files changed, 129 insertions(+), 16 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
>