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

[Szabolcs Nagy]

On 22/02/2019 15:40, Andrey Konovalov wrote:
> On Fri, Feb 22, 2019 at 4:35 PM Szabolcs Nagy <Szabolcs.Nagy@xxxxxxx> wrote:
>>
>> 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.
> 
> You're right. I'll expand the cover letter in the next version to
> describe this more accurately. The patchset however contains "mm,
> arm64: untag user pointers in mm/gup.c" that should take care of futex
> pointers.

the robust mutex list pointer is not a futex pointer,
i'm not sure how the mm/gup.c patch helps.

>>
>>> 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
>>>
>>