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 ) 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")
When passing tagged pointers to syscalls, there's a special case of such a pointer being passed to one of the memory syscalls (mmap, mprotect, etc.). These syscalls don't do memory accesses but rather deal with memory ranges, hence an untagged pointer is better suited.
This patchset extends tagged pointer support to non-memory syscalls. This is done by reusing the untagged_addr macro to untag 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.
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  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. 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 .
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: 1 From 502466b9652c57a23af3bd72124144319212f30b Mon Sep 17 00:00:00 2001 - Added annotations for user pointer casts found by sparse. 1 From 502466b9652c57a23af3bd72124144319212f30b Mon Sep 17 00:00:00 2001 - Rebased onto 050cdc6c (4.19-rc1+). 1 From 502466b9652c57a23af3bd72124144319212f30b Mon Sep 17 00:00:00 2001
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 (8): arm64: add type casts to untagged_addr macro uaccess: add untagged_addr definition for other arches arm64: untag user addresses in access_ok and __uaccess_mask_ptr mm, arm64: untag user addresses in mm/gup.c lib, arm64: untag addrs passed to strncpy_from_user and strnlen_user fs, arm64: untag user address in copy_mount_options 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 | 14 +++++++---- fs/namespace.c | 2 +- include/linux/uaccess.h | 4 +++ lib/strncpy_from_user.c | 2 ++ lib/strnlen_user.c | 2 ++ mm/gup.c | 4 +++ 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 ++++++++++++++ 11 files changed, 80 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