=== 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/5f960cb10f56ec2017c128ef9d16060e...
[3] https://lkml.org/lkml/2018/12/10/402
[4] https://community.arm.com/processors/b/blog/posts/arm-a-profile-architecture...
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