Comment 8 for bug 1921948

Richard Henderson <email address hidden> writes:

> We were incorrectly assuming that only the first byte of an MTE access
> is checked against the tags. But per the ARM, unaligned accesses are
> pre-decomposed into single-byte accesses. So by the time we reach the
> actual MTE check in the ARM pseudocode, all accesses are aligned.
>
> Therefore, the first failure is always either the first byte of the
> access, or the first byte of the granule.
>
> In addition, some of the arithmetic is off for last-first -> count.
> This does not become directly visible until a later patch that passes
> single bytes into this function, so ptr == ptr_last.
>
> Buglink: https://bugs.launchpad.net/bugs/1921948

Minor note: you can Cc: Bug 1921948 <email address hidden> to
automatically copy patches to the appropriate bugs which is useful if
you don't have the Cc for the reporter.

Anyway I'm trying to get the kasas unit tests running as a way of
testing this (and maybe expanding with a version of Andrey's test). I
suspect this may be a PEBCAC issue but I built an MTE enabled kernel
with:

  CONFIG_HAVE_ARCH_KASAN=y
  CONFIG_HAVE_ARCH_KASAN_SW_TAGS=y
  CONFIG_HAVE_ARCH_KASAN_HW_TAGS=y
  CONFIG_CC_HAS_KASAN_GENERIC=y
  CONFIG_KASAN=y
  # CONFIG_KASAN_GENERIC is not set
  CONFIG_KASAN_HW_TAGS=y
  CONFIG_KASAN_STACK=1
  CONFIG_KASAN_KUNIT_TEST=m
  CONFIG_TEST_KASAN_MODULE=m

and was able to boot it. But when I insmod the kasan tests:

  insmod test_kasan.ko

it looks like it just keeps looping failing on the same test:

  Ignoring spurious kernel translation fault at virtual address dead00000000010a
  WARNING: CPU: 0 PID: 1444 at arch/arm64/mm/fault.c:364 __do_kernel_fault+0xc4/0x1bc
  Modules linked in: test_kasan(+)
  CPU: 0 PID: 1444 Comm: kunit_try_catch Tainted: G B W 5.11.0-ajb-kasan #3
  Hardware name: linux,dummy-virt (DT)
  pstate: 60400009 (nZCv daif +PAN -UAO -TCO BTYPE=--)
  pc : __do_kernel_fault+0xc4/0x1bc
  lr : __do_kernel_fault+0xc4/0x1bc
  sp : ffffffc01191b900
  x29: ffffffc01191b900 x28: fcffff8001f7a880
  x27: fcffff8001c01e00 x26: 0000000000000000
  x25: 0000000000000001 x24: 00000000000000f4
  x23: 0000000020400009 x22: dead00000000010a
  x21: 0000000000000025 x20: ffffffc01191b9d0
  x19: 0000000097c08004 x18: 0000000000000000
  x17: 000000000000000a x16: 000017a83fb75794
  x15: 0000000000000030 x14: 6c656e72656b2073
  x13: ffffffc010e21be0 x12: 00000000000001aa
  x11: 000000000000008e x10: ffffffc010e2d930
  x9 : 000000000003a6d0 x8 : ffffffc010e21be0
  x7 : ffffffc010e2cbe0 x6 : 0000000000000d50
  x5 : ffffff8007f9c850 x4 : ffffffc01191b700
  x3 : 0000000000000001 x2 : 0000000000000000
  x1 : 0000000000000000 x0 : fcffff8001f7a880
  Call trace:
   __do_kernel_fault+0xc4/0x1bc
   do_translation_fault+0x98/0xb0
   do_mem_abort+0x44/0xb0
   el1_abort+0x40/0x6c
   el1_sync_handler+0x6c/0xb0
   el1_sync+0x70/0x100
   kasan_update_kunit_status+0x6c/0x1ac
   kasan_report_invalid_free+0x34/0xa0
   ____kasan_slab_free.constprop.0+0xf8/0x1a0
   __kasan_slab_free+0x10/0x20
   slab_free_freelist_hook+0xf8/0x1a0
   kfree+0x148/0x25c
   kunit_destroy_resource+0x15c/0x1bc
   string_stream_destroy+0x20/0x80
   kunit_do_assertion+0x190/0x1e4
   kmalloc_double_kzfree+0x158/0x190 [test_kasan]
   kunit_try_run_case+0x78/0xa4
   kunit_generic_run_threadfn_adapter+0x20/0x2c
   kthread+0x134/0x144
   ret_from_fork+0x10/0x38
  ---[ end trace 5acd02cdb9b3d3f0 ]---

but maybe I'm using the kunit tests wrong. It's my first time playing
with them.

> Signed-off-by: Richard Henderson <email address hidden>
> ---
> target/arm/mte_helper.c | 38 +++++++++++++++++---------------------
> 1 file changed, 17 insertions(+), 21 deletions(-)
>
> diff --git a/target/arm/mte_helper.c b/target/arm/mte_helper.c
> index 8be17e1b70..c87717127c 100644
> --- a/target/arm/mte_helper.c
> +++ b/target/arm/mte_helper.c
> @@ -757,10 +757,10 @@ uint64_t mte_checkN(CPUARMState *env, uint32_t desc,
> uint64_t ptr, uintptr_t ra)
> {
> int mmu_idx, ptr_tag, bit55;
> - uint64_t ptr_last, ptr_end, prev_page, next_page;
> - uint64_t tag_first, tag_end;
> - uint64_t tag_byte_first, tag_byte_end;
> - uint32_t esize, total, tag_count, tag_size, n, c;
> + uint64_t ptr_last, prev_page, next_page;
> + uint64_t tag_first, tag_last;
> + uint64_t tag_byte_first, tag_byte_last;
> + uint32_t total, tag_count, tag_size, n, c;
> uint8_t *mem1, *mem2;
> MMUAccessType type;
>
> @@ -779,29 +779,27 @@ uint64_t mte_checkN(CPUARMState *env, uint32_t desc,
>
> mmu_idx = FIELD_EX32(desc, MTEDESC, MIDX);
> type = FIELD_EX32(desc, MTEDESC, WRITE) ? MMU_DATA_STORE : MMU_DATA_LOAD;
> - esize = FIELD_EX32(desc, MTEDESC, ESIZE);
> total = FIELD_EX32(desc, MTEDESC, TSIZE);
>
> /* Find the addr of the end of the access, and of the last element. */
> - ptr_end = ptr + total;
> - ptr_last = ptr_end - esize;
> + ptr_last = ptr + total - 1;
>
> /* Round the bounds to the tag granule, and compute the number of tags. */
> tag_first = QEMU_ALIGN_DOWN(ptr, TAG_GRANULE);
> - tag_end = QEMU_ALIGN_UP(ptr_last, TAG_GRANULE);
> - tag_count = (tag_end - tag_first) / TAG_GRANULE;
> + tag_last = QEMU_ALIGN_DOWN(ptr_last, TAG_GRANULE);
> + tag_count = ((tag_last - tag_first) / TAG_GRANULE) + 1;
>
> /* Round the bounds to twice the tag granule, and compute the bytes. */
> tag_byte_first = QEMU_ALIGN_DOWN(ptr, 2 * TAG_GRANULE);
> - tag_byte_end = QEMU_ALIGN_UP(ptr_last, 2 * TAG_GRANULE);
> + tag_byte_last = QEMU_ALIGN_DOWN(ptr_last, 2 * TAG_GRANULE);
>
> /* Locate the page boundaries. */
> prev_page = ptr & TARGET_PAGE_MASK;
> next_page = prev_page + TARGET_PAGE_SIZE;
>
> - if (likely(tag_end - prev_page <= TARGET_PAGE_SIZE)) {
> + if (likely(tag_last - prev_page <= TARGET_PAGE_SIZE)) {
> /* Memory access stays on one page. */
> - tag_size = (tag_byte_end - tag_byte_first) / (2 * TAG_GRANULE);
> + tag_size = ((tag_byte_last - tag_byte_first) / (2 * TAG_GRANULE)) + 1;
> mem1 = allocation_tag_mem(env, mmu_idx, ptr, type, total,
> MMU_DATA_LOAD, tag_size, ra);
> if (!mem1) {
> @@ -815,9 +813,9 @@ uint64_t mte_checkN(CPUARMState *env, uint32_t desc,
> mem1 = allocation_tag_mem(env, mmu_idx, ptr, type, next_page - ptr,
> MMU_DATA_LOAD, tag_size, ra);
>
> - tag_size = (tag_byte_end - next_page) / (2 * TAG_GRANULE);
> + tag_size = ((tag_byte_last - next_page) / (2 * TAG_GRANULE)) + 1;
> mem2 = allocation_tag_mem(env, mmu_idx, next_page, type,
> - ptr_end - next_page,
> + ptr_last - next_page + 1,
> MMU_DATA_LOAD, tag_size, ra);
>
> /*
> @@ -838,15 +836,13 @@ uint64_t mte_checkN(CPUARMState *env, uint32_t desc,
> }
>
> /*
> - * If we failed, we know which granule. Compute the element that
> - * is first in that granule, and signal failure on that element.
> + * If we failed, we know which granule. For the first granule, the
> + * failure address is @ptr, the first byte accessed. Otherwise the
> + * failure address is the first byte of the nth granule.
> */
> if (unlikely(n < tag_count)) {
> - uint64_t fail_ofs;
> -
> - fail_ofs = tag_first + n * TAG_GRANULE - ptr;
> - fail_ofs = ROUND_UP(fail_ofs, esize);
> - mte_check_fail(env, desc, ptr + fail_ofs, ra);
> + uint64_t fault = (n == 0 ? ptr : tag_first + n * TAG_GRANULE);
> + mte_check_fail(env, desc, fault, ra);
> }
>
> done:

--
Alex Bennée