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arm64: mte: Allow PTRACE_PEEKMTETAGS access to the zero page 

The ptrace(PTRACE_PEEKMTETAGS) implementation checks whether the user
page has valid tags (mapped with PROT_MTE) by testing the PG_mte_tagged
page flag. If this bit is cleared, ptrace(PTRACE_PEEKMTETAGS) returns
-EIO.

A newly created (PROT_MTE) mapping points to the zero page which had its
tags zeroed during cpu_enable_mte(). If there were no prior writes to
this mapping, ptrace(PTRACE_PEEKMTETAGS) fails with -EIO since the zero
page does not have the PG_mte_tagged flag set.

Set PG_mte_tagged on the zero page when its tags are cleared during
boot. In addition, to avoid ptrace(PTRACE_PEEKMTETAGS) succeeding on
!PROT_MTE mappings pointing to the zero page, change the
__access_remote_tags() check to (vm_flags & VM_MTE) instead of
PG_mte_tagged.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Fixes: 34bfeea4a9 ("arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE")
Cc: <stable@vger.kernel.org> # 5.10.x
Cc: Will Deacon <will@kernel.org>
Reported-by: Luis Machado <luis.machado@linaro.org>
Tested-by: Luis Machado <luis.machado@linaro.org>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lore.kernel.org/r/20210210180316.23654-1-catalin.marinas@arm.com
master
Catalin Marinas 2021-02-10 18:03:16 +00:00
parent 22cd5edb2d
commit 68d54ceeec
2 changed files with 3 additions and 6 deletions
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6
arch/arm64/kernel/cpufeature.c
View File

@ -1701,16 +1701,12 @@ static void bti_enable(const struct arm64_cpu_capabilities *__unused)
#ifdef CONFIG_ARM64_MTE
static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap)
{
static bool cleared_zero_page = false;
/*
* Clear the tags in the zero page. This needs to be done via the
* linear map which has the Tagged attribute.
*/
if (!cleared_zero_page) {
cleared_zero_page = true;
if (!test_and_set_bit(PG_mte_tagged, &ZERO_PAGE(0)->flags))
mte_clear_page_tags(lm_alias(empty_zero_page));
}
kasan_init_hw_tags_cpu();
}

3
arch/arm64/kernel/mte.c
View File

@ -329,11 +329,12 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
* would cause the existing tags to be cleared if the page
* was never mapped with PROT_MTE.
*/
if (!test_bit(PG_mte_tagged, &page->flags)) {
if (!(vma->vm_flags & VM_MTE)) {
ret = -EOPNOTSUPP;
put_page(page);
break;
}
WARN_ON_ONCE(!test_bit(PG_mte_tagged, &page->flags));
/* limit access to the end of the page */
offset = offset_in_page(addr);