Re: [PATCH] arm64: mte: Allow PTRACE_PEEKMTETAGS access to the zero page

On Wed, Feb 10, 2021 at 03:52:18PM -0300, Luis Machado wrote:

On 2/10/21 3:03 PM, Catalin Marinas wrote:

quoted

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: 34bfeea4a9e9 ("arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE")
Cc: <redacted> # 5.10.x
Cc: Will Deacon <will@kernel.org>
Reported-by: Luis Machado <redacted>

[...]

Thanks. I gave this a try and it works as expected. So memory that is
PROT_MTE but has not been accessed yet can be inspected with PEEKMTETAGS
without getting an EIO back.

Thanks. I assume I can add your tested-by.

-- 
Catalin

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