Re: [PATCH] arm64: mte: Do not flag the zero page as PG_mte_tagged

[David Hildenbrand <david@redhat.com>]

On 24.09.25 14:31, Catalin Marinas wrote:
> Commit 68d54ceeec0e ("arm64: mte: Allow PTRACE_PEEKMTETAGS access to the
> zero page") attempted to fix ptrace() reading of tags from the zero page
> by marking it as PG_mte_tagged during cpu_enable_mte(). The same commit
> also changed the ptrace() tag access permission check to the VM_MTE vma
> flag while turning the page flag test into a WARN_ON_ONCE().
> 
> Attempting to set the PG_mte_tagged flag early with
> CONFIG_DEFERRED_STRUCT_PAGE_INIT enabled may either hang (after commit
> d77e59a8fccd "arm64: mte: Lock a page for MTE tag initialisation") or
> have the flags cleared later during page_alloc_init_late(). In addition,
> pages_identical() -> memcmp_pages() will reject any comparison with the
> zero page as it is marked as tagged.
> 
> Partially revert the above commit to avoid setting PG_mte_tagged on the
> zero page. Update the __access_remote_tags() warning on untagged pages
> to ignore the zero page since it is known to have the tags initialised.
> 
> Note that all user mapping of the zero page are marked as pte_special().
> The arm64 set_pte_at() will not call mte_sync_tags() on such pages, so
> PG_mte_tagged will remain cleared.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Fixes: 68d54ceeec0e ("arm64: mte: Allow PTRACE_PEEKMTETAGS access to the zero page")
> Reported-by: Gergely Kovacs <Gergely.Kovacs2@arm.com>
> Cc: <stable@vger.kernel.org> # 5.10.x
> Cc: Will Deacon <will@kernel.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Lance Yang <lance.yang@linux.dev>
> ---
> 
> For reference, discussion on page merging here:
> 
> https://lore.kernel.org/r/aNKJ5glToE4hMhWA@arm.com
> 
> The deferred struct page init problem was reported by Gergely offline.
> 
> Given that we've had this bug for over four years and it was only
> recently noticed, I think we should merge it at -rc1, give it a bit more
> time in -next in case it breaks anything (not likely but you never know,
> MTE interaction with the mm code is always surprising ;)).

Agreed :)

LGTM

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb