On Mon, May 17, 2021 at 01:32:32PM +0100, Steven Price wrote:

mte_sync_tags() used test_and_set_bit() to set the PG_mte_tagged flag
before restoring/zeroing the MTE tags. However if another thread were to
race and attempt to sync the tags on the same page before the first
thread had completed restoring/zeroing then it would see the flag is
already set and continue without waiting. This would potentially expose
the previous contents of the tags to user space, and cause any updates
that user space makes before the restoring/zeroing has completed to
potentially be lost.

Since this code is run from atomic contexts we can't just lock the page
during the process. Instead implement a new (global) spinlock to protect
the mte_sync_page_tags() function.

Fixes: 34bfeea4a9e9 ("arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE")
Signed-off-by: Steven Price <steven.price@arm.com>

Other than the missing spinlock initialisation, the patch looks fine to
me.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

(though I'll probably queue it as a fix, waiting a couple of days for
comments)

-- 
Catalin

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