Re: [PATCH v16 1/7] arm64: mte: Handle race when synchronising tags

From: Marc Zyngier
Date: Fri Jun 18 2021 - 11:43:01 EST

On 2021-06-18 15:40, Catalin Marinas wrote:
On Fri, Jun 18, 2021 at 02:28:20PM +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")
Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Steven Price <steven.price@xxxxxxx>

Although I reviewed this patch, I think we should drop it from this
series and restart the discussion with the Chromium guys on what/if they
need PROT_MTE with MAP_SHARED. It currently breaks if you have two
PROT_MTE mappings but if they are ok with only one of the mappings being
PROT_MTE, I'm happy to just document it.

Not sure whether subsequent patches depend on it though.

I'd certainly like it to be independent of the KVM series, specially
as this series is pretty explicit that this MTE lock is not required
for KVM.

This will require some rework of patch #2, I believe. And while we're
at it, a rebase on 5.13-rc4 wouldn't hurt, as both patches #3 and #5
conflict with it...


Jazz is not dead. It just smells funny...