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

[Steven Price <steven.price@arm.com>]

On 18/06/2021 16:42, Marc Zyngier wrote:
> 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@arm.com>
>>> Signed-off-by: Steven Price <steven.price@arm.com>
>>
>> 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.

Sure, since KVM no longer uses the lock we don't have the dependency -
so I'll drop the first patch.

> 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...

Yeah there will be minor conflicts in patch #2 - but nothing major. I'll
rebase as requested at the same time.

Thanks,

Steve