Re: [PATCH v12 1/8] arm64: mte: Handle race when synchronising tags

[Steven Price]

On 17/05/2021 15:03, Marc Zyngier wrote:
> Hi Steven,

Hi Marc,

> On Mon, 17 May 2021 13:32:32 +0100,
> Steven Price <steven.price@arm.com> 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>
>> ---
>> ---
>>  arch/arm64/kernel/mte.c | 21 ++++++++++++++++++---
>>  1 file changed, 18 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
>> index 125a10e413e9..c88e778c2fa9 100644
>> --- a/arch/arm64/kernel/mte.c
>> +++ b/arch/arm64/kernel/mte.c
>> @@ -25,6 +25,7 @@
>>  u64 gcr_kernel_excl __ro_after_init;
>>  
>>  static bool report_fault_once = true;
>> +static spinlock_t tag_sync_lock;
> 
> What initialises this spinlock? Have you tried this with lockdep? I'd
> expect it to be defined with DEFINE_SPINLOCK(), which always does the
> right thing.

You of course are absolute right, and this will blow up with lockdep.
Sorry about that. DEFINE_SPINLOCK() solves the problem.

Thanks,

Steve