Re: [PATCH] crypto: ccp: Fix SNP panic notifier unregistration

["Kalra, Ashish" <ashish.kalra@amd.com>]

Hello Tom,

On 6/4/2025 10:57 AM, Tom Lendacky wrote:
> On 6/2/25 14:10, Ashish Kalra wrote:
>> From: Ashish Kalra <ashish.kalra@amd.com>
>>
>> Panic notifiers are invoked with RCU read lock held and when the
>> SNP panic notifier tries to unregister itself from the panic
>> notifier callback itself it causes a deadlock as notifier
>> unregistration does RCU synchronization.
> 
> You mean that during a panic, __sev_snp_shutdown_locked() is trying to
> unregister the notifier?

Yes.

This is the code path :

snp_shutdown_on_panic() ->
__sev_firmware_shutdown() ->
__sev_snp_shutdown_locked() ->
atomic_notifier_chain_unregister(.., &snp_panic_notifier)

So atomic_notifier_chain_unregister() is being invoked from the panic notifier (context) itself.

> 
> Wouldn't it be better to check if a panic is in progress and not try to
> perform the unregister?

Yes, actually it will be easier to do that by simply checking the panic parameter in
__sev_snp_shutdown_locked().

> 
> Or, is snp_panic_notifier() resilient enough to just always have it
> registered / unregistered on module load/unload?
> 

For registration it makes more sense to do that only if SNP is being initialized and as part of
__sev_snp_init_locked(), for unregistration see notes below.

> Also, wouldn't a better check be snp_panic_notifier.next != NULL during
> sev_pci_exit()?

Actually i can't use snp_initialized here as it will always be false.

But i also can't use snp_panic_notifier.next != NULL, because if it has already been unregistered then
snp_panic_notifier.next may be non-NULL as unregister() would have chained it to the next notifier
on the chain.

Actually i can simply call atomic_notifier_chain_unregister() unconditionally during module unload as it
handles the case of the specific notifier already unregistered and/or not added to the chain.

Thanks,
Ashish
 
> 
> Thanks,
> Tom
> 
>>
>> Fix SNP panic notifier to unregister itself during module unload
>> if SNP is initialized.
>>
>> Fixes: 19860c3274fb ("crypto: ccp - Register SNP panic notifier only if SNP is enabled")
>> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
>> ---
>>  drivers/crypto/ccp/sev-dev.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
>> index 8fb94c5f006a..942d93da1136 100644
>> --- a/drivers/crypto/ccp/sev-dev.c
>> +++ b/drivers/crypto/ccp/sev-dev.c
>> @@ -1787,9 +1787,6 @@ static int __sev_snp_shutdown_locked(int *error, bool panic)
>>  	sev->snp_initialized = false;
>>  	dev_dbg(sev->dev, "SEV-SNP firmware shutdown\n");
>>  
>> -	atomic_notifier_chain_unregister(&panic_notifier_list,
>> -					 &snp_panic_notifier);
>> -
>>  	/* Reset TMR size back to default */
>>  	sev_es_tmr_size = SEV_TMR_SIZE;
>>  
>> @@ -2562,4 +2559,8 @@ void sev_pci_exit(void)
>>  		return;
>>  
>>  	sev_firmware_shutdown(sev);
>> +
>> +	if (sev->snp_initialized)
>> +		atomic_notifier_chain_unregister(&panic_notifier_list,
>> +						 &snp_panic_notifier);
>>  }