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.

Code flow for SNP panic notifier:
snp_shutdown_on_panic() ->
__sev_firmware_shutdown() ->
__sev_snp_shutdown_locked() ->
atomic_notifier_chain_unregister(.., &snp_panic_notifier)

Fix SNP panic notifier to unregister itself during SNP shutdown
only if panic is not in progress.

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 | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
index 8fb94c5f006a..0ba5fe3b7883 100644
--- a/drivers/crypto/ccp/sev-dev.c
+++ b/drivers/crypto/ccp/sev-dev.c
@@ -1787,8 +1787,9 @@ 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);
+	if (!panic)
+		atomic_notifier_chain_unregister(&panic_notifier_list,
+						 &snp_panic_notifier);
 
 	/* Reset TMR size back to default */
 	sev_es_tmr_size = SEV_TMR_SIZE;
-- 
2.34.1

On 6/5/25 15:38, 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.
> 
> Code flow for SNP panic notifier:
> snp_shutdown_on_panic() ->
> __sev_firmware_shutdown() ->
> __sev_snp_shutdown_locked() ->
> atomic_notifier_chain_unregister(.., &snp_panic_notifier)
> 
> Fix SNP panic notifier to unregister itself during SNP shutdown
> only if panic is not in progress.
> 
> Fixes: 19860c3274fb ("crypto: ccp - Register SNP panic notifier only if SNP is enabled")
> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>

Looks good code-wise. A nice comment that explains the deadlock if
called during a panic would be good to add above the check for panic.

With that added:

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
>  drivers/crypto/ccp/sev-dev.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
> index 8fb94c5f006a..0ba5fe3b7883 100644
> --- a/drivers/crypto/ccp/sev-dev.c
> +++ b/drivers/crypto/ccp/sev-dev.c
> @@ -1787,8 +1787,9 @@ 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);
> +	if (!panic)
> +		atomic_notifier_chain_unregister(&panic_notifier_list,
> +						 &snp_panic_notifier);
>  
>  	/* Reset TMR size back to default */
>  	sev_es_tmr_size = SEV_TMR_SIZE;