Re: [PATCH] KEYS: Fix add_key() race with keyring restriction

[Jarkko Sakkinen]

On Wed, Sep 30, 2026 at 01:18:01PM +0900, tjdqudcks0424@xxxxxxxxx wrote:
> From: 성병찬 <tjdqudcks0424@xxxxxxxxx>
> 
> __key_create_or_update() snapshots keyring->restrict_link before taking
> the destination keyring's semaphore.  keyring_restrict() installs a
> restriction while holding that semaphore.
> 
> This allows a writer to observe no restriction, wait for the keyring
> owner to install a reject-all restriction and return successfully, and
> then link a key using the stale NULL snapshot.  The writer only needs
> write permission on the destination keyring.
> 
> Move the restrict_link read after __key_link_lock() and
> __key_link_begin().  The read and the subsequent restriction check are
> then serialized with restriction installation by keyring->sem.
> 
> The race was reproduced on v7.2.8 in 19 executions where restriction
> installation returned before the link completed.  All 19 linked the key
> despite the reject-all restriction.  With this change, 312 executions
> reached the same ordering and every add_key() call failed with -EPERM.
> 
> The issue was found by manual concurrency analysis assisted by AI-based
> analysis and independently verified with a QEMU reproducer and kernel
> instrumentation.
> 
> Fixes: 5ac7eace2d00 ("KEYS: Add a facility to restrict new links into a keyring")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: 성병찬 <tjdqudcks0424@xxxxxxxxx>
> ---
>  security/keys/key.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/security/keys/key.c b/security/keys/key.c
> index b34a64d81d47..a438c4508595 100644
> --- a/security/keys/key.c
> +++ b/security/keys/key.c
> @@ -840,9 +840,6 @@ static key_ref_t __key_create_or_update(key_ref_t keyring_ref,
>  
>  	key_check(keyring);
>  
> -	if (!(flags & KEY_ALLOC_BYPASS_RESTRICTION))
> -		restrict_link = keyring->restrict_link;
> -
>  	key_ref = ERR_PTR(-ENOTDIR);
>  	if (keyring->type != &key_type_keyring)
>  		goto error_put_type;
> @@ -880,6 +877,9 @@ static key_ref_t __key_create_or_update(key_ref_t keyring_ref,
>  		goto error_link_end;
>  	}
>  
> +	if (!(flags & KEY_ALLOC_BYPASS_RESTRICTION))
> +		restrict_link = keyring->restrict_link;
> +
>  	if (restrict_link && restrict_link->check) {
>  		ret = restrict_link->check(keyring, index_key.type,
>  					   &prep.payload, restrict_link->key);
> 
> base-commit: 6f8319e3e9a44dd537d17f41565a8453c560a581
> -- 
> 2.43.0
> 


Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>

Thanks.

Br, Jarkko