Re: [PATCH] keys: Protect the update method check against type removal

[Jarkko Sakkinen]

On Thu, Oct 29, 2026 at 07:58:06PM +0800, Cen Zhang wrote:
> key_update() reads key->type->update, ->def_datalen and ->preparse
> before it takes key->sem. A reference on the key does not pin the
> module that provides its type, so KEYCTL_UPDATE can race with the
> unloading of that module, for example dns_resolver:
> 
>   KEYCTL_UPDATE task                  Module unload and key GC
>   ------------------                  ------------------------
>   keyctl_update_key()
>     key_update()
>       key_permission()
>       load key->type
>                                       exit_dns_resolver()
>                                         unregister_key_type()
>                                           GC: down_write(&key->sem)
>                                           GC: retype to key_type_dead
>                                           GC: up_write(&key->sem)
>                                         unregister_key_type() returns
>                                       free_module()
>       read saved type's update member
> 
> The final access reads freed module memory. With the updating task
> held after the key->type load, it faults:
> 
>   BUG: unable to handle page fault for address: fffffbfff804107f
>   #PF: supervisor read access in kernel mode
>   RIP: 0010:key_update+0x12d/0x550
>   Call Trace:
>    keyctl_update_key+0x6b/0x110
>    __do_sys_keyctl+0x21d/0x4f0
> 
> This is the same problem that commit 925c84e3cc21 ("keys: Protect the
> read method check against type removal") fixed for KEYCTL_READ.

This is useful bit, thanks.

> 
> The encrypted and trusted types can be built as modules and have an
> update method. For them there is a second window: the GC can move the

How is the second window related to the fact that encrypted and trusted
can be built as LKMs and where is "there"?

> key to key_type_dead after the method check and before key_update()
> takes key->sem. key_update() then calls the NULL update method of
> key_type_dead.

I don't understand this paragrap overall. Needs a rewrite.

> 
> Take key->sem before the first access to key->type and hold it until
> the preparsed payload has been freed. Once a key exists, only the GC
> changes key->type, and it does so with key->sem held for writing.

+1

> unregister_key_type() does not return before the GC has done that for
> every key of the type that is still in use, so the type stays valid
> for the whole update. If the GC has already moved the key to
> key_type_dead when key_update() gets key->sem, there is no update
> method and key_update() returns -EOPNOTSUPP.

I don't understand this but the difference to the previous case is that
it would be better to be just deleted.

> 
> Assisted-by: LLM

Please run checkpatch.pl --strict for the next version.

> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Cen Zhang <zzzccc427@xxxxxxxxx>
> ---
>  security/keys/key.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/security/keys/key.c b/security/keys/key.c
> index e88548f1a704..bac2333089fc 100644
> --- a/security/keys/key.c
> +++ b/security/keys/key.c
> @@ -1101,9 +1101,13 @@ int key_update(key_ref_t key_ref, const void *payload, size_t plen)
>  	if (ret < 0)
>  		return ret;
>  
> +	/* the GC cannot change the key's type while we hold the semaphore */

I don't understand this.

> +	down_write(&key->sem);
> +
>  	/* attempt to update it if supported */
> +	ret = -EOPNOTSUPP;
>  	if (!key->type->update)
> -		return -EOPNOTSUPP;
> +		goto out;
>  
>  	memset(&prep, 0, sizeof(prep));
>  	prep.data = payload;
> @@ -1116,8 +1120,6 @@ int key_update(key_ref_t key_ref, const void *payload, size_t plen)
>  			goto error;
>  	}
>  
> -	down_write(&key->sem);
> -
>  	ret = key->type->update(key, &prep);
>  	if (ret == 0) {
>  		/* Updating a negative key positively instantiates it */
> @@ -1125,11 +1127,11 @@ int key_update(key_ref_t key_ref, const void *payload, size_t plen)
>  		notify_key(key, NOTIFY_KEY_UPDATED, 0);
>  	}
>  
> -	up_write(&key->sem);
> -
>  error:
>  	if (key->type->preparse)
>  		key->type->free_preparse(&prep);
> +out:
> +	up_write(&key->sem);
>  	return ret;
>  }
>  EXPORT_SYMBOL(key_update);
> 
> base-commit: 9a2a4c51dccc3168c3dc949fcbaa360a478fa228
> -- 
> 2.43.0
> 

Br, Jarkko