[PATCH v2] keys: big_key: Remove the redundant quota reservation

[sashiko-bot at kernel.org]

> A big_key is charged a fixed 16 bytes of key quota, regardless of its
> payload size.  big_key_update() nevertheless reserves the new raw payload
> length before destroying the old payload, and generic_key_instantiate()
> then reserves the fixed quota length.
> 
> For an update shorter than 16 bytes, the first reservation temporarily
> returns quota.  Another key operation by the same user can consume that
> quota before generic_key_instantiate() reserves it again.  If the second
> reservation fails, the key remains positive after its old payload has been
> destroyed.  Reading a buffer-backed key or revoking a shmem-backed key then
> dereferences a NULL payload pointer.
> 
> On a KASAN build, reading a buffer-backed key reports:
> 
>   BUG: KASAN: null-ptr-deref in big_key_read+0x110/0x310
> [ ... ]
> 
> Fixes: b6f61c314649 ("keys: Implement update for the big_key type")
> Cc: stable at vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Daehyeon Ko <4ncienth at gmail.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261017152123.1200429-1-4ncienth@gmail.com?part=1