Re: [PATCH] block/keyslot-manager: prevent crash when num_slots=1

[Satya Tangirala <satyat@google.com>]

On Tue, Nov 10, 2020 at 06:14:27PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> If there is only one keyslot, then blk_ksm_init() computes
> slot_hashtable_size=1 and log_slot_ht_size=0.  This causes
> blk_ksm_find_keyslot() to crash later because it uses
> hash_ptr(key, log_slot_ht_size) to find the hash bucket containing the
> key, and hash_ptr() doesn't support the bits == 0 case.
> 
> Fix this by making the hash table always have at least 2 buckets.
> 
> Tested by running:
> 
>     kvm-xfstests -c ext4 -g encrypt -m inlinecrypt \
>                  -o blk-crypto-fallback.num_keyslots=1
> 
> Fixes: 1b2628397058 ("block: Keyslot Manager for Inline Encryption")
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  block/keyslot-manager.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/block/keyslot-manager.c b/block/keyslot-manager.c
> index 35abcb1ec051d..0a5b2772324ad 100644
> --- a/block/keyslot-manager.c
> +++ b/block/keyslot-manager.c
> @@ -103,6 +103,13 @@ int blk_ksm_init(struct blk_keyslot_manager *ksm, unsigned int num_slots)
>  	spin_lock_init(&ksm->idle_slots_lock);
>  
>  	slot_hashtable_size = roundup_pow_of_two(num_slots);
> +
> +	/*
> +	 * hash_ptr() assumes bits != 0, so ensure the hash table has at least 2
> +	 * buckets.  This only makes a difference when there is only 1 keyslot.
> +	 */
> +	slot_hashtable_size = max(slot_hashtable_size, 2U);
> +
>  	ksm->log_slot_ht_size = ilog2(slot_hashtable_size);
>  	ksm->slot_hashtable = kvmalloc_array(slot_hashtable_size,
>  					     sizeof(ksm->slot_hashtable[0]),
> 
> base-commit: f8394f232b1eab649ce2df5c5f15b0e528c92091
> -- 
> 2.29.2
> 
Looks good to me. Please feel free to add
Reviewed-by: Satya Tangirala <satyat@google.com>