Re: [PATCH 6.18.y] dm-pcache: validate seg_id fields from persistent memory

[Greg KH <greg@kroah.com>]

On Wed, Sep 09, 2026 at 08:18:11PM -0400, Sasha Levin wrote:
> From: Bryam Vargas <hexlabsecurity@proton.me>
> 
> [ Upstream commit 90c990a68460d7b5720e5634cf650eccdf0f4098 ]
> 
> cache_pos_decode(), cache_key_decode() and the last-kset branches of
> cache_replay(), the writeback worker and the GC worker take a cache
> segment id from the cache device metadata and index cache->segments[]
> with it without checking it against cache->n_segs. That metadata is only
> CRC-protected with a fixed public seed, so whoever supplies the cache
> device on a table load (CAP_SYS_ADMIN) controls the id; an out-of-range
> value forms a wild pcache_cache_segment pointer that is dereferenced and
> written through -- an out-of-bounds read and write driven by on-disk data.
> 
> Add cache_seg_id_valid() and reject an out-of-range id at each decode
> site, failing the operation with -EIO instead of indexing past the array.
> Bound the id against the initialized-segment count (cache_info.n_segs)
> rather than the physical device total. A forged cache_info.n_segs below
> seg_num otherwise leaves segments[cache_info.n_segs..seg_num) as zeroed
> structs whose data pointer is NULL, so a forged id in that window would
> still be dereferenced. A later patch guarantees cache_info.n_segs <=
> seg_num, and a driver-created cache sets the two equal, so valid images
> are unaffected.
> 
> Fixes: 1d57628ff95b ("dm-pcache: add persistent cache target in device-mapper")
> Cc: stable@vger.kernel.org
> Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> [ adjusted cache_replay() context to preserve the existing hop-limit check before segment-ID validation ]
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>  drivers/md/dm-pcache/cache.c           |  4 ++++
>  drivers/md/dm-pcache/cache.h           | 15 +++++++++++++++
>  drivers/md/dm-pcache/cache_gc.c        | 16 ++++++++++++++--
>  drivers/md/dm-pcache/cache_key.c       | 11 +++++++++++
>  drivers/md/dm-pcache/cache_writeback.c | 23 +++++++++++++++++++----
>  5 files changed, 63 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/md/dm-pcache/cache.c b/drivers/md/dm-pcache/cache.c
> index 887157fde68da..502acf2451779 100644
> --- a/drivers/md/dm-pcache/cache.c
> +++ b/drivers/md/dm-pcache/cache.c
> @@ -118,6 +118,9 @@ int cache_pos_decode(struct pcache_cache *cache,
>  	if (!latest_addr)
>  		return -EIO;
>  
> +	if (!cache_seg_id_valid(cache, latest.cache_seg_id))
> +		return -EIO;
> +
>  	pos->cache_seg = &cache->segments[latest.cache_seg_id];
>  
>  	if (latest.seg_off >= pos->cache_seg->segment.data_size)
> @@ -158,6 +161,7 @@ static int cache_init(struct dm_pcache *pcache)
>  	cache->cache_dev = &pcache->cache_dev;
>  	cache->n_segs = cache_dev->seg_num;
>  	atomic_set(&cache->gc_errors, 0);
> +	atomic_set(&cache->writeback_errors, 0);
>  	spin_lock_init(&cache->seg_map_lock);
>  	spin_lock_init(&cache->key_head_lock);
>  
> diff --git a/drivers/md/dm-pcache/cache.h b/drivers/md/dm-pcache/cache.h
> index afc112b79496a..d9e3e09f18e32 100644
> --- a/drivers/md/dm-pcache/cache.h
> +++ b/drivers/md/dm-pcache/cache.h
> @@ -180,6 +180,7 @@ struct pcache_cache {
>  		u32 advance;
>  		int ret;
>  	} writeback_ctx;
> +	atomic_t		writeback_errors;
>  
>  	char gc_kset_onmedia_buf[PCACHE_KSET_ONMEDIA_SIZE_MAX];
>  	struct delayed_work	gc_work;
> @@ -420,6 +421,20 @@ static inline bool cache_seg_is_ctrl_seg(u32 cache_seg_id)
>  	return (cache_seg_id == 0);
>  }
>  
> +/**
> + * cache_seg_id_valid - Validate a cache segment id read from the cache device.
> + * @cache: Pointer to the pcache_cache structure.
> + * @cache_seg_id: Segment id decoded from on-media metadata.
> + *
> + * On-media segment ids are only protected by a CRC, which an attacker who can
> + * format the cache device computes over their chosen value. Reject any id that
> + * would index cache->segments[] out of bounds before it is dereferenced.
> + */
> +static inline bool cache_seg_id_valid(struct pcache_cache *cache, u32 cache_seg_id)
> +{
> +	return cache_seg_id < cache->cache_info.n_segs;
> +}
> +
>  /**
>   * cache_key_cutfront - Cuts a specified length from the front of a cache key.
>   * @key: Pointer to pcache_cache_key structure.
> diff --git a/drivers/md/dm-pcache/cache_gc.c b/drivers/md/dm-pcache/cache_gc.c
> index 3088cfef951bc..a167892be7cef 100644
> --- a/drivers/md/dm-pcache/cache_gc.c
> +++ b/drivers/md/dm-pcache/cache_gc.c
> @@ -74,11 +74,17 @@ static bool need_gc(struct pcache_cache *cache, struct pcache_cache_pos *dirty_t
>   * @cache: Pointer to the pcache_cache structure.
>   * @kset_onmedia: Pointer to the kset_onmedia structure for the last kset.
>   */
> -static void last_kset_gc(struct pcache_cache *cache, struct pcache_cache_kset_onmedia *kset_onmedia)
> +static int last_kset_gc(struct pcache_cache *cache, struct pcache_cache_kset_onmedia *kset_onmedia)
>  {
>  	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
>  	struct pcache_cache_segment *cur_seg, *next_seg;
>  
> +	if (!cache_seg_id_valid(cache, kset_onmedia->next_cache_seg_id)) {
> +		pcache_dev_err(pcache, "invalid next_cache_seg_id %u in gc (n_segs %u)\n",
> +				kset_onmedia->next_cache_seg_id, cache->n_segs);
> +		return -EIO;
> +	}
> +
>  	cur_seg = cache->key_tail.cache_seg;
>  
>  	next_seg = &cache->segments[kset_onmedia->next_cache_seg_id];
> @@ -94,6 +100,8 @@ static void last_kset_gc(struct pcache_cache *cache, struct pcache_cache_kset_on
>  	spin_lock(&cache->seg_map_lock);
>  	__clear_bit(cur_seg->cache_seg_id, cache->seg_map);
>  	spin_unlock(&cache->seg_map_lock);
> +
> +	return 0;
>  }
>  
>  void pcache_cache_gc_fn(struct work_struct *work)
> @@ -130,7 +138,11 @@ void pcache_cache_gc_fn(struct work_struct *work)
>  			if (dirty_tail.cache_seg == key_tail.cache_seg)
>  				break;
>  
> -			last_kset_gc(cache, kset_onmedia);
> +			ret = last_kset_gc(cache, kset_onmedia);
> +			if (ret) {
> +				atomic_inc(&cache->gc_errors);
> +				return;
> +			}
>  			continue;
>  		}
>  
> diff --git a/drivers/md/dm-pcache/cache_key.c b/drivers/md/dm-pcache/cache_key.c
> index a3bb1f95afb90..47338a4df36fc 100644
> --- a/drivers/md/dm-pcache/cache_key.c
> +++ b/drivers/md/dm-pcache/cache_key.c
> @@ -94,6 +94,12 @@ int cache_key_decode(struct pcache_cache *cache,
>  	key->off = key_onmedia->off;
>  	key->len = key_onmedia->len;
>  
> +	if (!cache_seg_id_valid(cache, key_onmedia->cache_seg_id)) {
> +		pcache_dev_err(pcache, "invalid cache_seg_id %u in cache key (n_segs %u)\n",
> +				key_onmedia->cache_seg_id, cache->n_segs);
> +		return -EIO;
> +	}
> +
>  	key->cache_pos.cache_seg = &cache->segments[key_onmedia->cache_seg_id];
>  	key->cache_pos.seg_off = key_onmedia->cache_seg_off;
>  
> @@ -801,6 +807,11 @@ int cache_replay(struct pcache_cache *cache)
>  				goto out;
>  			}
>  
> +			if (!cache_seg_id_valid(cache, kset_onmedia->next_cache_seg_id)) {
> +				ret = -EIO;
> +				goto out;
> +			}
> +
>  			next_seg = &cache->segments[kset_onmedia->next_cache_seg_id];
>  
>  			pos->cache_seg = next_seg;
> diff --git a/drivers/md/dm-pcache/cache_writeback.c b/drivers/md/dm-pcache/cache_writeback.c
> index 3df8687e2382f..c34db79fcefb4 100644
> --- a/drivers/md/dm-pcache/cache_writeback.c
> +++ b/drivers/md/dm-pcache/cache_writeback.c
> @@ -196,12 +196,18 @@ static int cache_kset_insert_tree(struct pcache_cache *cache, struct pcache_cach
>  	return ret;
>  }
>  
> -static void last_kset_writeback(struct pcache_cache *cache,
> +static int last_kset_writeback(struct pcache_cache *cache,
>  		struct pcache_cache_kset_onmedia *last_kset_onmedia)
>  {
>  	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
>  	struct pcache_cache_segment *next_seg;
>  
> +	if (!cache_seg_id_valid(cache, last_kset_onmedia->next_cache_seg_id)) {
> +		pcache_dev_err(pcache, "invalid next_cache_seg_id %u in writeback (n_segs %u)\n",
> +				last_kset_onmedia->next_cache_seg_id, cache->n_segs);
> +		return -EIO;
> +	}
> +
>  	pcache_dev_debug(pcache, "last kset, next: %u\n", last_kset_onmedia->next_cache_seg_id);
>  
>  	next_seg = &cache->segments[last_kset_onmedia->next_cache_seg_id];
> @@ -211,6 +217,8 @@ static void last_kset_writeback(struct pcache_cache *cache,
>  	cache->dirty_tail.seg_off = 0;
>  	cache_encode_dirty_tail(cache);
>  	mutex_unlock(&cache->dirty_tail_lock);
> +
> +	return 0;
>  }
>  
>  void cache_writeback_fn(struct work_struct *work)
> @@ -229,6 +237,9 @@ void cache_writeback_fn(struct work_struct *work)
>  	if (pcache_is_stopping(pcache))
>  		goto unlock;
>  
> +	if (atomic_read(&cache->writeback_errors))
> +		goto unlock;
> +
>  	kset_onmedia = (struct pcache_cache_kset_onmedia *)cache->wb_kset_onmedia_buf;
>  
>  	mutex_lock(&cache->dirty_tail_lock);
> @@ -241,15 +252,19 @@ void cache_writeback_fn(struct work_struct *work)
>  	}
>  
>  	if (kset_onmedia->flags & PCACHE_KSET_FLAGS_LAST) {
> -		last_kset_writeback(cache, kset_onmedia);
> +		ret = last_kset_writeback(cache, kset_onmedia);
> +		if (ret) {
> +			atomic_inc(&cache->writeback_errors);
> +			goto unlock;
> +		}
>  		delay = 0;
>  		goto queue_work;
>  	}
>  
>  	ret = cache_kset_insert_tree(cache, kset_onmedia);
>  	if (ret) {
> -		delay = PCACHE_CACHE_WRITEBACK_INTERVAL;
> -		goto queue_work;
> +		atomic_inc(&cache->writeback_errors);
> +		goto unlock;
>  	}
>  
>  	cache_wb_tree_writeback(cache, get_kset_onmedia_size(kset_onmedia));
> -- 
> 2.53.0
> 
> 

Did not apply :(