[PATCH v2 10/11] dm-pcache: validate the persisted dirty_tail chain at load

[Bryam Vargas via B4 Relay]

From: Bryam Vargas <hexlabsecurity@xxxxxxxxx>

The writeback worker follows the persisted dirty_tail chain, which is
decoded from the cache device independently of the key_tail chain that
cache_replay() walks and bounds. A crafted image, whose on-media fields are
authenticated only by a crc32c with a fixed seed, can aim dirty_tail at a
chain of last ksets that never terminates, so cache_writeback_fn() re-arms
itself with no delay forever.

Walk the dirty_tail chain once at load with the same hop cap cache_replay()
uses and fail the table load with -EIO if it does not reach an end within
n_segs hops.

Fixes: 1d57628ff95b ("dm-pcache: add persistent cache target in device-mapper")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Bryam Vargas <hexlabsecurity@xxxxxxxxx>
---
 drivers/md/dm-pcache/cache.c     |  7 ++++
 drivers/md/dm-pcache/cache.h     |  2 ++
 drivers/md/dm-pcache/cache_key.c | 69 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 78 insertions(+)

diff --git a/drivers/md/dm-pcache/cache.c b/drivers/md/dm-pcache/cache.c
index a94eadb7affd..b0b3e21677de 100644
--- a/drivers/md/dm-pcache/cache.c
+++ b/drivers/md/dm-pcache/cache.c
@@ -202,6 +202,7 @@ static int cache_tail_init(struct pcache_cache *cache)
 {
 	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
 	bool new_cache = !(cache->cache_info.flags & PCACHE_CACHE_FLAGS_INIT_DONE);
+	int ret;
 
 	if (new_cache) {
 		__set_bit(0, cache->seg_map);
@@ -218,6 +219,12 @@ static int cache_tail_init(struct pcache_cache *cache)
 			pcache_dev_err(pcache, "Corrupted key tail or dirty tail.\n");
 			return -EIO;
 		}
+
+		ret = cache_verify_dirty_tail(cache);
+		if (ret) {
+			pcache_dev_err(pcache, "dirty tail chain does not terminate (crafted cache image?)\n");
+			return ret;
+		}
 	}
 
 	return 0;
diff --git a/drivers/md/dm-pcache/cache.h b/drivers/md/dm-pcache/cache.h
index d9e3e09f18e3..8809ec5ae943 100644
--- a/drivers/md/dm-pcache/cache.h
+++ b/drivers/md/dm-pcache/cache.h
@@ -666,6 +666,8 @@ static inline int cache_decode_dirty_tail(struct pcache_cache *cache)
 				&cache->dirty_tail_index);
 }
 
+int cache_verify_dirty_tail(struct pcache_cache *cache);
+
 int pcache_cache_init(void);
 void pcache_cache_exit(void);
 #endif /* _PCACHE_CACHE_H */
diff --git a/drivers/md/dm-pcache/cache_key.c b/drivers/md/dm-pcache/cache_key.c
index f3ce319037be..1caea11a61a3 100644
--- a/drivers/md/dm-pcache/cache_key.c
+++ b/drivers/md/dm-pcache/cache_key.c
@@ -858,6 +858,75 @@ int cache_replay(struct pcache_cache *cache)
 	return ret;
 }
 
+/*
+ * cache_verify_dirty_tail - reject a persisted dirty_tail whose last-kset
+ * chain does not terminate.
+ *
+ * dirty_tail is decoded independently of the key_tail chain cache_replay()
+ * walks, so replay's hop cap does not cover it. A crafted chain that loops
+ * back on itself makes the writeback worker re-arm forever; walk it once here
+ * with the same cap and fail the load if it does not end within n_segs hops.
+ */
+int cache_verify_dirty_tail(struct pcache_cache *cache)
+{
+	struct pcache_cache_pos pos;
+	struct pcache_cache_kset_onmedia *kset_onmedia;
+	u32 to_copy, last_hops = 0, count = 0;
+	int ret = 0;
+
+	kset_onmedia = kzalloc(PCACHE_KSET_ONMEDIA_SIZE_MAX, GFP_KERNEL);
+	if (!kset_onmedia)
+		return -ENOMEM;
+
+	cache_pos_copy(&pos, &cache->dirty_tail);
+
+	while (true) {
+		to_copy = min(PCACHE_KSET_ONMEDIA_SIZE_MAX, cache_seg_remain(&pos));
+		ret = copy_mc_to_kernel(kset_onmedia, cache_pos_addr(&pos), to_copy);
+		if (ret) {
+			ret = -EIO;
+			goto out;
+		}
+
+		/* A missing, short or corrupt kset is the normal end of the chain. */
+		if (!kset_onmedia_valid(kset_onmedia) ||
+		    kset_onmedia->crc != cache_kset_crc(kset_onmedia)) {
+			ret = 0;
+			goto out;
+		}
+
+		if (kset_onmedia->flags & PCACHE_KSET_FLAGS_LAST) {
+			if (!cache_seg_id_valid(cache, kset_onmedia->next_cache_seg_id)) {
+				ret = -EIO;
+				goto out;
+			}
+
+			if (++last_hops > cache->n_segs) {
+				ret = -EIO;
+				goto out;
+			}
+
+			pos.cache_seg = &cache->segments[kset_onmedia->next_cache_seg_id];
+			pos.seg_off = 0;
+			continue;
+		}
+
+		if (get_kset_onmedia_size(kset_onmedia) > cache_seg_remain(&pos)) {
+			ret = -EIO;
+			goto out;
+		}
+
+		cache_pos_advance(&pos, get_kset_onmedia_size(kset_onmedia));
+		if (++count > 512) {
+			cond_resched();
+			count = 0;
+		}
+	}
+out:
+	kfree(kset_onmedia);
+	return ret;
+}
+
 int cache_tree_init(struct pcache_cache *cache, struct pcache_cache_tree *cache_tree, u32 n_subtrees)
 {
 	int ret;

-- 
2.43.0