[pbs-devel] [PATCH proxmox-backup] chunk store: fix race window between chunk stat and gc cleanup

[Christian Ebner]

Sweeping of unused chunks during garbage collection checks their
atime to distinguish between chunks being in-use and chunks no
longer being used. While garbage collection does lock the chunk
store by guarding its mutex before reading file stats and deleting
unused chunks, the conditional touch did not do this before updating
the chunks atime (thereby also checking the presence).

Therefore there is a race window between the chunks metadata being
read and the chunk being removed, but the chunk being touched
in-between.

The race is however rare, as for this to happen the chunk must be
older than the cutoff time and not be referenced by any index file,
otherwise the atime would be updated during phase 1 already.

Fix by guarding the chunk store mutex before touching a chunk.

Signed-off-by: Christian Ebner <c.ebner at proxmox.com>
---
 pbs-datastore/src/chunk_store.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/pbs-datastore/src/chunk_store.rs b/pbs-datastore/src/chunk_store.rs
index ba7618e40..d21db4a71 100644
--- a/pbs-datastore/src/chunk_store.rs
+++ b/pbs-datastore/src/chunk_store.rs
@@ -217,6 +217,7 @@ impl ChunkStore {
         assert!(self.locker.is_some());
 
         let (chunk_path, _digest_str) = self.chunk_path(digest);
+        let _lock = self.mutex.lock();
         self.cond_touch_path(&chunk_path, assert_exists)
     }
 
-- 
2.47.3