[PATCH v2 4/6] staging: erofs: avoid loop in submit chains

[Gao Xiang]

As reported by erofs-utils fuzzer, 2 conditions
can happen in corrupted images, which can cause
unexpected behaviors.
 - access the same pcluster one more time;
 - access the tail end pcluster again, e.g.
            _ access again (will trigger tail merging)
           |
     1 2 3 1 2             ->   1 2 3 1
     |_ tail end of the chain    \___/ (unexpected behavior)
Let's detect and avoid them now.

Reviewed-by: Chao Yu <yuch...@huawei.com>
Signed-off-by: Gao Xiang <gaoxian...@huawei.com>
---
Hi Greg,

 It seems that you picked up [PATCH 4/6], could you replace it
 with this v2? It seems that I missed a condition here, which
 can be observed after a much longer fuzzing on corrupted
 compressed images. Or you could just drop this [PATCH 4/6]
 patch when you apply to staging-next since those patches are
 independent.

Thanks you very much,
Gao Xiang

 drivers/staging/erofs/zdata.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/erofs/zdata.c b/drivers/staging/erofs/zdata.c
index 2d7aaf98f7de..5f8d3ac0e813 100644
--- a/drivers/staging/erofs/zdata.c
+++ b/drivers/staging/erofs/zdata.c
@@ -132,7 +132,7 @@ enum z_erofs_collectmode {
 struct z_erofs_collector {
        struct z_erofs_pagevec_ctor vector;
 
-       struct z_erofs_pcluster *pcl;
+       struct z_erofs_pcluster *pcl, *tailpcl;
        struct z_erofs_collection *cl;
        struct page **compressedpages;
        z_erofs_next_pcluster_t owned_head;
@@ -353,6 +353,11 @@ static struct z_erofs_collection *cllookup(struct 
z_erofs_collector *clt,
                return NULL;
 
        pcl = container_of(grp, struct z_erofs_pcluster, obj);
+       if (clt->owned_head == &pcl->next || pcl == clt->tailpcl) {
+               DBG_BUGON(1);
+               erofs_workgroup_put(grp);
+               return ERR_PTR(-EFSCORRUPTED);
+       }
 
        cl = z_erofs_primarycollection(pcl);
        if (unlikely(cl->pageofs != (map->m_la & ~PAGE_MASK))) {
@@ -379,7 +384,13 @@ static struct z_erofs_collection *cllookup(struct 
z_erofs_collector *clt,
                }
        }
        mutex_lock(&cl->lock);
+       /* used to check tail merging loop due to corrupted images */
+       if (clt->owned_head == Z_EROFS_PCLUSTER_TAIL)
+               clt->tailpcl = pcl;
        clt->mode = try_to_claim_pcluster(pcl, &clt->owned_head);
+       /* clean tailpcl if the current owned_head is Z_EROFS_PCLUSTER_TAIL */
+       if (clt->owned_head == Z_EROFS_PCLUSTER_TAIL)
+               clt->tailpcl = NULL;
        clt->pcl = pcl;
        clt->cl = cl;
        return cl;
@@ -432,6 +443,9 @@ static struct z_erofs_collection *clregister(struct 
z_erofs_collector *clt,
                kmem_cache_free(pcluster_cachep, pcl);
                return ERR_PTR(-EAGAIN);
        }
+       /* used to check tail merging loop due to corrupted images */
+       if (clt->owned_head == Z_EROFS_PCLUSTER_TAIL)
+               clt->tailpcl = pcl;
        clt->owned_head = &pcl->next;
        clt->pcl = pcl;
        clt->cl = cl;
-- 
2.17.1