[PATCH v2 1/2] smb: client: harden DFS cache against invalid target hints

[Fredric Cover <fredric.cover.lkernel@gmail.com>]

Currently, get_tgt_name() returns ERR_PTR(-ENOENT) when ce->tgthint is
NULL, and dfs_cache_noreq_update_tgthint() assumes ce->tgthint is always
valid.

In preparation for clearing ce->tgthint in free_tgts(), harden callers
of get_tgt_name() against ERR_PTR results and harden
dfs_cache_noreq_update_tgthint() against NULL pointer dereferences.

Cc: stable@vger.kernel.org
Signed-off-by: Fredric Cover <fredric.cover.lkernel@gmail.com>
---
 fs/smb/client/dfs_cache.c | 31 ++++++++++++++++++++++++-------
 1 file changed, 24 insertions(+), 7 deletions(-)

diff --git a/fs/smb/client/dfs_cache.c b/fs/smb/client/dfs_cache.c
index 8cd93cd2f00f..c9ace4326a35 100644
--- a/fs/smb/client/dfs_cache.c
+++ b/fs/smb/client/dfs_cache.c
@@ -869,13 +869,22 @@ int dfs_cache_find(const unsigned int xid, struct cifs_ses *ses, const struct nl
 		goto out_free_path;
 	}
 
-	if (ref)
-		rc = setup_referral(path, ce, ref, get_tgt_name(ce));
-	else
+	if (ref) {
+		char *target = get_tgt_name(ce);
+
+		if (IS_ERR(target)) {
+			rc = PTR_ERR(target);
+			goto out_unlock;
+		}
+		rc = setup_referral(path, ce, ref, target);
+	} else {
 		rc = 0;
+	}
+
 	if (!rc && tgt_list)
 		rc = get_targets(ce, tgt_list);
 
+out_unlock:
 	up_read(&htable_rw_lock);
 
 out_free_path:
@@ -915,10 +924,17 @@ int dfs_cache_noreq_find(const char *path, struct dfs_info3_param *ref,
 		goto out_unlock;
 	}
 
-	if (ref)
-		rc = setup_referral(path, ce, ref, get_tgt_name(ce));
-	else
+	if (ref) {
+		char *target = get_tgt_name(ce);
+
+		if (IS_ERR(target)) {
+			rc = PTR_ERR(target);
+			goto out_unlock;
+		}
+		rc = setup_referral(path, ce, ref, target);
+	} else {
 		rc = 0;
+	}
 	if (!rc && tgt_list)
 		rc = get_targets(ce, tgt_list);
 
@@ -959,7 +975,8 @@ void dfs_cache_noreq_update_tgthint(const char *path, const struct dfs_cache_tgt
 
 	t = READ_ONCE(ce->tgthint);
 
-	if (unlikely(!strcasecmp(it->it_name, t->name)))
+	/* Check 't' in case ce->tgthint was cleared by free_tgts() */
+	if (t && unlikely(!strcasecmp(it->it_name, t->name)))
 		goto out_unlock;
 
 	list_for_each_entry(t, &ce->tlist, list) {
-- 
2.53.0