CIFS: fix deadlock in cached root handling

Prevent deadlock between open_shroot() and
cifs_mark_open_files_invalid() by releasing the lock before entering
SMB2_open, taking it again after and checking if we still need to use
the result.

Link: https://lore.kernel.org/linux-cifs/684ed01c-cbca-2716-bc28-b0a59a0f8521@prodrive-technologies.com/T/#u
Fixes: 3d4ef9a15343 ("smb3: fix redundant opens on root")
Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>

1 files changed, 45 insertions, 1 deletions

diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
index 0cdc4e47ca87..fed75e1646c1 100644
--- a/fs/cifs/smb2ops.c
+++ b/fs/cifs/smb2ops.c
@@ -694,8 +694,51 @@ int open_shroot(unsigned int xid, struct cifs_tcon *tcon, struct cifs_fid *pfid)
 
         smb2_set_related(&rqst[1]);
 
+        /*
+         * We do not hold the lock for the open because in case
+         * SMB2_open needs to reconnect, it will end up calling
+         * cifs_mark_open_files_invalid() which takes the lock again
+         * thus causing a deadlock
+         */
+
+        mutex_unlock(&tcon->crfid.fid_mutex);
         rc = compound_send_recv(xid, ses, flags, 2, rqst,
                                 resp_buftype, rsp_iov);
+        mutex_lock(&tcon->crfid.fid_mutex);
+
+        /*
+         * Now we need to check again as the cached root might have
+         * been successfully re-opened from a concurrent process
+         */
+
+        if (tcon->crfid.is_valid) {
+                /* work was already done */
+
+                /* stash fids for close() later */
+                struct cifs_fid fid = {
+                        .persistent_fid = pfid->persistent_fid,
+                        .volatile_fid = pfid->volatile_fid,
+                };
+
+                /*
+                 * caller expects this func to set pfid to a valid
+                 * cached root, so we copy the existing one and get a
+                 * reference.
+                 */
+                memcpy(pfid, tcon->crfid.fid, sizeof(*pfid));
+                kref_get(&tcon->crfid.refcount);
+
+                mutex_unlock(&tcon->crfid.fid_mutex);
+
+                if (rc == 0) {
+                        /* close extra handle outside of crit sec */
+                        SMB2_close(xid, tcon, fid.persistent_fid, fid.volatile_fid);
+                }
+                goto oshr_free;
+        }
+
+        /* Cached root is still invalid, continue normaly */
+
         if (rc)
                 goto oshr_exit;
 
@@ -729,8 +772,9 @@ int open_shroot(unsigned int xid, struct cifs_tcon *tcon, struct cifs_fid *pfid)
                                 (char *)&tcon->crfid.file_all_info))
                 tcon->crfid.file_all_info_is_valid = 1;
 
- oshr_exit:
+oshr_exit:
         mutex_unlock(&tcon->crfid.fid_mutex);
+oshr_free:
         SMB2_open_free(&rqst[0]);
         SMB2_query_info_free(&rqst[1]);
         free_rsp_buf(resp_buftype[0], rsp_iov[0].iov_base);