CIFS: Fix retry mid list corruption on reconnects

When the client hits reconnect it iterates over the mid
pending queue marking entries for retry and moving them
to a temporary list to issue callbacks later without holding
GlobalMid_Lock. In the same time there is no guarantee that
mids can't be removed from the temporary list or even
freed completely by another thread. It may cause a temporary
list corruption:

[  430.454897] list_del corruption. prev->next should be ffff98d3a8f316c0, but was 2e885cb266355469
[  430.464668] ------------[ cut here ]------------
[  430.466569] kernel BUG at lib/list_debug.c:51!
[  430.468476] invalid opcode: 0000 [#1] SMP PTI
[  430.470286] CPU: 0 PID: 13267 Comm: cifsd Kdump: loaded Not tainted 5.4.0-rc3+ #19
[  430.473472] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
[  430.475872] RIP: 0010:__list_del_entry_valid.cold+0x31/0x55
...
[  430.510426] Call Trace:
[  430.511500]  cifs_reconnect+0x25e/0x610 [cifs]
[  430.513350]  cifs_readv_from_socket+0x220/0x250 [cifs]
[  430.515464]  cifs_read_from_socket+0x4a/0x70 [cifs]
[  430.517452]  ? try_to_wake_up+0x212/0x650
[  430.519122]  ? cifs_small_buf_get+0x16/0x30 [cifs]
[  430.521086]  ? allocate_buffers+0x66/0x120 [cifs]
[  430.523019]  cifs_demultiplex_thread+0xdc/0xc30 [cifs]
[  430.525116]  kthread+0xfb/0x130
[  430.526421]  ? cifs_handle_standard+0x190/0x190 [cifs]
[  430.528514]  ? kthread_park+0x90/0x90
[  430.530019]  ret_from_fork+0x35/0x40

Fix this by obtaining extra references for mids being retried
and marking them as MID_DELETED which indicates that such a mid
has been dequeued from the pending list.

Also move mid cleanup logic from DeleteMidQEntry to
_cifs_mid_q_entry_release which is called when the last reference
to a particular mid is put. This allows to avoid any use-after-free
of response buffers.

The patch needs to be backported to stable kernels. A stable tag
is not mentioned below because the patch doesn't apply cleanly
to any actively maintained stable kernel.

Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-and-tested-by: David Wysochanski <dwysocha@redhat.com>
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

2 files changed, 32 insertions, 20 deletions

diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
index bdea4b3e8005..ccaa8bad336f 100644
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -564,9 +564,11 @@ cifs_reconnect(struct TCP_Server_Info *server)
         spin_lock(&GlobalMid_Lock);
         list_for_each_safe(tmp, tmp2, &server->pending_mid_q) {
                 mid_entry = list_entry(tmp, struct mid_q_entry, qhead);
+                kref_get(&mid_entry->refcount);
                 if (mid_entry->mid_state == MID_REQUEST_SUBMITTED)
                         mid_entry->mid_state = MID_RETRY_NEEDED;
                 list_move(&mid_entry->qhead, &retry_list);
+                mid_entry->mid_flags |= MID_DELETED;
         }
         spin_unlock(&GlobalMid_Lock);
         mutex_unlock(&server->srv_mutex);
@@ -576,6 +578,7 @@ cifs_reconnect(struct TCP_Server_Info *server)
                 mid_entry = list_entry(tmp, struct mid_q_entry, qhead);
                 list_del_init(&mid_entry->qhead);
                 mid_entry->callback(mid_entry);
+                cifs_mid_q_entry_release(mid_entry);
         }
 
         if (cifs_rdma_enabled(server)) {
@@ -895,8 +898,10 @@ dequeue_mid(struct mid_q_entry *mid, bool malformed)
         if (mid->mid_flags & MID_DELETED)
                 printk_once(KERN_WARNING
                             "trying to dequeue a deleted mid\n");
-        else
+        else {
                 list_del_init(&mid->qhead);
+                mid->mid_flags |= MID_DELETED;
+        }
         spin_unlock(&GlobalMid_Lock);
 }
 
@@ -966,8 +971,10 @@ static void clean_demultiplex_info(struct TCP_Server_Info *server)
                 list_for_each_safe(tmp, tmp2, &server->pending_mid_q) {
                         mid_entry = list_entry(tmp, struct mid_q_entry, qhead);
                         cifs_dbg(FYI, "Clearing mid 0x%llx\n", mid_entry->mid);
+                        kref_get(&mid_entry->refcount);
                         mid_entry->mid_state = MID_SHUTDOWN;
                         list_move(&mid_entry->qhead, &dispose_list);
+                        mid_entry->mid_flags |= MID_DELETED;
                 }
                 spin_unlock(&GlobalMid_Lock);
 
@@ -977,6 +984,7 @@ static void clean_demultiplex_info(struct TCP_Server_Info *server)
                         cifs_dbg(FYI, "Callback mid 0x%llx\n", mid_entry->mid);
                         list_del_init(&mid_entry->qhead);
                         mid_entry->callback(mid_entry);
+                        cifs_mid_q_entry_release(mid_entry);
                 }
                 /* 1/8th of sec is more than enough time for them to exit */
                 msleep(125);
diff --git a/fs/cifs/transport.c b/fs/cifs/transport.c
index 308ad0f495e1..ca3de62688d6 100644
--- a/fs/cifs/transport.c
+++ b/fs/cifs/transport.c
@@ -86,22 +86,8 @@ AllocMidQEntry(const struct smb_hdr *smb_buffer, struct TCP_Server_Info *server)
 
 static void _cifs_mid_q_entry_release(struct kref *refcount)
 {
-        struct mid_q_entry *mid = container_of(refcount, struct mid_q_entry,
-                                               refcount);
-
-        mempool_free(mid, cifs_mid_poolp);
-}
-
-void cifs_mid_q_entry_release(struct mid_q_entry *midEntry)
-{
-        spin_lock(&GlobalMid_Lock);
-        kref_put(&midEntry->refcount, _cifs_mid_q_entry_release);
-        spin_unlock(&GlobalMid_Lock);
-}
-
-void
-DeleteMidQEntry(struct mid_q_entry *midEntry)
-{
+        struct mid_q_entry *midEntry =
+                        container_of(refcount, struct mid_q_entry, refcount);
 #ifdef CONFIG_CIFS_STATS2
         __le16 command = midEntry->server->vals->lock_cmd;
         __u16 smb_cmd = le16_to_cpu(midEntry->command);
@@ -166,6 +152,19 @@ DeleteMidQEntry(struct mid_q_entry *midEntry)
                 }
         }
 #endif
+
+        mempool_free(midEntry, cifs_mid_poolp);
+}
+
+void cifs_mid_q_entry_release(struct mid_q_entry *midEntry)
+{
+        spin_lock(&GlobalMid_Lock);
+        kref_put(&midEntry->refcount, _cifs_mid_q_entry_release);
+        spin_unlock(&GlobalMid_Lock);
+}
+
+void DeleteMidQEntry(struct mid_q_entry *midEntry)
+{
         cifs_mid_q_entry_release(midEntry);
 }
 
@@ -173,8 +172,10 @@ void
 cifs_delete_mid(struct mid_q_entry *mid)
 {
         spin_lock(&GlobalMid_Lock);
-        list_del_init(&mid->qhead);
-        mid->mid_flags |= MID_DELETED;
+        if (!(mid->mid_flags & MID_DELETED)) {
+                list_del_init(&mid->qhead);
+                mid->mid_flags |= MID_DELETED;
+        }
         spin_unlock(&GlobalMid_Lock);
 
         DeleteMidQEntry(mid);
@@ -872,7 +873,10 @@ cifs_sync_mid_result(struct mid_q_entry *mid, struct TCP_Server_Info *server)
                 rc = -EHOSTDOWN;
                 break;
         default:
-                list_del_init(&mid->qhead);
+                if (!(mid->mid_flags & MID_DELETED)) {
+                        list_del_init(&mid->qhead);
+                        mid->mid_flags |= MID_DELETED;
+                }
                 cifs_server_dbg(VFS, "%s: invalid mid state mid=%llu state=%d\n",
                          __func__, mid->mid, mid->mid_state);
                 rc = -EIO;