cifs: fix strcat buffer overflow and reduce raciness in smb21_set_oplock_level()

Change strcat to strncpy in the "None" case to fix a buffer overflow
when cinode->oplock is reset to 0 by another thread accessing the same
cinode. It is never valid to append "None" to any other message.

Consolidate multiple writes to cinode->oplock to reduce raciness.

Signed-off-by: Christoph Probst <kernel@probst.it>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>

1 files changed, 8 insertions, 6 deletions

diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
index 860dd1696830..4313cb04549e 100644
--- a/fs/cifs/smb2ops.c
+++ b/fs/cifs/smb2ops.c
@@ -2974,26 +2974,28 @@ smb21_set_oplock_level(struct cifsInodeInfo *cinode, __u32 oplock,
                        unsigned int epoch, bool *purge_cache)
 {
         char message[5] = {0};
+        unsigned int new_oplock = 0;
 
         oplock &= 0xFF;
         if (oplock == SMB2_OPLOCK_LEVEL_NOCHANGE)
                 return;
 
-        cinode->oplock = 0;
         if (oplock & SMB2_LEASE_READ_CACHING_HE) {
-                cinode->oplock |= CIFS_CACHE_READ_FLG;
+                new_oplock |= CIFS_CACHE_READ_FLG;
                 strcat(message, "R");
         }
         if (oplock & SMB2_LEASE_HANDLE_CACHING_HE) {
-                cinode->oplock |= CIFS_CACHE_HANDLE_FLG;
+                new_oplock |= CIFS_CACHE_HANDLE_FLG;
                 strcat(message, "H");
         }
         if (oplock & SMB2_LEASE_WRITE_CACHING_HE) {
-                cinode->oplock |= CIFS_CACHE_WRITE_FLG;
+                new_oplock |= CIFS_CACHE_WRITE_FLG;
                 strcat(message, "W");
         }
-        if (!cinode->oplock)
-                strcat(message, "None");
+        if (!new_oplock)
+                strncpy(message, "None", sizeof(message));
+
+        cinode->oplock = new_oplock;
         cifs_dbg(FYI, "%s Lease granted on inode %p\n", message,
                  &cinode->vfs_inode);
 }