[PATCH 2/2] smb: client: compare owner and group SIDs using their encoded lengths

[liuyong <pkfxxxing@gmail.com>]

populate_new_aces() compares owner and group SIDs over sizeof(struct
smb_sid), including storage for all 15 possible subauthorities. Received
SIDs contain only their encoded subauthorities. The comparison can read
past the security descriptor, as shown by the supplied KASAN report for
a 44-byte descriptor during chmod.

The input SIDs have already been checked by sid_from_sd(). Require equal
subauthority counts, then compare only the fixed SID header and the
encoded subauthorities with memcmp(). The count check ensures that both
validated SIDs contain the number of bytes being compared.

These issues were discovered and fixed by Qihoo 360's Tulongfeng
multi-agent system.

Fixes: 0f22053e811c ("cifs: Fix unix perm bits to cifsacl conversion for "other" bits.")
Cc: stable@vger.kernel.org
Signed-off-by: liuyong <pkfxxxing@gmail.com>
---
 fs/smb/client/cifsacl.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/smb/client/cifsacl.c b/fs/smb/client/cifsacl.c
index f9f61c758..e420943bd 100644
--- a/fs/smb/client/cifsacl.c
+++ b/fs/smb/client/cifsacl.c
@@ -1134,7 +1134,10 @@ static void populate_new_aces(char *nacl_base,
 	 * updated in the inode.
 	 */
 
-	if (!memcmp(pownersid, pgrpsid, sizeof(struct smb_sid))) {
+	if (pownersid->num_subauth == pgrpsid->num_subauth &&
+	    !memcmp(pownersid, pgrpsid,
+		    CIFS_SID_BASE_SIZE +
+		    pownersid->num_subauth * sizeof(__le32))) {
 		/*
 		 * Case when owner and group SIDs are the same.
 		 * Set the more restrictive of the two modes.