[PATCH] smb: client: fix DACL-rewrite heap overflow in id_mode_to_cifs_acl()

[Bjoern Doebel <doebel@amazon.de>]

Budget the destination buffer for the worst case in both branches:
every rewritten ACE may take sizeof(struct smb_ace) bytes (which
already accounts for an smb_sid with SID_MAX_SUB_AUTHORITIES
sub-authorities), plus the smb_acl header that
replace_sids_and_copy_aces() emits.

Fixes: bc3e9dd9d104 ("cifs: Change SIDs in ACEs while transferring file ownership.")
Cc: stable@vger.kernel.org
Signed-off-by: Bjoern Doebel <doebel@amazon.de>
Assisted-by: Kiro:claude-opus-4.6
---
 fs/smb/client/cifsacl.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/smb/client/cifsacl.c b/fs/smb/client/cifsacl.c
index 07cf0e5782337..6d572dd995d79 100644
--- a/fs/smb/client/cifsacl.c
+++ b/fs/smb/client/cifsacl.c
@@ -1812,11 +1812,13 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
 				cifs_put_tlink(tlink);
 				return rc;
 			}
-			if (mode_from_sid)
-				nsecdesclen +=
-					le16_to_cpu(dacl_ptr->num_aces) * sizeof(struct smb_ace);
-			else /* cifsacl */
-				nsecdesclen += le16_to_cpu(dacl_ptr->size);
+			/*
+			 * Worst case: every ACE is rewritten with a new SID of
+			 * SID_MAX_SUB_AUTHORITIES sub-auths -> sizeof(smb_ace) each,
+			 * plus the smb_acl header replace_sids_and_copy_aces() emits.
+			 */
+			nsecdesclen += sizeof(struct smb_acl) +
+				le16_to_cpu(dacl_ptr->num_aces) * sizeof(struct smb_ace);
 		}
 	}
 
-- 
2.50.1