FAILED: patch "[PATCH] smb: client: fix heap overflow in DACL owner/group rewrite" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 0ee150794c75bcd0be0e24ff3394f433cbae18cc
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026091607-trouble-defuse-0631@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 0ee150794c75bcd0be0e24ff3394f433cbae18cc Mon Sep 17 00:00:00 2001
From: Bjoern Doebel <[email protected]>
Date: Tue, 8 Sep 2026 16:10:00 +0000
Subject: [PATCH] smb: client: fix heap overflow in DACL owner/group rewrite

When id_mode_to_cifs_acl rewrites an existing DACL, it allocates a buffer sized according to the on-disk DACL length reported by dacl_ptr->size. However, replace_sids_and_copy_aces may rewrite each ACE with a new owner/group SID obtained from the cifs.idmap upcall. Those SIDs can have up to SID_MAX_SUB_AUTHORITIES (15) sub-authorities, making each ACE up to 76 bytes (sizeof(struct smb_ace)).

If the original DACL contains short SIDs (e.g., 1 sub-authority) while the replacement SIDs are long, the rewritten ACEs overflow the allocation.

Fix this by always budgeting for worst-case SID expansion: allocate sizeof(struct smb_acl) plus num_aces * sizeof(struct smb_ace), which covers the smb_acl header and room for every ACE at maximum SID size. This replaces the previous split logic that used dacl_ptr->size for cifsacl mounts but num_aces * sizeof(struct smb_ace) for mode_from_sid
mounts: both paths can trigger the same rewrite and need the same headroom.

KASAN reports this as:

  BUG: KASAN: slab-out-of-bounds in build_sec_desc+0x1e8a/0x2680 [cifs]
  Write of size 4 at addr ffff8881a5e25374 by task chown/5298
  ...
  The buggy address is located 0 bytes to the right of
   allocated 884-byte region [ffff8881a5e25000, ffff8881a5e25374)

index def8908dd7e9..3e96e151df35 100644
@@ -1837,11 +1837,13 @@ 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);
 		}
 	}
 

From: Bjoern Doebel <[email protected]>

[ Upstream commit 0ee150794c75bcd0be0e24ff3394f433cbae18cc ]

When id_mode_to_cifs_acl rewrites an existing DACL, it allocates a buffer sized according to the on-disk DACL length reported by dacl_ptr->size. However, replace_sids_and_copy_aces may rewrite each ACE with a new owner/group SID obtained from the cifs.idmap upcall. Those SIDs can have up to SID_MAX_SUB_AUTHORITIES (15) sub-authorities, making each ACE up to 76 bytes (sizeof(struct smb_ace)).

If the original DACL contains short SIDs (e.g., 1 sub-authority) while the replacement SIDs are long, the rewritten ACEs overflow the allocation.

Fix this by always budgeting for worst-case SID expansion: allocate sizeof(struct smb_acl) plus num_aces * sizeof(struct smb_ace), which covers the smb_acl header and room for every ACE at maximum SID size. This replaces the previous split logic that used dacl_ptr->size for cifsacl mounts but num_aces * sizeof(struct smb_ace) for mode_from_sid
mounts: both paths can trigger the same rewrite and need the same headroom.

KASAN reports this as:

  BUG: KASAN: slab-out-of-bounds in build_sec_desc+0x1e8a/0x2680 [cifs]
  Write of size 4 at addr ffff8881a5e25374 by task chown/5298
  ...
  The buggy address is located 0 bytes to the right of
   allocated 884-byte region [ffff8881a5e25000, ffff8881a5e25374)

---
 fs/cifs/cifsacl.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

index 1b52e0c88877a..c921d6e2dd51b 100644
@@ -1729,11 +1729,13 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
 				cifs_put_tlink(tlink);
 				return rc;
 			}
-			if (mode_from_sid)
-				nsecdesclen +=
-					le32_to_cpu(dacl_ptr->num_aces) * sizeof(struct cifs_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(cifs_ace) each,
+			 * plus the cifs_acl header replace_sids_and_copy_aces() emits.
+			 */
+			nsecdesclen += sizeof(struct cifs_acl) +
+				le32_to_cpu(dacl_ptr->num_aces) * sizeof(struct cifs_ace);
 		}
 	}
 

2.53.0