[PATCH] smb: client: fix heap overflow in DACL owner/group rewrite

[Bjoern Doebel <doebel@amazon.de>]

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)

Cc: stable@vger.kernel.org
Fixes: 5c3564852c58 ("cifs: Minimize the number of cifs_acl memory allocations")
Assisted-by: Kiro:claude-opus-4.6
Signed-off-by: Bjoern Doebel <doebel@amazon.de>

---
v2:
- Reword commit message to be more descriptive of what is happening
---
---
 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 12005f46307de..2d785a3039585 100644
--- a/fs/smb/client/cifsacl.c
+++ b/fs/smb/client/cifsacl.c
@@ -1815,11 +1815,13 @@ 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