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

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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: bc3e9dd9d1 ("cifs: Change SIDs in ACEs while transferring file ownership.")
Assisted-by: Kiro:claude-opus-4.6
Signed-off-by: Bjoern Doebel <doebel@amazon.de>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Fixes: 5c3564852c58 ("cifs: Minimize the number of cifs_acl memory allocations")
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
This commit is contained in:
Bjoern Doebel 2026-09-08 16:10:00 +00:00 • committed by Paulo Alcantara
commit 0ee150794c
1 changed files with 7 additions and 5 deletions
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