CVE-2026-53390

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: fix out-of-bounds read in smb_check_perm_dacl()

The permission-check ACE walk in smb_check_perm_dacl() validates the ACE

header size and caps sid.num_subauth at SID_MAX_SUB_AUTHORITIES, but it

never checks that ace->size is actually large enough to contain

num_subauth sub-authorities before compare_sids() dereferences them.

CIFS_SID_BASE_SIZE covers the SID header up to but excluding the

sub_auth[] array, and offsetof(struct smb_ace, sid) is the ACE header,

so the existing guards only guarantee the 8-byte SID base, i.e. zero

sub-authorities. compare_sids() then reads ace->sid.sub_auth[i] for

i < min(local_sid->num_subauth, ace->sid.num_subauth). The local

comparison SIDs (sid_everyone, sid_unix_NFS_mode, and the id_to_sid()

result) always have at least one sub-authority, and an attacker controls

the ACE revision and authority bytes (which lie within the in-bounds SID

base), so they can match one of those SIDs and force the sub_auth read.

A crafted ACE with size == 16 and num_subauth >= 1 placed at the tail of

the security descriptor therefore causes a heap out-of-bounds read of up

to SID_MAX_SUB_AUTHORITIES * sizeof(__le32) bytes past the pntsd

allocation. The security descriptor is loaded by ksmbd_vfs_get_sd_xattr()

into a buffer sized exactly to the on-disk data (kzalloc(sd_size) in

ndr_decode_v4_ntacl()), so the read lands past the allocation. The

malformed descriptor can be stored verbatim via SMB2_SET_INFO (the DACL

is not normalised before being written to the security.NTACL xattr) and

the read fires on a subsequent SMB2_CREATE access check, making this

reachable by an authenticated client on a share that uses ACL xattrs.

Add the missing num_subauth-versus-ace_size check, mirroring the

identical guards already present in the sibling parsers parse_dacl() and

smb_inherit_dacl().