CVE-2026-53390

Summary

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().

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/988c93d3bba066d8669143e6ec30bb2be9608d53
• https://git.kernel.org/stable/c/36599894fa8536fefdf1e296c0af71b8b7226859
• https://git.kernel.org/stable/c/d5c81a095c86fe507c032d08f3a8cfc518444927
• https://git.kernel.org/stable/c/7627ff8c4f9919f14de562b0160ab4ec9d80b1f7
• https://git.kernel.org/stable/c/e36e35660adb9b8ef1435ac359151dda5f094c55
• https://git.kernel.org/stable/c/1ef06004ed4bd6d3ed8c840d9d1a376b66d4935b

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