CVE-2026-31611ksmbd: require 3 sub-authorities before reading sub_auth[2]

Official description

In the Linux kernel, the following vulnerability has been resolved: ksmbd: require 3 sub-authorities before reading sub_auth[2] parse_dacl() compares each ACE SID against sid_unix_NFS_mode and on match reads sid.sub_auth[2] as the file mode. If sid_unix_NFS_mode is the prefix S-1-5-88-3 with num_subauth = 2 then compare_sids() compares only min(num_subauth, 2) sub-authorities so a client SID with num_subauth = 2 and sub_auth = {88, 3} will match. If num_subauth = 2 and the ACE is placed at the very end of the security descriptor, sub_auth[2] will be 4 bytes past end_of_acl. The out-of-band bytes will then be masked to the low 9 bits and applied as the file's POSIX mode, probably not something that is good to have happen. Fix this up by forcing the SID to actually carry a third sub-authority before reading it at all.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/cf2148b880fb7c0fcd727202dbc4fd5d6998b9c2
• https://git.kernel.org/stable/c/b5b5d5936a50497fb151c0b122899a6894721c2b
• https://git.kernel.org/stable/c/08f9e6d899b5c834bbcc239eae1bed58d9b15d2c
• https://git.kernel.org/stable/c/d2454f4a002d08560a60f214f392e6491cf11560
• https://git.kernel.org/stable/c/46bbcd3ebfb3549c8da1838fc4493e79bd3241e7
• https://git.kernel.org/stable/c/9401f86a224f37b50e6a3ccf1d46a70d5ef8af0a
• https://git.kernel.org/stable/c/53370cf9090777774e07fd9a8ebce67c6cc333ab
• https://cert-portal.siemens.com/productcert/html/ssa-784472.html