parse_dacl() treats an ACE SID matching sid_unix_NFS_mode as an NFS
mode SID and reads sid.sub_auth[2] to recover the mode bits.

That assumes the ACE carries three subauthorities, but compare_sids()
only compares min(a, b) subauthorities.  A malicious server can return
an ACE with num_subauth = 2 and sub_auth[] = {88, 3}, which still
matches sid_unix_NFS_mode and then drives the sub_auth[2] read four
bytes past the end of the ACE.

Require num_subauth >= 3 before treating the ACE as an NFS mode SID.
This keeps the fix local to the special-SID mode path without changing
compare_sids() semantics for the rest of cifsacl.

Fixes: e2f8fbfb8d09 ("cifs: get mode bits from special sid on stat")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 fs/smb/client/cifsacl.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/smb/client/cifsacl.c b/fs/smb/client/cifsacl.c
index c920039d733c..a62c8a733779 100644
--- a/fs/smb/client/cifsacl.c
+++ b/fs/smb/client/cifsacl.c
@@ -831,6 +831,7 @@ static void parse_dacl(struct smb_acl *pdacl, char *end_of_acl,
 			dump_ace(ppace[i], end_of_acl);
 #endif
 			if (mode_from_special_sid &&
+			    ppace[i]->sid.num_subauth >= 3 &&
 			    (compare_sids(&(ppace[i]->sid),
 					  &sid_unix_NFS_mode) == 0)) {
 				/*
-- 
2.53.0

This patch wouldn't apply due to a conflict with "smb: client:
validate the whole DACL before rewriting it in cifsacl"

Had to change "end_of_acl" to "end_of_dacl"

See attached.

On Mon, Apr 20, 2026 at 10:14 AM Michael Bommarito
<michael.bommarito@gmail.com> wrote:
>
> parse_dacl() treats an ACE SID matching sid_unix_NFS_mode as an NFS
> mode SID and reads sid.sub_auth[2] to recover the mode bits.
>
> That assumes the ACE carries three subauthorities, but compare_sids()
> only compares min(a, b) subauthorities.  A malicious server can return
> an ACE with num_subauth = 2 and sub_auth[] = {88, 3}, which still
> matches sid_unix_NFS_mode and then drives the sub_auth[2] read four
> bytes past the end of the ACE.
>
> Require num_subauth >= 3 before treating the ACE as an NFS mode SID.
> This keeps the fix local to the special-SID mode path without changing
> compare_sids() semantics for the rest of cifsacl.
>
> Fixes: e2f8fbfb8d09 ("cifs: get mode bits from special sid on stat")
> Cc: stable@vger.kernel.org
> Assisted-by: Claude:claude-opus-4-6
> Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
> ---
>  fs/smb/client/cifsacl.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/fs/smb/client/cifsacl.c b/fs/smb/client/cifsacl.c
> index c920039d733c..a62c8a733779 100644
> --- a/fs/smb/client/cifsacl.c
> +++ b/fs/smb/client/cifsacl.c
> @@ -831,6 +831,7 @@ static void parse_dacl(struct smb_acl *pdacl, char *end_of_acl,
>                         dump_ace(ppace[i], end_of_acl);
>  #endif
>                         if (mode_from_special_sid &&
> +                           ppace[i]->sid.num_subauth >= 3 &&
>                             (compare_sids(&(ppace[i]->sid),
>                                           &sid_unix_NFS_mode) == 0)) {
>                                 /*
> --
> 2.53.0
>
>


-- 
Thanks,

Steve