In the Linux kernel, the following vulnerability has been resolved: smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/

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

smb: client: clear setuid/setgid bit on write with cifsacl/modefromsid/posix extensions

When a file has the setuid or setgid bit set and is written to, the VFS
strips those bits and issues a setattr with ATTR_KILL_SUID/ATTR_KILL_SGID
together with an ATTR_MODE carrying the already-cleared mode.

Both cifs_setattr_unix() and cifs_setattr_nounix() unconditionally dropped
ATTR_MODE in that case:

 1/* skip mode change if it's just for clearing setuid/setgid */
 2if (attrs->ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID))
 3	attrs->ia_valid &= ~ATTR_MODE;

This is fine for the default mount, where the mode is only emulated via
the DOS read-only attribute and cannot represent the setuid/setgid bits
anyway. However, with the "cifsacl" or "modefromsid" mount options the
mode is stored on the server through an ACL (id_mode_to_cifs_acl()), with
the SMB3.1.1 POSIX extensions the mode is sent to the server directly,
and with the SMB1 Unix extensions (cifs_setattr_unix) the mode is sent
via CIFSSMBUnixSetPathInfo(). In all those cases dropping ATTR_MODE means
the cleared mode is never pushed to the server, so the setuid/setgid bit
survives the write.

This is a security issue: on local filesystems the setuid bit is stripped
when a file is written, but over these cifs.ko mounts the bit persists on
the server, potentially allowing an unexpected privilege escalation on
subsequent execution.

Fix this in two places:

1. cifs_setattr_nounix(): only take the "skip mode change" shortcut
   when the mode is emulated via the DOS read-only attribute (i.e.
   neither cifsacl/modefromsid nor the SMB3.1.1 POSIX extensions are
   in effect), so that the cleared mode is propagated to the server
   in the ACL / POSIX cases.

2. cifs_setattr_unix(): this function is only called when Unix
   extensions are in effect, so the mode is always stored on the
   server. Remove the shortcut entirely so that the cleared mode is
   always pushed.