posix_acl: Clear SGID bit when setting file permissions

When file permissions are modified via chmod(2) and the user is not in
the owning group or capable of CAP_FSETID, the setgid bit is cleared in
inode_change_ok().  Setting a POSIX ACL via setxattr(2) sets the file
permissions as well as the new ACL, but doesn't clear the setgid bit in
a similar way; this allows to bypass the check in chmod(2).  Fix that.

References: CVE-2016-7097
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

1 files changed, 4 insertions, 5 deletions

diff --git a/fs/jffs2/acl.c b/fs/jffs2/acl.c
index bc2693d56298..2a0f2a1044c1 100644
--- a/fs/jffs2/acl.c
+++ b/fs/jffs2/acl.c
@@ -233,9 +233,10 @@ int jffs2_set_acl(struct inode *inode, struct posix_acl *acl, int type)
         case ACL_TYPE_ACCESS:
                 xprefix = JFFS2_XPREFIX_ACL_ACCESS;
                 if (acl) {
-                        umode_t mode = inode->i_mode;
-                        rc = posix_acl_equiv_mode(acl, &mode);
-                        if (rc < 0)
+                        umode_t mode;
+
+                        rc = posix_acl_update_mode(inode, &mode, &acl);
+                        if (rc)
                                 return rc;
                         if (inode->i_mode != mode) {
                                 struct iattr attr;
@@ -247,8 +248,6 @@ int jffs2_set_acl(struct inode *inode, struct posix_acl *acl, int type)
                                 if (rc < 0)
                                         return rc;
                         }
-                        if (rc == 0)
-                                acl = NULL;
                 }
                 break;
         case ACL_TYPE_DEFAULT: