ovl: fix UAF in ovl_dentry_update_reval by moving dput() in ovl_link_up

JIRA: https://issues.redhat.com/browse/RHEL-93461
CVE: CVE-2025-21887

commit c84e125
Author: Vasiliy Kovalev <kovalev@altlinux.org>
Date:   Sat Feb 15 00:51:48 2025 +0300

    ovl: fix UAF in ovl_dentry_update_reval by moving dput() in ovl_link_up

    The issue was caused by dput(upper) being called before
    ovl_dentry_update_reval(), while upper->d_flags was still
    accessed in ovl_dentry_remote().

    Move dput(upper) after its last use to prevent use-after-free.

    BUG: KASAN: slab-use-after-free in ovl_dentry_remote fs/overlayfs/util.c:162 [inline]
    BUG: KASAN: slab-use-after-free in ovl_dentry_update_reval+0xd2/0xf0 fs/overlayfs/util.c:167

    Call Trace:
     <TASK>
     __dump_stack lib/dump_stack.c:88 [inline]
     dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:114
     print_address_description mm/kasan/report.c:377 [inline]
     print_report+0xc3/0x620 mm/kasan/report.c:488
     kasan_report+0xd9/0x110 mm/kasan/report.c:601
     ovl_dentry_remote fs/overlayfs/util.c:162 [inline]
     ovl_dentry_update_reval+0xd2/0xf0 fs/overlayfs/util.c:167
     ovl_link_up fs/overlayfs/copy_up.c:610 [inline]
     ovl_copy_up_one+0x2105/0x3490 fs/overlayfs/copy_up.c:1170
     ovl_copy_up_flags+0x18d/0x200 fs/overlayfs/copy_up.c:1223
     ovl_rename+0x39e/0x18c0 fs/overlayfs/dir.c:1136
     vfs_rename+0xf84/0x20a0 fs/namei.c:4893
    ...
     </TASK>

    Fixes: b07d5cc ("ovl: update of dentry revalidate flags after copy up")
    Reported-by: syzbot+316db8a1191938280eb6@syzkaller.appspotmail.com
    Closes: https://syzkaller.appspot.com/bug?extid=316db8a1191938280eb6
    Signed-off-by: Vasiliy Kovalev <kovalev@altlinux.org>
    Link: https://lore.kernel.org/r/20250214215148.761147-1-kovalev@altlinux.org
    Reviewed-by: Amir Goldstein <amir73il@gmail.com>
    Signed-off-by: Christian Brauner <brauner@kernel.org>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot

fs/overlayfs/copy_up.c

@@ -620,14 +620,14 @@ static int ovl_link_up(struct ovl_copy_up_ctx *c)
 	err = PTR_ERR(upper);
 	if (!IS_ERR(upper)) {
 		err = ovl_do_link(ofs, ovl_dentry_upper(c->dentry), udir, upper);
-		dput(upper);
 
 		if (!err) {
 			/* Restore timestamps on parent (best effort) */
 			ovl_set_timestamps(ofs, upperdir, &c->pstat);
 			ovl_dentry_set_upper_alias(c->dentry);
 			ovl_dentry_update_reval(c->dentry, upper);
 		}
+		dput(upper);
 	}
 	inode_unlock(udir);
 	if (err)