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

jira VULN-55376
cve CVE-2025-21887
commit-author Vasiliy Kovalev <[email protected]>
commit c84e125

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: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=316db8a1191938280eb6
	Signed-off-by: Vasiliy Kovalev <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
	Reviewed-by: Amir Goldstein <[email protected]>
	Signed-off-by: Christian Brauner <[email protected]>
(cherry picked from commit c84e125)
	Signed-off-by: Marcin Wcisło <[email protected]>

Author: pvts-mat

fs/overlayfs/copy_up.c

@@ -532,14 +532,14 @@ 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)