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: b07d5cc93e1b ("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>

diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
index 0c28e5fa340775d7b4cd5d90920d51b045302454..d7310fcf38881e42de1151ca95b12d271eff06d2 100644 (file)
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -618,7 +618,6 @@ 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) */
@@ -626,6 +625,7 @@ static int ovl_link_up(struct ovl_copy_up_ctx *c)
                        ovl_dentry_set_upper_alias(c->dentry);
                        ovl_dentry_update_reval(c->dentry, upper);
                }
+               dput(upper);
        }
        inode_unlock(udir);
        if (err)