ACK: [SRU][N][PATCH 1/1] ovl: fix UAF in ovl_dentry_update_reval by moving dput() in ovl_link_up

[Thibault Ferrante]

Acked-by: Thibault Ferrante <thibault.ferrante at canonical.com>


On 16/07/2025 20:33, Manuel Diewald wrote:
> From: Vasiliy Kovalev <kovalev at altlinux.org>
> 
> 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 at syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=316db8a1191938280eb6
> Signed-off-by: Vasiliy Kovalev <kovalev at altlinux.org>
> Link: https://lore.kernel.org/r/20250214215148.761147-1-kovalev@altlinux.org
> Reviewed-by: Amir Goldstein <amir73il at gmail.com>
> Signed-off-by: Christian Brauner <brauner at kernel.org>
> (cherry picked from commit c84e125fff2615b4d9c259e762596134eddd2f27)
> CVE-2025-21887
> Signed-off-by: Manuel Diewald <manuel.diewald at canonical.com>
> ---
>   fs/overlayfs/copy_up.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index 5dce4ffb5567..2730ecb1bd70 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -619,7 +619,6 @@ 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) */
> @@ -627,6 +626,7 @@ 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)