Re: [PATCH v4 4/4] ovl: Optimize override/revert creds

[Amir Goldstein <amir73il@xxxxxxxxx>]

On Wed, Nov 13, 2024 at 8:30 PM Vinicius Costa Gomes
<vinicius.gomes@xxxxxxxxx> wrote:
>
> Amir Goldstein <amir73il@xxxxxxxxx> writes:
>
> > On Thu, Nov 7, 2024 at 1:57 AM Vinicius Costa Gomes
> > <vinicius.gomes@xxxxxxxxx> wrote:
>
> [...]
>
> >
> > Vinicius,
> >
> > While testing fanotify with LTP tests (some are using overlayfs),
> > kmemleak consistently reports the problems below.
> >
> > Can you see the bug, because I don't see it.
> > Maybe it is a false positive...
>
> Hm, if the leak wasn't there before and we didn't touch anything related to
> prepare_creds(), I think that points to the leak being real.
>
> But I see your point, still not seeing it.
>
> This code should be equivalent to the code we have now (just boot
> tested):
>
> ----
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 136a2c7fb9e5..7ebc2fd3097a 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -576,8 +576,7 @@ static int ovl_setup_cred_for_create(struct dentry *dentry, struct inode *inode,
>          * We must be called with creator creds already, otherwise we risk
>          * leaking creds.
>          */
> -       WARN_ON_ONCE(override_creds(override_cred) != ovl_creds(dentry->d_sb));
> -       put_cred(override_cred);
> +       WARN_ON_ONCE(override_creds_light(override_cred) != ovl_creds(dentry->d_sb));
>
>         return 0;
>  }
> ----
>
> Does it change anything? (I wouldn't think so, just to try something)

No, but I think this does:

--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -576,7 +576,8 @@ static int ovl_setup_cred_for_create(struct dentry
*dentry, struct inode *inode,
         * We must be called with creator creds already, otherwise we risk
         * leaking creds.
         */
-       WARN_ON_ONCE(override_creds(override_cred) != ovl_creds(dentry->d_sb));
+       old_cred = override_creds(override_cred);
+       WARN_ON_ONCE(old_cred != ovl_creds(dentry->d_sb));
        put_cred(override_cred);

        return 0;

Compiler optimized out override_creds(override_cred)? :-/

However, this is not enough.

Dropping the ref of the new creds is going to drop the refcount to zero,
so that is incorrect, we need to return the reference to the new creds
explicitly to the callers. I will send a patch.

Thanks,
Amir.