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

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Message-ID: <CAOQ4uxiXt4Y=fP+nUfbKkf46of4em633Dmd+iUCFyB=5ijvHdw@mail.gmail.com>
Date: Thu, 14 Nov 2024 12:01:42 +0100
From: Amir Goldstein <amir73il@...il.com>
To: Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc: brauner@...nel.org, miklos@...redi.hu, hu1.chen@...el.com, 
	malini.bhandaru@...el.com, tim.c.chen@...el.com, mikko.ylinen@...el.com, 
	linux-unionfs@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/4] ovl: Optimize override/revert creds

On Thu, Nov 14, 2024 at 9:56 AM Amir Goldstein <amir73il@...il.com> wrote:
>
> On Wed, Nov 13, 2024 at 8:30 PM Vinicius Costa Gomes
> <vinicius.gomes@...el.com> wrote:
> >
> > Amir Goldstein <amir73il@...il.com> writes:
> >
> > > On Thu, Nov 7, 2024 at 1:57 AM Vinicius Costa Gomes
> > > <vinicius.gomes@...el.com> 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:
>

Vinicius,

Sorry, your fix is correct. I did not apply it properly when I tested.

I edited the comment as follows and applied on top of the patch
that I just sent [1]:


-       put_cred(override_creds(override_cred));
+
+       /*
+        * Caller is going to match this with revert_creds_light() and drop
+        * reference on the returned creds.
+        * We must be called with creator creds already, otherwise we risk
+        * leaking creds.
+        */
+       old_cred = override_creds_light(override_cred);
+       WARN_ON_ONCE(old_cred != ovl_creds(dentry->d_sb));

        return override_cred;

Thanks,
Amir.

[1] https://lore.kernel.org/linux-unionfs/20241114100536.628162-1-amir73il@gmail.com/