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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAOQ4uxguV9SkFihaCcyk1tADNJs4gb8wrA7J3SVYaNnzGhLusw@mail.gmail.com>
Date: Thu, 14 Nov 2024 09:56:04 +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 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:

--- 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.