linux-kernel - Re: [PATCH next] audit: fix potential double free on error path from fsnotify_add_inode

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Message-ID: <CAHC9VhSw0zJWQEm-GCzQy_-4-X-yVuzvX1wBFcYMkG1mOXy+BA@mail.gmail.com>
Date:   Mon, 22 Aug 2022 10:34:15 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Gaosheng Cui <cuigaosheng1@...wei.com>, eparis@...hat.com,
        mszeredi@...hat.com, amir73il@...il.com, linux-audit@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH next] audit: fix potential double free on error path from fsnotify_add_inode_mark

On Mon, Aug 22, 2022 at 4:50 AM Jan Kara <jack@...e.cz> wrote:
> On Mon 22-08-22 10:29:05, Gaosheng Cui wrote:
> > Audit_alloc_mark() assign pathname to audit_mark->path, on error path
> > from fsnotify_add_inode_mark(), fsnotify_put_mark will free memory
> > of audit_mark->path, but the caller of audit_alloc_mark will free
> > the pathname again, so there will be double free problem.
> >
> > Fix this by resetting audit_mark->path to NULL pointer on error path
> > from fsnotify_add_inode_mark().
> >
> > Fixes: 7b1293234084d ("fsnotify: Add group pointer in fsnotify_init_mark()")
> > Signed-off-by: Gaosheng Cui <cuigaosheng1@...wei.com>
>
> Good spotting! The patch looks good to me. Feel free to add:
>
> Reviewed-by: Jan Kara <jack@...e.cz>
>
> > ---
> >  kernel/audit_fsnotify.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/kernel/audit_fsnotify.c b/kernel/audit_fsnotify.c
> > index 6432a37ac1c9..c565fbf66ac8 100644
> > --- a/kernel/audit_fsnotify.c
> > +++ b/kernel/audit_fsnotify.c
> > @@ -102,6 +102,7 @@ struct audit_fsnotify_mark *audit_alloc_mark(struct audit_krule *krule, char *pa
> >
> >       ret = fsnotify_add_inode_mark(&audit_mark->mark, inode, 0);
> >       if (ret < 0) {
> > +             audit_mark->path = NULL;
> >               fsnotify_put_mark(&audit_mark->mark);

As I'm tracing the code path from audit through fsnotify, and back
into audit, I'm wondering if we still have a problem.  When
fsnotify_add_inode_mark() fails it will end up freeing not just
audit_mark->path, but audit_mark itself via audit_fsnotify_mark_free()
(via a call into fsnotify_put_mark()), yes?  If that is the case, I
think the better fix would simply be to just remove the
fsnotify_put_mark() call and add a small comment in this error patch
mentioning that fsnotify_put_mark() will release audit_mark on error.

Thoughts?

> >               audit_mark = ERR_PTR(ret);
> >       }

-- 
paul-moore.com