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

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Message-ID: <CAHC9VhTpLqvZ7PCi3kn1NE6Sq967wLZA1MgnONUVYa0+j_EXbA@mail.gmail.com>
Date:   Mon, 22 Aug 2022 18:49:44 -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 1:59 PM Jan Kara <jack@...e.cz> wrote:
> On Mon 22-08-22 13:39:23, Paul Moore wrote:
> > On Mon, Aug 22, 2022 at 11:20 AM Jan Kara <jack@...e.cz> wrote:
> > > On Mon 22-08-22 10:34:15, Paul Moore wrote:
> > > > 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?
> > >
> > > I don't think so. fsnotify_add_mark_locked() will call fsnotify_put_mark()
> > > but that is just a counter part to fsnotify_get_mark() a few lines above.
> > > The caller of fsnotify_add_inode_mark() still holds its own mark reference
> > > which prevents mark from being freed.
> >
> > Okay, that sounds reasonable, but I'm still looking for a code path
> > that only frees audit_mark:path and not the audit_mark itself.  What
> > am I not seeing?
>
> The callers of audit_alloc_mark() call kfree(path) if audit_alloc_mark()
> returns error (which is a sensible thing because in some cases path indeed
> needs freeing).

Of course!  Thanks for that, I think I got a bit of tunnel vision on
this for some reason.

I'll merge this into audit/stable-6.0 now and once testing is complete
I'll send it up to Linus.  Thanks everyone!

-- 
paul-moore.com