linux-kernel - Re: linux-next boot error: general protection fault in tomoyo_get_local

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Date:   Fri, 15 May 2020 13:16:59 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Alexey Gladkov <gladkov.alexey@...il.com>,
        syzbot <syzbot+c1af344512918c61362c@...kaller.appspotmail.com>,
        jmorris@...ei.org, linux-kernel@...r.kernel.org,
        linux-next@...r.kernel.org, linux-security-module@...r.kernel.org,
        serge@...lyn.com, sfr@...b.auug.org.au,
        syzkaller-bugs@...glegroups.com,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: linux-next boot error: general protection fault in tomoyo_get_local_path

Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> writes:

> This is
>
>         if (sb->s_magic == PROC_SUPER_MAGIC && *pos == '/') {
>                 char *ep;
>                 const pid_t pid = (pid_t) simple_strtoul(pos + 1, &ep, 10);
>                 struct pid_namespace *proc_pidns = proc_pid_ns(d_inode(dentry)); // <= here
>
>                 if (*ep == '/' && pid && pid ==
>                     task_tgid_nr_ns(current, proc_pidns)) {
>
> which was added by commit c59f415a7cb6e1e1 ("Use proc_pid_ns() to get pid_namespace from the proc superblock").
>
> @@ -161,9 +162,10 @@ static char *tomoyo_get_local_path(struct dentry *dentry, char * const buffer,
>         if (sb->s_magic == PROC_SUPER_MAGIC && *pos == '/') {
>                 char *ep;
>                 const pid_t pid = (pid_t) simple_strtoul(pos + 1, &ep, 10);
> +               struct pid_namespace *proc_pidns = proc_pid_ns(d_inode(dentry));
>
>                 if (*ep == '/' && pid && pid ==
> -                   task_tgid_nr_ns(current, sb->s_fs_info)) {
> +                   task_tgid_nr_ns(current, proc_pidns)) {
>                         pos = ep - 5;
>                         if (pos < buffer)
>                                 goto out;
>
> Alexey and Eric, any clue?

Looking at the stack backtrace this is happening as part of creating a
file or a device node.  The dentry that is passed in most likely
comes from d_alloc_parallel.  So we have d_inode == NULL.

I want to suggest doing the very simple fix:

-	if (sb->s_magic == PROC_SUPER_MAGIC && *pos == '/') {
+	if (sb->s_magic == PROC_SUPER_MAGIC && *pos == '/' && denty->d_inode) {

But I don't know if there are any other security hooks early in lookup,
that could be called for an already existing dentry.

So it looks like we need a version proc_pid_ns that works for a dentry,
or a superblock.

Alex do you think you can code up an patch against my proc-next branch
to fix this?

Eric