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

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Message-ID: <72cb7aea-92bd-d71b-2f8a-63881a35fad8@i-love.sakura.ne.jp>
Date:   Sat, 16 May 2020 00:36:28 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Alexey Gladkov <gladkov.alexey@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Cc:     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

On 2020/05/16 0:18, Tetsuo Handa wrote:
> 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?
> 

A similar bug (racing inode destruction with open() on proc filesystem) was fixed as
commit 6f7c41374b62fd80 ("tomoyo: Don't use nifty names on sockets."). Then, it might
not be safe to replace dentry->d_sb->s_fs_info with dentry->d_inode->i_sb->s_fs_info .