Re: [PATCH] proc: s_fs_info may be NULL when proc_kill_sb is called

Eric W. Biederman
2020-06-10T17:17:10Z

Alexey Gladkov <gladkov.alexey@gmail.com> writes:

> syzbot found that proc_fill_super() fails before filling up sb->s_fs_info,
> deactivate_locked_super() will be called and sb->s_fs_info will be NULL.
> The proc_kill_sb() does not expect fs_info to be NULL which is wrong.

For the case where s_fs_info is never allocated this looks correct.
That is because generic_shutdown_super has a special for !sb->s_root.

However for the existing cases I can't convince myself that it is safe
to change the order we free the pid namespace and free fs_info.

There is a lot of code that can run while generic_shutdown_super is
running and purging all of the inodes. We have crazy things like
proc_flush_pid that might care, as well proc_evict_inode.

I haven't found anything that actually references fs_info or actually
depends on the pid namespace living longer than the proc inode but it
would be really easy to miss something.

Can you send a v2 version does not change the order things are freed in
for the case where we do allocate fs_info. That will make it trivially
safe to apply.

Otherwise this looks like a very good patch.

Thank you,
Eric

> Link: https://lore.kernel.org/lkml/0000000000002d7ca605a7b8b1c5@google.com
> Reported-by: syzbot+4abac52934a48af5ff19@syzkaller.appspotmail.com
> Fixes: fa10fed30f25 ("proc: allow to mount many instances of proc in one pid namespace")
> Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
> ---
> fs/proc/root.c | 15 +++++++++------
> 1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/fs/proc/root.c b/fs/proc/root.c
> index ffebed1999e5..a715eb9f196a 100644
> --- a/fs/proc/root.c
> +++ b/fs/proc/root.c
> @@ -264,15 +264,18 @@ static void proc_kill_sb(struct super_block *sb)
> {
> struct proc_fs_info *fs_info = proc_sb_info(sb);
>
> - if (fs_info->proc_self)
> - dput(fs_info->proc_self);
> + if (fs_info) {
> + if (fs_info->proc_self)
> + dput(fs_info->proc_self);
>
> - if (fs_info->proc_thread_self)
> - dput(fs_info->proc_thread_self);
> + if (fs_info->proc_thread_self)
> + dput(fs_info->proc_thread_self);
> +
> + put_pid_ns(fs_info->pid_ns);
> + kfree(fs_info);
> + }
>
> kill_anon_super(sb);
> - put_pid_ns(fs_info->pid_ns);
> - kfree(fs_info);
> }
>
> static struct file_system_type proc_fs_type = {