linux-kernel - Re: [PATCH mm] VFS: seq_file: ensure ->from is valid.

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Date:   Sat, 7 Jul 2018 05:50:00 +0200
From:   Jann Horn <jannh@...gle.com>
To:     neilb@...e.com
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-doc@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH mm] VFS: seq_file: ensure ->from is valid.

On Sat, Jul 7, 2018 at 5:29 AM NeilBrown <neilb@...e.com> wrote:
> Previous patch ("VFS: simplify seq_file iteration code and interface")
> removed code to set ->from to zero when ->count is zero, as ->from is
> dead at that time.  However it didn't ensure ->from was set properly
> whenever ->count becomes non-zero.
> This can only happen when ->show() is called.  Of the three places it
> is called one already has ->from set to zero.  The other two are
> fixed by setting from to zero after fully flushing the buffer (at which
> point ->count will also be zero).
>
> Reported-by: Jann Horn <jannh@...gle.com>
> Signed-off-by: NeilBrown <neilb@...e.com>

Tested-by: Jann Horn <jannh@...gle.com>

> ---
>  fs/seq_file.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index fd82585ab50f..1dea7a8a5255 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -220,6 +220,7 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
>                         goto Done;
>         }
>         /* we need at least one record in buffer */
> +       m->from = 0;
>         p = m->op->start(m, &m->index);
>         while (1) {
>                 err = PTR_ERR(p);

This looks correct to me. I have also tested that with this patch
applied, my crasher doesn't work anymore.