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Date: Thu, 20 Oct 2016 19:04:39 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Leon Yu <chianglungyu@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>, Kees Cook <keescook@chromium.org>,
        Michal Hocko <mhocko@suse.com>, John Stultz <john.stultz@linaro.org>,
        Mateusz Guzik <mguzik@redhat.com>,
        Janis Danisevskis <jdanis@google.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] proc: fix NULL dereference when reading
        /proc/<pid>/auxv
Message-ID: <20161020170438.GA13724@redhat.com>
References: <1476885580-7612-1-git-send-email-chianglungyu@gmail.com> <1476966200-14457-1-git-send-email-chianglungyu@gmail.com>
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On 10/20, Leon Yu wrote:
>
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1014,6 +1014,9 @@ static ssize_t auxv_read(struct file *file, char __user *buf,
>  {
>  	struct mm_struct *mm = file->private_data;
>  	unsigned int nwords = 0;
> +
> +	if (!mm)
> +		return 0;
>  	do {
>  		nwords += 2;
>  	} while (mm->saved_auxv[nwords - 2] != 0); /* AT_NULL */

Michal disagrees and I won't argue with his patch which makes __mem_open()
fail if ->mm == NULL. Even if I don't really understand why should we change
the old behaviour, this _can_ break or at least confuse something/someone.

However, even if we do the change above, personally I do think we should
fix the trivial bug first, then surprise the user-space.

Acked-by: Oleg Nesterov <oleg@redhat.com>