linux-kernel - Re: [PATCH] proc: nommu: /proc/<pid>/maps: release mmap read lock

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Date:   Thu, 14 Sep 2023 10:02:03 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Ben Wolsieffer <ben.wolsieffer@...ring.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Greg Ungerer <gerg@...inux.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Giulio Benetti <giulio.benetti@...ettiengineering.com>,
        Ben Wolsieffer <Ben.Wolsieffer@...ring.com>
Subject: Re: [PATCH] proc: nommu: /proc/<pid>/maps: release mmap read lock

On Thu, 14 Sep 2023 12:30:20 -0400 Ben Wolsieffer <ben.wolsieffer@...ring.com> wrote:

> The no-MMU implementation of /proc/<pid>/map doesn't normally release
> the mmap read lock, because it uses !IS_ERR_OR_NULL(_vml) to determine
> whether to release the lock. Since _vml is NULL when the end of the
> mappings is reached, the lock is not released.
> 
> This code was incorrectly adapted from the MMU implementation, which
> at the time released the lock in m_next() before returning the last entry.
> 
> The MMU implementation has diverged further from the no-MMU version
> since then, so this patch brings their locking and error handling into
> sync, fixing the bug and hopefully avoiding similar issues in the
> future.

Thanks.  Is this bug demonstrable from userspace?  If so, how?