linux-kernel - Re: [PATCH] FS: fix stack-out-of-bounds wanning

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Message-ID: <20170509002634.GU29622@ZenIV.linux.org.uk>
Date:   Tue, 9 May 2017 01:26:35 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Shaohua Li <shli@...com>
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        Kernel-team@...com
Subject: Re: [PATCH] FS: fix stack-out-of-bounds wanning

On Mon, May 08, 2017 at 04:42:50PM -0700, Shaohua Li wrote:
> I'm seeing below with lastet upstream. Commit
> 5ecda13(generic_file_read_iter(): make use of iov_iter_revert())
> directly pass iter to ->direct_IO(), ->direct_io already advances count
> bytes but return -EIOCBQUEUED. In this case, count is bigger than
> iov_iter_count(iter). We really want to revert count -
> iov_iter_count(iter) instead of the opposite.

Already discussed; the patch in my tree is that braino fix + sanity check
in iov_iter_revert(), to prevent anything similar in the future.

The reason it wasn't caught in tests (without KASAN, that is) is that
to get really obvious breakage out of that, you need a short read from
->direct_IO() *and* fallback to cached read after it.  -EIOCBQUEUED
obviously excludes the latter, so the bug is almost certain to remain quiet.
There is a non-zero chance of actually running into unmapped memory while
doing that iov_iter_revert(), but it's fairly low.  Unfortunately, since
that allowed the damn thing to remain unnoticed...

Will be in tonight's pull request.