linux-kernel - Re: [PATCH 4/7] pipe: fix off-by-one error when checking buffer limits

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Date:   Mon, 8 Jan 2018 07:42:00 +0100
From:   Willy Tarreau <w@....eu>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH 4/7] pipe: fix off-by-one error when checking buffer
 limits

On Sun, Jan 07, 2018 at 09:35:39PM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> With pipe-user-pages-hard set to 'N', users were actually only allowed
> up to 'N - 1' buffers; and likewise for pipe-user-pages-soft.
> 
> Fix this to allow up to 'N' buffers, as would be expected.

Interesting. I was a bit surprized at first and found that this was
changed by b0b91d1 ("pipe: fix limit checking in pipe_set_size()").
Prior to this fix, only already allocated pipes were counted. After
the fix, an allocation attempt was made before checking the size. So
I think that your fix is needed in stable versions which backported
the commit above.

> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Acked-by: Willy Tarreau <w@....eu>

Thanks,
Willy