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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org,
        "Linus Torvalds" <torvalds@linux-foundation.org>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
        "Vegard Nossum" <vegard.nossum@oracle.com>,
        "Tetsuo Handa" <penguin-kernel@I-love.SAKURA.ne.jp>,
        "Jens Axboe" <axboe@fb.com>, "Willy Tarreau" <w@1wt.eu>,
        "Al Viro" <viro@zeniv.linux.org.uk>, "" <socketpair@gmail.com>
Date:   Thu, 07 Jun 2018 15:05:21 +0100
Message-ID: <lsq.1528380321.867389047@decadent.org.uk>
Subject: [PATCH 3.16 206/410] pipe: cap initial pipe capacity according to
 pipe-max-size limit
In-Reply-To: <lsq.1528380320.647747352@decadent.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

3.16.57-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>

commit 086e774a57fba4695f14383c0818994c0b31da7c upstream.

This is a patch that provides behavior that is more consistent, and
probably less surprising to users. I consider the change optional, and
welcome opinions about whether it should be applied.

By default, pipes are created with a capacity of 64 kiB.  However,
/proc/sys/fs/pipe-max-size may be set smaller than this value.  In this
scenario, an unprivileged user could thus create a pipe whose initial
capacity exceeds the limit. Therefore, it seems logical to cap the
initial pipe capacity according to the value of pipe-max-size.

The test program shown earlier in this patch series can be used to
demonstrate the effect of the change brought about with this patch:

    # cat /proc/sys/fs/pipe-max-size
    1048576
    # sudo -u mtk ./test_F_SETPIPE_SZ 1
    Initial pipe capacity: 65536
    # echo 10000 > /proc/sys/fs/pipe-max-size
    # cat /proc/sys/fs/pipe-max-size
    16384
    # sudo -u mtk ./test_F_SETPIPE_SZ 1
    Initial pipe capacity: 16384
    # ./test_F_SETPIPE_SZ 1
    Initial pipe capacity: 65536

The last two executions of 'test_F_SETPIPE_SZ' show that pipe-max-size
caps the initial allocation for a new pipe for unprivileged users, but
not for privileged users.

Link: http://lkml.kernel.org/r/31dc7064-2a17-9c5b-1df1-4e3012ee992c@gmail.com
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Reviewed-by: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: <socketpair@gmail.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Jens Axboe <axboe@fb.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 fs/pipe.c | 3 +++
 1 file changed, 3 insertions(+)

--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -617,6 +617,9 @@ struct pipe_inode_info *alloc_pipe_info(
 	if (pipe == NULL)
 		goto out_free_uid;
 
+	if (pipe_bufs * PAGE_SIZE > pipe_max_size && !capable(CAP_SYS_RESOURCE))
+		pipe_bufs = pipe_max_size >> PAGE_SHIFT;
+
 	user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
 
 	if (too_many_pipe_buffers_soft(user_bufs)) {