linux-kernel - [PATCH 3.16 212/410] pipe: actually allow root to exceed the pipe buffer limits

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Message-ID: <lsq.1528380321.251987497@decadent.org.uk>
Date:   Thu, 07 Jun 2018 15:05:21 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        "Mikulas Patocka" <mpatocka@...hat.com>,
        "Eric Biggers" <ebiggers@...gle.com>, "Willy Tarreau" <w@....eu>,
        "Alexander Viro" <viro@...iv.linux.org.uk>,
        "Joe Lawrence" <joe.lawrence@...hat.com>,
        "Michael Kerrisk" <mtk.manpages@...il.com>,
        "Kees Cook" <keescook@...omium.org>
Subject: [PATCH 3.16 212/410] pipe: actually allow root to exceed the pipe
 buffer limits

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

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

From: Eric Biggers <ebiggers@...gle.com>

commit 85c2dd5473b2718b4b63e74bfeb1ca876868e11f upstream.

pipe-user-pages-hard and pipe-user-pages-soft are only supposed to apply
to unprivileged users, as documented in both Documentation/sysctl/fs.txt
and the pipe(7) man page.

However, the capabilities are actually only checked when increasing a
pipe's size using F_SETPIPE_SZ, not when creating a new pipe.  Therefore,
if pipe-user-pages-hard has been set, the root user can run into it and be
unable to create pipes.  Similarly, if pipe-user-pages-soft has been set,
the root user can run into it and have their pipes limited to 1 page each.

Fix this by allowing the privileged override in both cases.

Link: http://lkml.kernel.org/r/20180111052902.14409-4-ebiggers3@gmail.com
Fixes: 759c01142a5d ("pipe: limit the per-user amount of pages allocated in pipes")
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
Acked-by: Kees Cook <keescook@...omium.org>
Acked-by: Joe Lawrence <joe.lawrence@...hat.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: "Luis R . Rodriguez" <mcgrof@...nel.org>
Cc: Michael Kerrisk <mtk.manpages@...il.com>
Cc: Mikulas Patocka <mpatocka@...hat.com>
Cc: Willy Tarreau <w@....eu>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 fs/pipe.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -601,6 +601,11 @@ static bool too_many_pipe_buffers_hard(u
 	return pipe_user_pages_hard && user_bufs >= pipe_user_pages_hard;
 }
 
+static bool is_unprivileged_user(void)
+{
+	return !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN);
+}
+
 struct pipe_inode_info *alloc_pipe_info(void)
 {
 	struct pipe_inode_info *pipe;
@@ -617,12 +622,12 @@ struct pipe_inode_info *alloc_pipe_info(
 
 	user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
 
-	if (too_many_pipe_buffers_soft(user_bufs)) {
+	if (too_many_pipe_buffers_soft(user_bufs) && is_unprivileged_user()) {
 		user_bufs = account_pipe_buffers(user, pipe_bufs, 1);
 		pipe_bufs = 1;
 	}
 
-	if (too_many_pipe_buffers_hard(user_bufs))
+	if (too_many_pipe_buffers_hard(user_bufs) && is_unprivileged_user())
 		goto out_revert_acct;
 
 	pipe->bufs = kcalloc(pipe_bufs, sizeof(struct pipe_buffer),
@@ -1053,7 +1058,7 @@ static long pipe_set_size(struct pipe_in
 	if (nr_pages > pipe->buffers &&
 			(too_many_pipe_buffers_hard(user_bufs) ||
 			 too_many_pipe_buffers_soft(user_bufs)) &&
-			!capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) {
+			is_unprivileged_user()) {
 		ret = -EPERM;
 		goto out_revert_acct;
 	}