linux-kernel - [PATCH 3/7] pipe: actually allow root to exceed the pipe buffer limits

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Date:   Sun,  7 Jan 2018 21:35:38 -0800
From:   Eric Biggers <ebiggers3@...il.com>
To:     linux-fsdevel@...r.kernel.org
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Willy Tarreau <w@....eu>,
        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>,
        stable@...r.kernel.org
Subject: [PATCH 3/7] pipe: actually allow root to exceed the pipe buffer limits

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

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.

Fixes: 759c01142a5d ("pipe: limit the per-user amount of pages allocated in pipes")
Cc: stable@...r.kernel.org
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
 fs/pipe.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index d0dec5e7ef33..847ecc388820 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -613,6 +613,11 @@ static bool too_many_pipe_buffers_hard(unsigned long user_bufs)
 	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;
@@ -629,12 +634,12 @@ struct pipe_inode_info *alloc_pipe_info(void)
 
 	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),
@@ -1065,7 +1070,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
 	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;
 	}
-- 
2.15.1