linux-kernel - [PATCH 3.16 078/114] proc: prevent accessing /proc/<PID>/environ until it's ready

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Date:	Mon, 13 Jun 2016 19:36:37 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Emese Revfy" <re.emese@...il.com>,
	"Jarod Wilson" <jarod@...hat.com>,
	"Cyrill Gorcunov" <gorcunov@...nvz.org>,
	"Al Viro" <viro@...iv.linux.org.uk>,
	"Mathias Krause" <minipli@...glemail.com>,
	"Mateusz Guzik" <mguzik@...hat.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Pax Team" <pageexec@...email.hu>,
	"Alexey Dobriyan" <adobriyan@...il.com>
Subject: [PATCH 3.16 078/114] proc: prevent accessing /proc/<PID>/environ
 until it's ready

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

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

From: Mathias Krause <minipli@...glemail.com>

commit 8148a73c9901a8794a50f950083c00ccf97d43b3 upstream.

If /proc/<PID>/environ gets read before the envp[] array is fully set up
in create_{aout,elf,elf_fdpic,flat}_tables(), we might end up trying to
read more bytes than are actually written, as env_start will already be
set but env_end will still be zero, making the range calculation
underflow, allowing to read beyond the end of what has been written.

Fix this as it is done for /proc/<PID>/cmdline by testing env_end for
zero.  It is, apparently, intentionally set last in create_*_tables().

This bug was found by the PaX size_overflow plugin that detected the
arithmetic underflow of 'this_len = env_end - (env_start + src)' when
env_end is still zero.

The expected consequence is that userland trying to access
/proc/<PID>/environ of a not yet fully set up process may get
inconsistent data as we're in the middle of copying in the environment
variables.

Fixes: https://forums.grsecurity.net/viewtopic.php?f=3&t=4363
Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=116461
Signed-off-by: Mathias Krause <minipli@...glemail.com>
Cc: Emese Revfy <re.emese@...il.com>
Cc: Pax Team <pageexec@...email.hu>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Mateusz Guzik <mguzik@...hat.com>
Cc: Alexey Dobriyan <adobriyan@...il.com>
Cc: Cyrill Gorcunov <gorcunov@...nvz.org>
Cc: Jarod Wilson <jarod@...hat.com>
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/proc/base.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -809,7 +809,8 @@ static ssize_t environ_read(struct file
 	int ret = 0;
 	struct mm_struct *mm = file->private_data;

-	if (!mm)
+	/* Ensure the process spawned far enough to have an environment. */
+	if (!mm || !mm->env_end)
 		return 0;

 	page = (char *)__get_free_page(GFP_TEMPORARY);