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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, "Mateusz Guzik" <mguzik@redhat.com>,
        "Linus Torvalds" <torvalds@linux-foundation.org>,
        "Pax Team" <pageexec@freemail.hu>, "Al Viro" <viro@zeniv.linux.org.uk>,
        "Emese Revfy" <re.emese@gmail.com>, "Jarod Wilson" <jarod@redhat.com>,
        "Alexey Dobriyan" <adobriyan@gmail.com>,
        "Mathias Krause" <minipli@googlemail.com>,
        "Cyrill Gorcunov" <gorcunov@openvz.org>
Date: Sun, 12 Jun 2016 22:34:42 +0100
Message-ID: <lsq.1465767282.56820918@decadent.org.uk>
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Subject: [PATCH 3.2 40/46] proc: prevent accessing /proc/<PID>/environ
 until it's ready
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3.2.81-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Mathias Krause <minipli@googlemail.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@googlemail.com>
Cc: Emese Revfy <re.emese@gmail.com>
Cc: Pax Team <pageexec@freemail.hu>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Mateusz Guzik <mguzik@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 fs/proc/base.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -905,7 +905,8 @@ static ssize_t environ_read(struct file
 
 	mm = mm_for_maps(task);
 	ret = PTR_ERR(mm);
-	if (!mm || IS_ERR(mm))
+	/* Ensure the process spawned far enough to have an environment. */
+	if (!mm || IS_ERR(mm) || !mm->env_end)
 		goto out_free;
 
 	ret = 0;