[precise][PATCH 1/1] proc: prevent accessing /proc/<PID>/environ until it's ready

[Luis Henriques]

From: Mathias Krause <minipli at googlemail.com>

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 at googlemail.com>
Cc: Emese Revfy <re.emese at gmail.com>
Cc: Pax Team <pageexec at freemail.hu>
Cc: Al Viro <viro at zeniv.linux.org.uk>
Cc: Mateusz Guzik <mguzik at redhat.com>
Cc: Alexey Dobriyan <adobriyan at gmail.com>
Cc: Cyrill Gorcunov <gorcunov at openvz.org>
Cc: Jarod Wilson <jarod at redhat.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
CVE-2016-7916
(backported from commit 8148a73c9901a8794a50f950083c00ccf97d43b3)
[ luis: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/proc/base.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 92690313a450..70d4920281e1 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -956,7 +956,8 @@ static ssize_t environ_read(struct file *file, char __user *buf,
 
 	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;