[PATCH 3.10 109/143] proc: prevent accessing /proc/<PID>/environ until it's ready

From: Willy Tarreau
Date: Sun Jun 05 2016 - 06:37:18 EST

From: Mathias Krause <minipli@xxxxxxxxxxxxxx>

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

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@xxxxxxxxxxxxxx>
Cc: Emese Revfy <re.emese@xxxxxxxxx>
Cc: Pax Team <pageexec@xxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Mateusz Guzik <mguzik@xxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Cc: Jarod Wilson <jarod@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Willy Tarreau <w@xxxxxx>
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 7b5d453..e5160b7 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -844,7 +844,8 @@ static ssize_t environ_read(struct file *file, char __user *buf,
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);