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

[Mathias Krause]

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.

Reported-at: https://forums.grsecurity.net/viewtopic.php?f=3&t=4363
Bugzilla: 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>
---
 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 4f764c2ac1a5..45f2162e55b2 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -955,7 +955,8 @@ static ssize_t environ_read(struct file *file, char __user *buf,
 	struct mm_struct *mm = file->private_data;
 	unsigned long env_start, env_end;
 
-	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);
-- 
1.7.10.4