Patch "proc: prevent accessing /proc/<PID>/environ until it's ready" has been added to the 4.5-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
proc-prevent-accessing-proc-pid-environ-until-it-s-ready.patch
and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mathias Krause <[email protected]>
Date: Thu, 5 May 2016 16:22:26 -0700
Subject: proc: prevent accessing /proc/<PID>/environ until it's ready

From: Mathias Krause <[email protected]>

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.

---
 fs/proc/base.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

@@ -955,7 +955,8 @@ static ssize_t environ_read(struct file
 	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);

Patches currently in stable-queue which might be from [email protected] are

queue-4.5/proc-prevent-accessing-proc-pid-environ-until-it-s-ready.patch