proc: environ_read() make sure offset points to environment address range

Currently the following offset and environment address range check in
environ_read() of /proc/<pid>/environ is buggy:

  int this_len = mm->env_end - (mm->env_start + src);
  if (this_len <= 0)
    break;

Large or negative offsets on /proc/<pid>/environ converted to 'unsigned
long' may pass this check since '(mm->env_start + src)' can overflow and
'this_len' will be positive.

This can turn /proc/<pid>/environ to act like /proc/<pid>/mem since
(mm->env_start + src) will point and read from another VMA.

There are two fixes here plus some code cleaning:

1) Fix the overflow by checking if the offset that was converted to
   unsigned long will always point to the [mm->env_start, mm->env_end]
   address range.

2) Remove the truncation that was made to the result of the check,
   storing the result in 'int this_len' will alter its value and we can
   not depend on it.

For kernels that have commit b409e578 ("proc: clean up
/proc/<pid>/environ handling") which adds the appropriate ptrace check and
saves the 'mm' at ->open() time, this is not a security issue.

This patch is taken from the grsecurity patch since it was just made
available.
Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Brad Spengler <spender@grsecurity.net>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -827,15 +827,16 @@ static ssize_t environ_read(struct file *file, char __user *buf,
 	if (!atomic_inc_not_zero(&mm->mm_users))
 		goto free;
 	while (count > 0) {
-		int this_len, retval, max_len;
-
-		this_len = mm->env_end - (mm->env_start + src);
+		size_t this_len, max_len;
+		int retval;
 
-		if (this_len <= 0)
+		if (src >= (mm->env_end - mm->env_start))
 			break;
 
-		max_len = (count > PAGE_SIZE) ? PAGE_SIZE : count;
-		this_len = (this_len > max_len) ? max_len : this_len;
+		this_len = mm->env_end - (mm->env_start + src);
+
+		max_len = min_t(size_t, PAGE_SIZE, count);
+		this_len = min(max_len, this_len);
 
 		retval = access_remote_vm(mm, (mm->env_start + src),
 			page, this_len, 0);