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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 b409e578d ("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>
This commit is contained in:
Djalal Harouni 2012-07-30 14:42:26 -07:00 committed by Linus Torvalds
parent 108ceeb020
commit e8905ec27e
1 changed files with 7 additions and 6 deletions
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13
fs/proc/base.c
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@ -827,15 +827,16 @@ static ssize_t environ_read(struct file *file, char __user *buf,
if (!atomic_inc_not_zero(&mm->mm_users)) if (!atomic_inc_not_zero(&mm->mm_users))
goto free; goto free;
while (count > 0) { while (count > 0) {
int this_len, retval, max_len; size_t this_len, max_len;
int retval;
if (src >= (mm->env_end - mm->env_start))
break;
this_len = mm->env_end - (mm->env_start + src); this_len = mm->env_end - (mm->env_start + src);
if (this_len <= 0) max_len = min_t(size_t, PAGE_SIZE, count);
break; this_len = min(max_len, this_len);
max_len = (count > PAGE_SIZE) ? PAGE_SIZE : count;
this_len = (this_len > max_len) ? max_len : this_len;
retval = access_remote_vm(mm, (mm->env_start + src), retval = access_remote_vm(mm, (mm->env_start + src),
page, this_len, 0); page, this_len, 0);