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

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

Signed-off-by: Mathias Krause <>
Cc: Emese Revfy <>
Cc: Pax Team <>
Cc: Al Viro <>
Cc: Mateusz Guzik <>
Cc: Alexey Dobriyan <>
Cc: Cyrill Gorcunov <>
Cc: Jarod Wilson <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
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minipli authored and torvalds committed May 5, 2016
1 parent 32a4e16 commit 8148a73c9901a8794a50f950083c00ccf97d43b3
Showing with 2 additions and 1 deletion.
  1. +2 −1 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);

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