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

[Mateusz Guzik]

On Thu, Apr 28, 2016 at 09:04:18PM +0200, Mathias Krause wrote:
> 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
> ---
>  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);

In this case get_cmdline in mm/util.c should also be patched for
completness. It tests for arg_end, but later accesses env_end.

-- 
Mateusz Guzik