ACK: [precise][PATCH 1/1] proc: prevent accessing /proc/<PID>/environ until it's ready

[Colin Ian King]

On 17/11/16 14:53, Luis Henriques wrote:
> From: Mathias Krause <minipli at googlemail.com>
> 
> 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.
> 
> Fixes: https://forums.grsecurity.net/viewtopic.php?f=3&t=4363
> Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=116461
> Signed-off-by: Mathias Krause <minipli at googlemail.com>
> Cc: Emese Revfy <re.emese at gmail.com>
> Cc: Pax Team <pageexec at freemail.hu>
> Cc: Al Viro <viro at zeniv.linux.org.uk>
> Cc: Mateusz Guzik <mguzik at redhat.com>
> Cc: Alexey Dobriyan <adobriyan at gmail.com>
> Cc: Cyrill Gorcunov <gorcunov at openvz.org>
> Cc: Jarod Wilson <jarod at redhat.com>
> Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> CVE-2016-7916
> (backported from commit 8148a73c9901a8794a50f950083c00ccf97d43b3)
> [ luis: adjusted context ]
> Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
> ---
>  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 92690313a450..70d4920281e1 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -956,7 +956,8 @@ static ssize_t environ_read(struct file *file, char __user *buf,
>  
>  	mm = mm_for_maps(task);
>  	ret = PTR_ERR(mm);
> -	if (!mm || IS_ERR(mm))
> +	/* Ensure the process spawned far enough to have an environment. */
> +	if (!mm || IS_ERR(mm) || !mm->env_end)
>  		goto out_free;
>  
>  	ret = 0;
> 

Looks good to me

Acked-by: Colin Ian King <colin.king at canonical.com>