Re: [PATCH 1/2] proc: restrict access to /proc/PID/io

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

(2011/06/24 21:08), Vasiliy Kulikov wrote:
> /proc/PID/io may be used for gathering private information.  E.g. for
> openssh and vsftpd daemons wchars/rchars may be used to learn the
> precise password length.  Restrict it to processes being able to ptrace
> the target process.
> 
> ptrace_may_access() is needed to prevent keeping open file descriptor of
> "io" file, executing setuid binary and gathering io information of the
> setuid'ed process.
> 
> Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>

This description seems makes sense to me. But Vasilly, I have one question.
Doesn't this change break iotop command or other userland tools?


> ---
>  fs/proc/base.c |    7 +++++--
>  1 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 14def99..5ae25d1 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2712,6 +2712,9 @@ static int do_io_accounting(struct task_struct *task, char *buffer, int whole)
>  	struct task_io_accounting acct = task->ioac;
>  	unsigned long flags;
>  
> +	if (!ptrace_may_access(task, PTRACE_MODE_READ))
> +		return -EACCES;
> +

I think this check need a comment. Usually procfs don't use ptrace_may_access() directly
(see mm_for_maps) because it's racy against exec(). However I think your code is ok.
because a few bytes io accounting leak has no big matter.