linux-kernel - Re: [patch] seccomp.2: Note that memory area is read-only

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Date:	Sat, 18 Apr 2015 17:29:43 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [patch] seccomp.2: Note that memory area is read-only

On Sat, Apr 18, 2015 at 04:37:35PM -0700, Kees Cook wrote:
> On Mon, Apr 6, 2015 at 8:29 AM, Michael Kerrisk (man-pages)
> <mtk.manpages@...il.com> wrote:
> > Hi Kees,
> >
> > I recently was asked about the point below, and had to go check the code
> > to be sure, since the man page said nothing. It would be good to have
> > a confirmation: the seccomp_data buffer supplied to a seccomp BPF program
> > is read-only, right? (That is, one can't write to the buffer in order to
> > change the arguments that a system call actually receives.)
> 
> That's correct. If BPF even allows changing the data, it's not copied
> back to the syscall when it runs. Anything wanting to do things like
> that would need to use ptrace to catch the call an directly modify the
> registers before continuing with the call.

Today BPF programs can only read from seccomp_data buffer.
I think it would be an interesting feature to allow overwrite of
syscall arguments by the program on the fly... sometime in the future.

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