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Date: Thu, 24 May 2012 19:54:50 +0200
From: "Indan Zupancic" <indan@....nu>
To: "Will Drewry" <wad@...omium.org>
Cc: linux-kernel@...r.kernel.org,
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"Will Drewry" <wad@...omium.org>
Subject: Re: [RFC PATCH 1/3] seccomp: Don't allow tracers to abuse RET_TRACE
On Thu, May 24, 2012 18:07, Will Drewry wrote:
> Ensure that consumers of the PTRACE_EVENT_SECCOMP notification
> cannot change the system call number for the traced task
> without it resulting in the system call being skipped.
>
> Traditionally, tracers will set the system call number to
> -1 to skip the system call. This behavior will work as expected
> but the tracer will be unable to remap the system call to a valid
> system call after the seccomp policy has been evaluated.
>
> Signed-off-by: Will Drewry <wad@...omium.org>
> ---
> kernel/seccomp.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index ee376be..33f0ad6 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -425,6 +425,10 @@ int __secure_computing(int this_syscall)
> */
> if (fatal_signal_pending(current))
> break;
> + /* Skip the system call if the tracer changed it. */
> + if (this_syscall !=
> + syscall_get_nr(current, task_pt_regs(current)))
> + goto skip;
> return 0;
> case SECCOMP_RET_ALLOW:
> return 0;
> --
This patch doesn't make any sense whatsoever. You can't know why a system
call was blocked by a seccomp filter, assuming it's always because of the
system call number is wrong.
Also, you don't check if an allowed system call is changed into a denied
one, so this doesn't protect against ptracers bypassing seccomp filters.
And one of the main points of PTRACE_EVENT_SECCOMP events was that it's
useful for cases that can't be handled or decided by the seccomp filter.
Then taking away the ability to change the syscall number makes it a lot
less useful.
Either do the seccomp test before or after ptrace, or both, but please
don't introduce ad hoc checks like this.
Greetings,
Indan
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