linux-kernel - Re: [PATCH 3.5 2/2] seccomp: Future-proof against silly tracers

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Date:	Tue, 17 Jul 2012 21:13:11 -0500
From:	Will Drewry <wad@...omium.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 3.5 2/2] seccomp: Future-proof against silly tracers

On Tue, Jul 17, 2012 at 6:19 PM, Andy Lutomirski <luto@...capital.net> wrote:
> Currently, if a tracer changes a syscall nr to __NR_future_enosys,
> behavior will differ between kernels that know about
> __NR_future_enosys (and return -ENOSYS) and older kernels (which
> return the value from pt_regs).  This is silly; we should just
> return -ENOSYS.
>
> This is unlikely to ever happen on x86 because the return value in
> pt_regs starts out as -ENOSYS, but a silly tracer can change that.
>
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> Cc: Will Drewry <wad@...omium.org>
> ---
>  arch/x86/include/asm/syscall.h |   11 +++++++++++
>  kernel/seccomp.c               |   15 +++++++++++++++
>  2 files changed, 26 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
> index 1ace47b..8191e057 100644
> --- a/arch/x86/include/asm/syscall.h
> +++ b/arch/x86/include/asm/syscall.h

Since this is used in an arch-wide location, the prototype and comment
should be in asm-generic/syscall.h too.

> @@ -70,6 +70,17 @@ static inline void syscall_set_return_value(struct task_struct *task,
>         regs->ax = (long) error ?: val;
>  }
>
> +static inline bool syscall_is_nr_future(struct task_struct *task,
> +                                       struct pt_regs *regs)
> +{
> +#ifdef CONFIG_IA32_EMULATION
> +       if (task_thread_info(task)->status & TS_COMPAT)
> +               return syscall_get_nr(task, regs) > __NR_ia32_syscall_max;
> +#endif
> +
> +       return syscall_get_nr(task, regs) > __NR_syscall_max;

I'm not sure how easy this will be to implement on some of the arches
where this data isn't bubbled up.  It'd be good if some non-x86 arch
maintainers chimed in (since x86 is easy and already works as expected
:).

> +}
> +
>  #ifdef CONFIG_X86_32
>
>  static inline void syscall_get_arguments(struct task_struct *task,
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 5af44b5..bd7527d 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -429,6 +429,21 @@ int __secure_computing(int this_syscall)
>                          */
>                         if (fatal_signal_pending(current))
>                                 break;
> +
> +                       if (syscall_is_nr_future(current, regs)) {
> +                               /*
> +                                * If the tracer selects a system call that
> +                                * only future kernels know about, we still need
> +                                * to execute that syscall by returning
> +                                * -ENOSYS.  (On x86, this only matters if the
> +                                * tracer changed the return value, which would
> +                                * be silly, but user code can be silly.)
> +                                */
> +                               syscall_set_return_value(current, regs,
> +                                                        -ENOSYS, 0);
> +                               goto skip;
> +                       }
> +
>                         if (syscall_get_nr(current, regs) < 0)
>                                 goto skip;  /* Explicit request to skip. */
>
> --
> 1.7.7.6
>

thanks!
will
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