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Re: [RFC] syscalls: Restore address limit after a syscall

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On Thu, Feb 9, 2017 at 10:33 AM, Thomas Garnier <thgarnie@google.com> wrote:
> This patch prevents a syscall to modify the address limit of the
> caller. The address limit is kept by the syscall wrapper and restored
> just after the syscall ends.
>
> For example, it would mitigation this bug:
>
> - https://bugs.chromium.org/p/project-zero/issues/detail?id=990
>
> Signed-off-by: Thomas Garnier <thgarnie@google.com>
> ---
> Based on next-20170209
> ---
>  include/linux/syscalls.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 91a740f6b884..a1b6a62a9849 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -198,7 +198,10 @@ extern struct trace_event_functions exit_syscall_print_funcs;
>         asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__));      \
>         asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__))       \
>         {                                                               \
> -               long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__));  \
> +               long ret;                                               \
> +               mm_segment_t fs = get_fs();                             \
> +               ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__));       \
> +               set_fs(fs);                                             \
>                 __MAP(x,__SC_TEST,__VA_ARGS__);                         \
>                 __PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__));       \
>                 return ret;                                             \
> --
> 2.11.0.483.g087da7b7c-goog
>

I have a memory of Andy looking at this before, and there was some
problem with how a bunch of compat code would set fs and then re-call
the syscall... but I can't quite find the conversation. Andy, do you
remember the details?

This seems like an entirely reasonable thing to enforce for syscalls,
though I'm sure there's a gotcha somewhere. :)

-Kees

-- 
Kees Cook
Pixel Security

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[RFC] syscalls: Restore address limit after a syscall Thomas Garnier <thgarnie@google.com> - 2017-02-09 19:40 +0100
  Re: [RFC] syscalls: Restore address limit after a syscall Kees Cook <keescook@chromium.org> - 2017-02-09 20:40 +0100
    Re: [RFC] syscalls: Restore address limit after a syscall Andy Lutomirski <luto@amacapital.net> - 2017-02-10 00:10 +0100
      Re: [RFC] syscalls: Restore address limit after a syscall Thomas Garnier <thgarnie@google.com> - 2017-02-10 00:50 +0100
        Re: [RFC] syscalls: Restore address limit after a syscall Andy Lutomirski <luto@amacapital.net> - 2017-02-10 06:50 +0100
          Re: [RFC] syscalls: Restore address limit after a syscall Russell King - ARM Linux <linux@armlinux.org.uk> - 2017-02-10 20:30 +0100
            Re: [RFC] syscalls: Restore address limit after a syscall Kees Cook <keescook@chromium.org> - 2017-02-10 22:00 +0100
              Re: [RFC] syscalls: Restore address limit after a syscall Russell King - ARM Linux <linux@armlinux.org.uk> - 2017-02-10 23:00 +0100

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