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

[Kees Cook]

On Thu, Feb 9, 2017 at 10:33 AM, Thomas Garnier <thgarnie@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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