Re: [PATCH 2/6] x86/ia32: Fix ia32_restore_sigcontext AC leak

Andy Lutomirski
2019-02-25T15:42:07Z

On Mon, Feb 25, 2019 at 4:53 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Don't call load_gs_index() with AC set; delay the segment setting
> until after the AC section.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> arch/x86/ia32/ia32_signal.c | 29 +++++++++++++++++++----------
> 1 file changed, 19 insertions(+), 10 deletions(-)
>
> --- a/arch/x86/ia32/ia32_signal.c
> +++ b/arch/x86/ia32/ia32_signal.c
> @@ -60,17 +60,21 @@
> regs->seg = GET_SEG(seg) | 3; \
> } while (0)
>
> -#define RELOAD_SEG(seg) { \
> - unsigned int pre = GET_SEG(seg); \
> - unsigned int cur = get_user_seg(seg); \
> - pre |= 3; \
> - if (pre != cur) \
> - set_user_seg(seg, pre); \
> +#define LOAD_SEG(seg) { \
> + pre_##seg = 3 | GET_SEG(seg); \
> + cur_##seg = get_user_seg(seg); \
> +}
> +
> +#define RELOAD_SEG(seg) { \
> + if (pre_##seg != cur_##seg) \
> + set_user_seg(seg, pre_##seg); \
> }

This is so tangled.

How about changing RELOAD_SEG to replace unsigned int pre =
GET_SEG(seg); with unsigned int pre = (seg); to make it less magic.
Then do:

unsigned int gs = GET_SEG(gs);

...

RELOAD_SEG(gs);

And now the code actually does what it looks like it does.