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

Andy Lutomirski
2019-02-25T16:29:19Z

> On Feb 25, 2019, at 8:10 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
>> On Mon, Feb 25, 2019 at 07:41:50AM -0800, Andy Lutomirski wrote:
>> 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.
>
> Is this what you mean?

Yes, except:

>
> ---
> diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
> index 321fe5f5d0e9..e04eeeddcc35 100644
> --- a/arch/x86/ia32/ia32_signal.c
> +++ b/arch/x86/ia32/ia32_signal.c
> @@ -53,17 +53,16 @@
> #define GET_SEG(seg) ({ \
> unsigned short tmp; \
> get_user_ex(tmp, &sc->seg); \
> - tmp; \
> + tmp | 3; \
> })
>

Drop this part.

> #define COPY_SEG_CPL3(seg) do { \
> - regs->seg = GET_SEG(seg) | 3; \
> + regs->seg = GET_SEG(seg); \
> } while (0)

And this.

Unfortunately, whether we want the | 3 varies by segment. For FS and GS, we definitely don’t want it, since 0 is a common and important value, and 3 is a deeply screwed up value. (3 is legal to *write* to GS, and it sticks, but IRET silently changes it to 0, because the original 386 designers (I assume) confused themselves.

>
> #define RELOAD_SEG(seg) { \
> - unsigned int pre = GET_SEG(seg); \
> + unsigned int pre = (seg); \
> unsigned int cur = get_user_seg(seg); \
> - pre |= 3; \
> if (pre != cur) \
> set_user_seg(seg, pre); \
> }
> @@ -72,6 +71,7 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
> struct sigcontext_32 __user *sc)
> {
> unsigned int tmpflags, err = 0;
> + u16 gs, fs, es, ds;
> void __user *buf;
> u32 tmp;
>
> @@ -79,16 +79,10 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
> current->restart_block.fn = do_no_restart_syscall;
>
> get_user_try {
> - /*
> - * Reload fs and gs if they have changed in the signal
> - * handler. This does not handle long fs/gs base changes in
> - * the handler, but does not clobber them at least in the
> - * normal case.
> - */
> - RELOAD_SEG(gs);
> - RELOAD_SEG(fs);
> - RELOAD_SEG(ds);
> - RELOAD_SEG(es);
> + gs = GET_SEG(gs);
> + fs = GET_SEG(fs);
> + ds = GET_SEG(ds);
> + es = GET_SEG(es);
>
> COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
> COPY(dx); COPY(cx); COPY(ip); COPY(ax);
> @@ -106,6 +100,17 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
> buf = compat_ptr(tmp);
> } get_user_catch(err);
>
> + /*
> + * Reload fs and gs if they have changed in the signal
> + * handler. This does not handle long fs/gs base changes in
> + * the handler, but does not clobber them at least in the
> + * normal case.
> + */
> + RELOAD_SEG(gs);
> + RELOAD_SEG(fs);
> + RELOAD_SEG(ds);
> + RELOAD_SEG(es);
> +
> err |= fpu__restore_sig(buf, 1);
>
> force_iret();

I