On Thu, Aug 13, 2015 at 1:18 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> Linux used to have nearly useless SS handling for 64-bit signals. Signal
> delivery to a 64-bit process would preserve the SS selector, but the
> selector wasn't saved in sigcontext. Sigreturn would then clobber SS.
> If the signal being delivered was due to a bad SS, signal delivery would
> fail and the task would be killed.
>
> As of Linux 4.1, it's fixed: signal delivery sets up a valid SS in the
> hardware SS selector, saves the old SS in the sigcontext, and restores it
> properly in sigreturn.
>
> DOSEMU had a curious workaround for the old behavior: it saved the
> hardware SS selector it was given during signal delivery and fudged
> RIP and CS so that sigreturn would return to a trampoline that
> restored the old RIP, CS, and, importantly, SS.
>
> The upshot is that the change in sigcontext had no effect on DOSEMU
> (DOSEMU doesn't care what was in sigcontext, and the fact that the
> old SS is presented to the trampoline in new kernels is irrelevant
> because the trampoline uses long mode), but the change in signal
> delivery caused DOSEMU's workaround to restore __USER_DS instead of
> the correct pre-signal SS value.
>
> Do our best to work around it: explicitly check whether the old SS
> is usable and leave it alone during signal delivery if it is.
> Sigreturn is unchanged.
>
> Reported-by: Stas Sergeev <stsp@xxxxxxx>
> Fixes: c6f2062935c8 ("x86/signal/64: Fix SS handling for signals delivered to 64-bit programs")
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
> + asm ("lar %[old_ss], %[ar]\n\t"
> + "jz 1f\n\t"
> + "xorl %[ar], %[ar]\n\t" /* If invalid, set ar = 0 */
> + "1:"
> + : [ar] "=r" (ar)
> + : [old_ss] "rm" ((u16)regs->ss));
> +
Now that I sent this...
I should learn to think very carefully before doubting Linus'
off-the-cuff intuition about the x86 instruction set. This can use
VERW after all, as long as it's careful to set RPL==3.
For purposes of testing DOSEMU testing, the version I sent should be
fine. For purposes of review, please pretend that I was sensible and
used VERW instead of LAR. This also means that the VMX fixlet is
optional, but I still think it's a good idea.
--Andy
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