On Thu, Aug 13, 2015 at 1:18 PM, Andy Lutomirski <l...@kernel.org> 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 <s...@list.ru> > Fixes: c6f2062935c8 ("x86/signal/64: Fix SS handling for signals delivered to > 64-bit programs") > Signed-off-by: Andy Lutomirski <l...@kernel.org> > ---
> + 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html