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Date: Mon, 27 Apr 2015 08:56:22 -0700 From: Andy Lutomirski <luto@...capital.net> To: Borislav Petkov <bp@...en8.de> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, Denys Vlasenko <vda.linux@...glemail.com>, Brian Gerst <brgerst@...il.com>, Denys Vlasenko <dvlasenk@...hat.com>, Ingo Molnar <mingo@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, Oleg Nesterov <oleg@...hat.com>, Frederic Weisbecker <fweisbec@...il.com>, Alexei Starovoitov <ast@...mgrid.com>, Will Drewry <wad@...omium.org>, Kees Cook <keescook@...omium.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue On Mon, Apr 27, 2015 at 8:46 AM, Borislav Petkov <bp@...en8.de> wrote: > On Mon, Apr 27, 2015 at 07:57:36AM -0700, Linus Torvalds wrote: >> On Mon, Apr 27, 2015 at 4:35 AM, Borislav Petkov <bp@...en8.de> wrote: >> > >> > /* >> > * Change top 16 bits to be the sign-extension of 47th bit, if this >> > * changed %rcx, it was not canonical. >> > */ >> > ALTERNATIVE "", \ >> > "shl $(64 - (47+1)), %rcx; \ >> > sar $(64 - (47+1)), %rcx; \ >> > cmpq %rcx, %r11; \ >> > jne opportunistic_sysret_failed", X86_BUG_SYSRET_CANON_RCX >> >> Guys, if we're looking at cycles for this, then don't do the "exact >> canonical test". and go back to just doing >> >> shr $__VIRTUAL_MASK_SHIFT, %rcx >> jnz opportunistic_sysret_failed >> >> which is much smaller. > > Right, what about the false positives: > > 17be0aec74fb ("x86/asm/entry/64: Implement better check for canonical addresses") > > ? We don't care? The false positives only matter for very strange workloads, e.g. vsyscall=native with old libc. If it's a measurable regression, we could revert it. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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