Messages in this thread | | | Date | Mon, 27 Apr 2015 17:46:31 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue |
| |
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@alien8.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?
> In fact, aim to make the conditional jump be a > two-byte one (jump forward to another jump if required - it's a > slow-path that doesn't matter at *all* for the taken case), and the > end result is just six bytes. That way you can use alternative to > replace it with one single noop on AMD.
Well, even with the non-optimal NOPs (we end up with 4 3-byte NOPs and one single-byte), we're still better than the unconditional JMP I had there before:
https://lkml.kernel.org/r/20150427143905.GK6774@pd.tnic
(you might want to look at the raw email - marc.info breaks lines)
I'll retest with the F16h NOPs to see whether there's any difference.
Thanks.
-- Regards/Gruss, Boris.
ECO tip #101: Trim your mails when you reply. --
| |