Messages in this thread | | | Date | Mon, 27 Apr 2015 12:04:44 -0400 | Subject | Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue | From | Brian Gerst <> |
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On Mon, Apr 27, 2015 at 11:56 AM, Andy Lutomirski <luto@amacapital.net> wrote: > On Mon, Apr 27, 2015 at 8:46 AM, Borislav Petkov <bp@alien8.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@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? > > 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
Another alternative is to do the canonical check in the paths that can set user RIP with an untrusted value, ie, sigreturn and exec.
-- Brian Gerst
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