Messages in this thread | | | Date | Mon, 27 Apr 2015 13:35:06 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue |
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On Mon, Apr 27, 2015 at 10:53:05AM +0200, Borislav Petkov wrote: > ALTERNATIVE "", > "shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx \ > sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx \ > cmpq %rcx, %r11 \ > jne opportunistic_sysret_failed" > X86_BUG_SYSRET_CANONICAL_RCX
Right, so I can do this:
/* * 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
If I use the __VIRTUAL_MASK_SHIFT macro *in* the ALTERNATIVE macro, I get some really cryptic gas error:
arch/x86/kernel/entry_64.S: Assembler messages: arch/x86/kernel/entry_64.S:441: Error: can't resolve `L0' {*ABS* section} - `L0' {*UND* section} scripts/Makefile.build:294: recipe for target 'arch/x86/kernel/entry_64.o' failed make[1]: *** [arch/x86/kernel/entry_64.o] Error 1 Makefile:1536: recipe for target 'arch/x86/kernel/entry_64.o' failed make: *** [arch/x86/kernel/entry_64.o] Error 2
but I guess we can simply use the naked "47" because a couple of lines above, we already have the sanity-check:
.ifne __VIRTUAL_MASK_SHIFT - 47 .error "virtual address width changed -- SYSRET checks need update" .endif
so we should be guarded just fine.
Anyway, if we do it this way, we get 17 NOPs added at build time which is the length of the 4 instructions:
ffffffff819ef40c: 48 c1 e1 10 shl $0x10,%rcx ffffffff819ef410: 48 c1 f9 10 sar $0x10,%rcx ffffffff819ef414: 49 39 cb cmp %rcx,%r11 ffffffff819ef417: 0f 85 ff 9c bc ff jne ffffffff815b911c <opportunistic_sysret_failed>
and the 17 NOPs should be optimized at boot time on AMD.
I was initially afraid that the JMP in the 4th line might be wrong but apparently since we're using a global label, gas/gcc does generate the offset properly (0xffffffff815b911c):
ffffffff815b911c <opportunistic_sysret_failed>: ffffffff815b911c: ff 15 fe a4 26 00 callq *0x26a4fe(%rip) # ffffffff81823620 <pv_cpu_ops+0x120> ffffffff815b9122: e9 01 09 00 00 jmpq ffffffff815b9a28 <restore_c_regs_and_iret> ffffffff815b9127: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1) ffffffff815b912e: 00 00
So, on to do some tracing.
-- Regards/Gruss, Boris.
ECO tip #101: Trim your mails when you reply. --
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