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Date: Wed, 20 Jan 2016 02:41:22 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Borislav Petkov <bp@...e.de>
CC: Andy Lutomirski <luto@...capital.net>,
Brian Gerst <brgerst@...il.com>,
the arch/x86 maintainers <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...nel.org>,
Denys Vlasenko <dvlasenk@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: static_cpu_has_safe: discard dynamic check after init
On January 20, 2016 2:33:45 AM PST, Borislav Petkov <bp@...e.de> wrote:
>On Tue, Jan 19, 2016 at 08:03:55PM -0800, H. Peter Anvin wrote:
>> On 01/19/16 05:57, Borislav Petkov wrote:
>> >
>> > old insn VA: 0xffffffff8108c0d0, CPU feat: X86_BUG_SYSRET_SS_ATTRS,
>size: 2, padlen: 0
>> > __switch_to:
>> > ffffffff8108c0d0: eb 70 jmp
>ffffffff8108c142
>> > repl insn: 0xffffffff8ca7a1a7, size: 0
>> >
>>
>> This is the only one I could possibly imagine mattering. Would it be
>> possible to get the disassembly here?
>
>Sure, I've kept the vmlinux, see below:
>
>ffffffff8108c0d0: eb 70 jmp ffffffff8108c142
><__switch_to+0x838>
>
>jumps to:
>
>ffffffff8108c142: 48 ff 05 b7 08 aa 0b incq 0xbaa08b7(%rip)
>
>which is something-gcov. In any case, it jumps over the SS fixup code:
>
> if (static_cpu_has_bug(X86_BUG_SYSRET_SS_ATTRS)) {
>
> ...
>
> unsigned short ss_sel;
> savesegment(ss, ss_sel);
> if (ss_sel != __KERNEL_DS)
> loadsegment(ss, __KERNEL_DS);
> }
>
>I guess we could fix that with an alternative_call() where @oldinstr is
>empty so that on !X86_BUG_SYSRET_SS_ATTRS machines we simply return and
>on the others, we do a CALL fixup_ss or so.
>
>We have to pay attention to clobbers, though, similar to the POPCNT fun
>in __arch_hweight32().
>
>ffffffff8108b90a <__switch_to>:
>ffffffff8108b90a: 55 push %rbp
>ffffffff8108b90b: 48 8d 87 c0 2d 00 00 lea
>0x2dc0(%rdi),%rax
>ffffffff8108b912: 48 c7 c1 80 f6 1d 00 mov $0x1df680,%rcx
>ffffffff8108b919: 48 89 e5 mov %rsp,%rbp
>ffffffff8108b91c: 41 57 push %r15
>ffffffff8108b91e: 45 31 ff xor %r15d,%r15d
>ffffffff8108b921: 41 56 push %r14
>
>...
>
>ffffffff8108c0c2: 48 ff 05 07 09 aa 0b incq 0xbaa0907(%rip)
> # ffffffff8cb2c9d0 <__gcov0.__switch_to+0xc0>
>ffffffff8108c0c9: 48 ff 05 08 09 aa 0b incq 0xbaa0908(%rip)
> # ffffffff8cb2c9d8 <__gcov0.__switch_to+0xc8>
>ffffffff8108c0d0: eb 70 jmp ffffffff8108c142
><__switch_to+0x838>
>ffffffff8108c0d2: 48 ff 05 07 09 aa 0b incq 0xbaa0907(%rip)
> # ffffffff8cb2c9e0 <__gcov0.__switch_to+0xd0>
>ffffffff8108c0d9: 48 ff 05 08 09 aa 0b incq 0xbaa0908(%rip)
> # ffffffff8cb2c9e8 <__gcov0.__switch_to+0xd8>
>ffffffff8108c0e0: 48 ff 05 11 09 aa 0b incq 0xbaa0911(%rip)
> # ffffffff8cb2c9f8 <__gcov0.__switch_to+0xe8>
>ffffffff8108c0e7: 66 8c d0 mov %ss,%ax
>ffffffff8108c0ea: 66 83 f8 18 cmp $0x18,%ax
>ffffffff8108c0ee: 75 17 jne ffffffff8108c107
><__switch_to+0x7fd>
>ffffffff8108c0f0: eb 57 jmp ffffffff8108c149
><__switch_to+0x83f>
>ffffffff8108c0f2: 48 ff 05 cf 08 aa 0b incq 0xbaa08cf(%rip)
> # ffffffff8cb2c9c8 <__gcov0.__switch_to+0xb8>
>ffffffff8108c0f9: e8 db b0 01 00 callq ffffffff810a71d9
><warn_pre_alternatives>
>ffffffff8108c0fe: 48 ff 05 eb 08 aa 0b incq 0xbaa08eb(%rip)
> # ffffffff8cb2c9f0 <__gcov0.__switch_to+0xe0>
>ffffffff8108c105: eb 3b jmp ffffffff8108c142
><__switch_to+0x838>
>ffffffff8108c107: 48 ff 05 fa 08 aa 0b incq 0xbaa08fa(%rip)
> # ffffffff8cb2ca08 <__gcov0.__switch_to+0xf8>
>ffffffff8108c10e: b8 18 00 00 00 mov $0x18,%eax
>ffffffff8108c113: 8e d0 mov %eax,%ss
>ffffffff8108c115: 48 ff 05 f4 08 aa 0b incq 0xbaa08f4(%rip)
> # ffffffff8cb2ca10 <__gcov0.__switch_to+0x100>
>ffffffff8108c11c: eb 2b jmp ffffffff8108c149
><__switch_to+0x83f>
>ffffffff8108c11e: 48 ff 05 9b 02 aa 0b incq 0xbaa029b(%rip)
> # ffffffff8cb2c3c0 <__gcov0.copy_xregs_to_kernel+0x30>
>ffffffff8108c125: e9 91 f9 ff ff jmpq ffffffff8108babb
><__switch_to+0x1b1>
>ffffffff8108c12a: 48 ff 05 bf 02 aa 0b incq 0xbaa02bf(%rip)
> # ffffffff8cb2c3f0 <__gcov0.copy_kernel_to_xregs+0x20>
>ffffffff8108c131: e9 84 fe ff ff jmpq ffffffff8108bfba
><__switch_to+0x6b0>
>ffffffff8108c136: 48 ff 05 43 02 aa 0b incq 0xbaa0243(%rip)
> # ffffffff8cb2c380 <__gcov0.copy_kernel_to_fxregs+0x20>
>ffffffff8108c13d: e9 db fe ff ff jmpq ffffffff8108c01d
><__switch_to+0x713>
>ffffffff8108c142: 48 ff 05 b7 08 aa 0b incq 0xbaa08b7(%rip)
> # ffffffff8cb2ca00 <__gcov0.__switch_to+0xf0>
>ffffffff8108c149: 48 83 c4 28 add $0x28,%rsp
>ffffffff8108c14d: 4c 89 e0 mov %r12,%rax
>ffffffff8108c150: 5b pop %rbx
>ffffffff8108c151: 41 5c pop %r12
>ffffffff8108c153: 41 5d pop %r13
>ffffffff8108c155: 41 5e pop %r14
>ffffffff8108c157: 41 5f pop %r15
>ffffffff8108c159: 5d pop %rbp
>ffffffff8108c15a: c3 retq
Ah. What would be even more of a win would be to rebias static_cpu_has_bug() so that the fallthrough case is the functional one. Easily done by reversing the labels.
--
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