Re: [PATCH] x86: static_cpu_has_safe: discard dynamic check after init

[H. Peter Anvin]

On January 20, 2016 2:33:45 AM PST, Borislav Petkov <bp@xxxxxxx> 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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