x86/retpoline: Simplify retpolines

stable inclusion
from stable-v5.10.133
commit 28ca351296742a9e7506a548acaf7ea3bc9feef0
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5PTAS
CVE: CVE-2022-29900,CVE-2022-23816,CVE-2022-29901

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=28ca351296742a9e7506a548acaf7ea3bc9feef0



--------------------------------

commit 11925185 upstream.

Due to:

  c9c324dc ("objtool: Support stack layout changes in alternatives")

it is now possible to simplify the retpolines.

Currently our retpolines consist of 2 symbols:

 - __x86_indirect_thunk_\reg: the compiler target
 - __x86_retpoline_\reg:  the actual retpoline.

Both are consecutive in code and aligned such that for any one register
they both live in the same cacheline:

  0000000000000000 <__x86_indirect_thunk_rax>:
   0:   ff e0                   jmpq   *%rax
   2:   90                      nop
   3:   90                      nop
   4:   90                      nop

  0000000000000005 <__x86_retpoline_rax>:
   5:   e8 07 00 00 00          callq  11 <__x86_retpoline_rax+0xc>
   a:   f3 90                   pause
   c:   0f ae e8                lfence
   f:   eb f9                   jmp    a <__x86_retpoline_rax+0x5>
  11:   48 89 04 24             mov    %rax,(%rsp)
  15:   c3                      retq
  16:   66 2e 0f 1f 84 00 00 00 00 00   nopw   %cs:0x0(%rax,%rax,1)

The thunk is an alternative_2, where one option is a JMP to the
retpoline. This was done so that objtool didn't need to deal with
alternatives with stack ops. But that problem has been solved, so now
it is possible to fold the entire retpoline into the alternative to
simplify and consolidate unused bytes:

  0000000000000000 <__x86_indirect_thunk_rax>:
   0:   ff e0                   jmpq   *%rax
   2:   90                      nop
   3:   90                      nop
   4:   90                      nop
   5:   90                      nop
   6:   90                      nop
   7:   90                      nop
   8:   90                      nop
   9:   90                      nop
   a:   90                      nop
   b:   90                      nop
   c:   90                      nop
   d:   90                      nop
   e:   90                      nop
   f:   90                      nop
  10:   90                      nop
  11:   66 66 2e 0f 1f 84 00 00 00 00 00        data16 nopw %cs:0x0(%rax,%rax,1)
  1c:   0f 1f 40 00             nopl   0x0(%rax)

Notice that since the longest alternative sequence is now:

   0:   e8 07 00 00 00          callq  c <.altinstr_replacement+0xc>
   5:   f3 90                   pause
   7:   0f ae e8                lfence
   a:   eb f9                   jmp    5 <.altinstr_replacement+0x5>
   c:   48 89 04 24             mov    %rax,(%rsp)
  10:   c3                      retq

17 bytes, we have 15 bytes NOP at the end of our 32 byte slot. (IOW, if
we can shrink the retpoline by 1 byte we can pack it more densely).

 [ bp: Massage commit message. ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210326151259.506071949@infradead.org


[bwh: Backported to 5.10:
 - Use X86_FEATRURE_RETPOLINE_LFENCE flag instead of
   X86_FEATURE_RETPOLINE_AMD, since the later renaming of this flag
   has already been applied
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lin Yujun <linyujun809@huawei.com>
Reviewed-by: Zhang Jianhua <chris.zjh@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>