commit f0cd7091cc upstream.
The software mitigation for BHI is to execute BHB clear sequence at syscall
entry, and possibly after a cBPF program. ITS mitigation thunks RETs in the
lower half of the cacheline. This causes the RETs in the BHB clear sequence
to be thunked as well, adding unnecessary branches to the BHB clear
sequence.
Since the sequence is in hot path, align the RET instructions in the
sequence to avoid thunking.
This is how disassembly clear_bhb_loop() looks like after this change:
0x44 <+4>: mov $0x5,%ecx
0x49 <+9>: call 0xffffffff81001d9b <clear_bhb_loop+91>
0x4e <+14>: jmp 0xffffffff81001de5 <clear_bhb_loop+165>
0x53 <+19>: int3
...
0x9b <+91>: call 0xffffffff81001dce <clear_bhb_loop+142>
0xa0 <+96>: ret
0xa1 <+97>: int3
...
0xce <+142>: mov $0x5,%eax
0xd3 <+147>: jmp 0xffffffff81001dd6 <clear_bhb_loop+150>
0xd5 <+149>: nop
0xd6 <+150>: sub $0x1,%eax
0xd9 <+153>: jne 0xffffffff81001dd3 <clear_bhb_loop+147>
0xdb <+155>: sub $0x1,%ecx
0xde <+158>: jne 0xffffffff81001d9b <clear_bhb_loop+91>
0xe0 <+160>: ret
0xe1 <+161>: int3
0xe2 <+162>: int3
0xe3 <+163>: int3
0xe4 <+164>: int3
0xe5 <+165>: lfence
0xe8 <+168>: pop %rbp
0xe9 <+169>: ret
Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
bd57853b86