diff options
| author | Pawan Gupta <pawan.kumar.gupta@linux.intel.com> | 2025-05-02 06:25:19 -0700 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2025-05-18 08:24:59 +0200 |
| commit | bd57853b863ea427c8fa25f7eb61f03cb6baf5ad (patch) | |
| tree | af4c4bca72bb37dc238c570063f6c4d160aa88ed | |
| parent | 9f132c0397df9cc073172560b6e4c975a4746c0e (diff) | |
x86/its: Align RETs in BHB clear sequence to avoid thunking
commit f0cd7091cc5a032c8870b4285305d9172569d126 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>
| -rw-r--r-- | arch/x86/entry/entry_64.S | 20 |
1 files changed, 17 insertions, 3 deletions
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 1b5be07f8669..9c6a110a52d4 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -1524,7 +1524,9 @@ SYM_CODE_END(rewind_stack_and_make_dead) * ORC to unwind properly. * * The alignment is for performance and not for safety, and may be safely - * refactored in the future if needed. + * refactored in the future if needed. The .skips are for safety, to ensure + * that all RETs are in the second half of a cacheline to mitigate Indirect + * Target Selection, rather than taking the slowpath via its_return_thunk. */ SYM_FUNC_START(clear_bhb_loop) push %rbp @@ -1534,10 +1536,22 @@ SYM_FUNC_START(clear_bhb_loop) call 1f jmp 5f .align 64, 0xcc + /* + * Shift instructions so that the RET is in the upper half of the + * cacheline and don't take the slowpath to its_return_thunk. + */ + .skip 32 - (.Lret1 - 1f), 0xcc ANNOTATE_INTRA_FUNCTION_CALL 1: call 2f - RET +.Lret1: RET .align 64, 0xcc + /* + * As above shift instructions for RET at .Lret2 as well. + * + * This should be ideally be: .skip 32 - (.Lret2 - 2f), 0xcc + * but some Clang versions (e.g. 18) don't like this. + */ + .skip 32 - 18, 0xcc 2: movl $5, %eax 3: jmp 4f nop @@ -1545,7 +1559,7 @@ SYM_FUNC_START(clear_bhb_loop) jnz 3b sub $1, %ecx jnz 1b - RET +.Lret2: RET 5: lfence pop %rbp RET |