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x86/bhi: Do not set BHI_DIS_S in 32-bit mode

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With the possibility of intra-mode BHI via cBPF, complete mitigation for
BHI is to use IBHF (history fence) instruction with BHI_DIS_S set. Since
this new instruction is only available in 64-bit mode, setting BHI_DIS_S in
32-bit mode is only a partial mitigation.

Do not set BHI_DIS_S in 32-bit mode so as to avoid reporting misleading
mitigated status. With this change IBHF won't be used in 32-bit mode, also
remove the CONFIG_X86_64 check from emit_spectre_bhb_barrier().

Suggested-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
This commit is contained in:
Pawan Gupta 2025-05-05 14:35:12 -07:00 committed by Dave Hansen
parent 9f725eec8f
commit 073fdbe02c
2 changed files with 6 additions and 5 deletions
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6
arch/x86/kernel/cpu/bugs.c
View File

@ -1697,11 +1697,11 @@ static void __init bhi_select_mitigation(void)
return;
}
/* Mitigate in hardware if supported */
if (spec_ctrl_bhi_dis())
if (!IS_ENABLED(CONFIG_X86_64))
return;
if (!IS_ENABLED(CONFIG_X86_64))
/* Mitigate in hardware if supported */
if (spec_ctrl_bhi_dis())
return;
if (bhi_mitigation == BHI_MITIGATION_VMEXIT_ONLY) {

5
arch/x86/net/bpf_jit_comp.c
View File

@ -1527,8 +1527,7 @@ static int emit_spectre_bhb_barrier(u8 **pprog, u8 *ip,
/* Insert IBHF instruction */
if ((cpu_feature_enabled(X86_FEATURE_CLEAR_BHB_LOOP) &&
cpu_feature_enabled(X86_FEATURE_HYPERVISOR)) ||
(cpu_feature_enabled(X86_FEATURE_CLEAR_BHB_HW) &&
IS_ENABLED(CONFIG_X86_64))) {
cpu_feature_enabled(X86_FEATURE_CLEAR_BHB_HW)) {
/*
* Add an Indirect Branch History Fence (IBHF). IBHF acts as a
* fence preventing branch history from before the fence from
@ -1538,6 +1537,8 @@ static int emit_spectre_bhb_barrier(u8 **pprog, u8 *ip,
* hardware that doesn't need or support it. The REP and REX.W
* prefixes are required by the microcode, and they also ensure
* that the NOP is unlikely to be used in existing code.
*
* IBHF is not a valid instruction in 32-bit mode.
*/
EMIT5(0xF3, 0x48, 0x0F, 0x1E, 0xF8); /* ibhf */
}