x86/spec_ctrl: Only set SPEC_CTRL_IBRS_FIRMWARE if IBRS is actually in use

Currently the SPEC_CTRL_IBRS_FIRMWARE flag always gets set as long as
IBRS is supported by the hardware. However, as best as can be determined
by the documention, if IBRS has been disabled (e.g., spectre_v2=off) then
SPEC_CTRL_IBRS_FIRMWARE should not be set:

nospectre_v2	[X86] Disable all mitigations for the Spectre variant 2
		(indirect branch prediction) vulnerability. System may
		allow data leaks with this option, which is equivalent
		to spectre_v2=off.

and:

spectre_v2=	[X86] Control mitigation of Spectre variant 2
		(indirect branch speculation) vulnerability.

		off  - unconditionally disable

Add a check in set_ibrs_firmware() to only set SPEC_CTRL_IBRS_FIRMWARE if
ibrs_disabled is not also set.

Orabug: 28274907

Signed-off-by: Patrick Colp <patrick.colp@oracle.com>
Reviewed-by: Kanth Ghatraju <kanth.ghatraju@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>

diff --git a/arch/x86/include/asm/spec_ctrl.h b/arch/x86/include/asm/spec_ctrl.h
index 1abaf122a6e8218bd65a606f8d0de9ab9c3007d2..c9016d4d3694faab043d637c6b7c287cdcec4e42 100644 (file)
--- a/arch/x86/include/asm/spec_ctrl.h
+++ b/arch/x86/include/asm/spec_ctrl.h
@@ -337,7 +337,7 @@ static inline void clear_ibrs_disabled(void)
 
 static inline void set_ibrs_firmware(void)
 {
-       if (ibrs_supported)
+       if (ibrs_supported && !ibrs_disabled)
                use_ibrs |= SPEC_CTRL_IBRS_FIRMWARE;
 }