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Commit 1b86883c authored by Konrad Rzeszutek Wilk's avatar Konrad Rzeszutek Wilk Committed by Thomas Gleixner
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x86/bugs: Read SPEC_CTRL MSR during boot and re-use reserved bits 

The 336996-Speculative-Execution-Side-Channel-Mitigations.pdf refers to all
the other bits as reserved. The Intel SDM glossary defines reserved as
implementation specific - aka unknown.

As such at bootup this must be taken it into account and proper masking for
the bits in use applied.

A copy of this document is available at
https://bugzilla.kernel.org/show_bug.cgi?id=199511



[ tglx: Made x86_spec_ctrl_base __ro_after_init ]

Suggested-by: default avatarJon Masters <jcm@redhat.com>
Signed-off-by: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarBorislav Petkov <bp@suse.de>
Reviewed-by: default avatarIngo Molnar <mingo@kernel.org>
parent d1059518

arch/x86/include/asm/nospec-branch.h

+20 −4
Original line number Original line Diff line number Diff line
@@ -217,6 +217,17 @@ enum spectre_v2_mitigation { 
	SPECTRE_V2_IBRS,

	SPECTRE_V2_IBRS,

};

};





/*

 * The Intel specification for the SPEC_CTRL MSR requires that we

 * preserve any already set reserved bits at boot time (e.g. for

 * future additions that this kernel is not currently aware of).

 * We then set any additional mitigation bits that we want

 * ourselves and always use this as the base for SPEC_CTRL.

 * We also use this when handling guest entry/exit as below.

 */

extern void x86_spec_ctrl_set(u64);

extern u64 x86_spec_ctrl_get_default(void);



extern char __indirect_thunk_start[];

extern char __indirect_thunk_start[];

extern char __indirect_thunk_end[];

extern char __indirect_thunk_end[];
@@ -254,8 +265,9 @@ void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature) 




static inline void indirect_branch_prediction_barrier(void)

static inline void indirect_branch_prediction_barrier(void)

{

{

	alternative_msr_write(MSR_IA32_PRED_CMD, PRED_CMD_IBPB,

	u64 val = PRED_CMD_IBPB;

			      X86_FEATURE_USE_IBPB);



	alternative_msr_write(MSR_IA32_PRED_CMD, val, X86_FEATURE_USE_IBPB);

}

}





/*

/*
@@ -266,14 +278,18 @@ static inline void indirect_branch_prediction_barrier(void) 
 */

 */

#define firmware_restrict_branch_speculation_start()			\

#define firmware_restrict_branch_speculation_start()			\

do {									\

do {									\

	u64 val = x86_spec_ctrl_get_default() | SPEC_CTRL_IBRS;		\

									\

	preempt_disable();						\

	preempt_disable();						\

	alternative_msr_write(MSR_IA32_SPEC_CTRL, SPEC_CTRL_IBRS,	\

	alternative_msr_write(MSR_IA32_SPEC_CTRL, val,			\

			      X86_FEATURE_USE_IBRS_FW);			\

			      X86_FEATURE_USE_IBRS_FW);			\

} while (0)

} while (0)





#define firmware_restrict_branch_speculation_end()			\

#define firmware_restrict_branch_speculation_end()			\

do {									\

do {									\

	alternative_msr_write(MSR_IA32_SPEC_CTRL, 0,			\

	u64 val = x86_spec_ctrl_get_default();				\

									\

	alternative_msr_write(MSR_IA32_SPEC_CTRL, val,			\

			      X86_FEATURE_USE_IBRS_FW);			\

			      X86_FEATURE_USE_IBRS_FW);			\

	preempt_enable();						\

	preempt_enable();						\

} while (0)

} while (0)

arch/x86/kernel/cpu/bugs.c

+28 −0
Original line number Original line Diff line number Diff line
@@ -28,6 +28,12 @@ 




static void __init spectre_v2_select_mitigation(void);

static void __init spectre_v2_select_mitigation(void);





/*

 * Our boot-time value of the SPEC_CTRL MSR. We read it once so that any

 * writes to SPEC_CTRL contain whatever reserved bits have been set.

 */

static u64 __ro_after_init x86_spec_ctrl_base;



void __init check_bugs(void)

void __init check_bugs(void)

{

{

	identify_boot_cpu();

	identify_boot_cpu();
@@ -37,6 +43,13 @@ void __init check_bugs(void) 
		print_cpu_info(&boot_cpu_data);

		print_cpu_info(&boot_cpu_data);

	}

	}





	/*

	 * Read the SPEC_CTRL MSR to account for reserved bits which may

	 * have unknown values.

	 */

	if (boot_cpu_has(X86_FEATURE_IBRS))

		rdmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);



	/* Select the proper spectre mitigation before patching alternatives */

	/* Select the proper spectre mitigation before patching alternatives */

	spectre_v2_select_mitigation();

	spectre_v2_select_mitigation();
@@ -95,6 +108,21 @@ static const char *spectre_v2_strings[] = { 




static enum spectre_v2_mitigation spectre_v2_enabled = SPECTRE_V2_NONE;

static enum spectre_v2_mitigation spectre_v2_enabled = SPECTRE_V2_NONE;





void x86_spec_ctrl_set(u64 val)

{

	if (val & ~SPEC_CTRL_IBRS)

		WARN_ONCE(1, "SPEC_CTRL MSR value 0x%16llx is unknown.\n", val);

	else

		wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base | val);

}

EXPORT_SYMBOL_GPL(x86_spec_ctrl_set);



u64 x86_spec_ctrl_get_default(void)

{

	return x86_spec_ctrl_base;

}

EXPORT_SYMBOL_GPL(x86_spec_ctrl_get_default);



#ifdef RETPOLINE

#ifdef RETPOLINE

static bool spectre_v2_bad_module;

static bool spectre_v2_bad_module;