Commit bc124170 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

x86/speculation/mds: Add mitigation control for MDS 



Now that the mitigations are in place, add a command line parameter to
control the mitigation, a mitigation selector function and a SMT update
mechanism.

This is the minimal straight forward initial implementation which just
provides an always on/off mode. The command line parameter is:

  mds=[full|off]

This is consistent with the existing mitigations for other speculative
hardware vulnerabilities.

The idle invocation is dynamically updated according to the SMT state of
the system similar to the dynamic update of the STIBP mitigation. The idle
mitigation is limited to CPUs which are only affected by MSBDS and not any
other variant, because the other variants cannot be mitigated on SMT
enabled systems.

Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarBorislav Petkov <bp@suse.de>
Reviewed-by: default avatarJon Masters <jcm@redhat.com>
Tested-by: default avatarJon Masters <jcm@redhat.com>
parent 07f07f55

Documentation/admin-guide/kernel-parameters.txt

+22 −0
Original line number Diff line number Diff line
@@ -2356,6 +2356,28 @@ 
			Format: <first>,<last>

			Specifies range of consoles to be captured by the MDA.



	mds=		[X86,INTEL]

			Control mitigation for the Micro-architectural Data

			Sampling (MDS) vulnerability.



			Certain CPUs are vulnerable to an exploit against CPU

			internal buffers which can forward information to a

			disclosure gadget under certain conditions.



			In vulnerable processors, the speculatively

			forwarded data can be used in a cache side channel

			attack, to access data to which the attacker does

			not have direct access.



			This parameter controls the MDS mitigation. The

			options are:



			full    - Enable MDS mitigation on vulnerable CPUs

			off     - Unconditionally disable MDS mitigation



			Not specifying this option is equivalent to

			mds=full.



	mem=nn[KMG]	[KNL,BOOT] Force usage of a specific amount of memory

			Amount of memory to be used when the kernel is not able

			to see the whole system memory or for test.

arch/x86/include/asm/processor.h

+5 −0
Original line number Diff line number Diff line
@@ -992,4 +992,9 @@ enum l1tf_mitigations { 


extern enum l1tf_mitigations l1tf_mitigation;



enum mds_mitigations {

	MDS_MITIGATION_OFF,

	MDS_MITIGATION_FULL,

};



#endif /* _ASM_X86_PROCESSOR_H */

arch/x86/kernel/cpu/bugs.c

+70 −0
Original line number Diff line number Diff line
@@ -37,6 +37,7 @@ 
static void __init spectre_v2_select_mitigation(void);

static void __init ssb_select_mitigation(void);

static void __init l1tf_select_mitigation(void);

static void __init mds_select_mitigation(void);



/* The base value of the SPEC_CTRL MSR that always has to be preserved. */

u64 x86_spec_ctrl_base;
@@ -108,6 +109,8 @@ void __init check_bugs(void) 


	l1tf_select_mitigation();



	mds_select_mitigation();



#ifdef CONFIG_X86_32

	/*

	 * Check whether we are able to run this kernel safely on SMP.
@@ -213,6 +216,50 @@ static void x86_amd_ssb_disable(void) 
		wrmsrl(MSR_AMD64_LS_CFG, msrval);

}



#undef pr_fmt

#define pr_fmt(fmt)	"MDS: " fmt



/* Default mitigation for L1TF-affected CPUs */

static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;



static const char * const mds_strings[] = {

	[MDS_MITIGATION_OFF]	= "Vulnerable",

	[MDS_MITIGATION_FULL]	= "Mitigation: Clear CPU buffers"

};



static void __init mds_select_mitigation(void)

{

	if (!boot_cpu_has_bug(X86_BUG_MDS)) {

		mds_mitigation = MDS_MITIGATION_OFF;

		return;

	}



	if (mds_mitigation == MDS_MITIGATION_FULL) {

		if (boot_cpu_has(X86_FEATURE_MD_CLEAR))

			static_branch_enable(&mds_user_clear);

		else

			mds_mitigation = MDS_MITIGATION_OFF;

	}

	pr_info("%s\n", mds_strings[mds_mitigation]);

}



static int __init mds_cmdline(char *str)

{

	if (!boot_cpu_has_bug(X86_BUG_MDS))

		return 0;



	if (!str)

		return -EINVAL;



	if (!strcmp(str, "off"))

		mds_mitigation = MDS_MITIGATION_OFF;

	else if (!strcmp(str, "full"))

		mds_mitigation = MDS_MITIGATION_FULL;



	return 0;

}

early_param("mds", mds_cmdline);



#undef pr_fmt

#define pr_fmt(fmt)     "Spectre V2 : " fmt
@@ -617,6 +664,26 @@ static void update_indir_branch_cond(void) 
		static_branch_disable(&switch_to_cond_stibp);

}



/* Update the static key controlling the MDS CPU buffer clear in idle */

static void update_mds_branch_idle(void)

{

	/*

	 * Enable the idle clearing if SMT is active on CPUs which are

	 * affected only by MSBDS and not any other MDS variant.

	 *

	 * The other variants cannot be mitigated when SMT is enabled, so

	 * clearing the buffers on idle just to prevent the Store Buffer

	 * repartitioning leak would be a window dressing exercise.

	 */

	if (!boot_cpu_has_bug(X86_BUG_MSBDS_ONLY))

		return;



	if (sched_smt_active())

		static_branch_enable(&mds_idle_clear);

	else

		static_branch_disable(&mds_idle_clear);

}



void arch_smt_update(void)

{

	/* Enhanced IBRS implies STIBP. No update required. */
@@ -638,6 +705,9 @@ void arch_smt_update(void) 
		break;

	}



	if (mds_mitigation == MDS_MITIGATION_FULL)

		update_mds_branch_idle();



	mutex_unlock(&spec_ctrl_mutex);

}

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