Attack vector controls (part 1)

[PATCH v5 02/16] x86/bugs: Restructure TAA mitigation

Posted by David Kaplan 7 months, 4 weeks ago

Restructure TAA mitigation to use select/update/apply functions to
create consistent vulnerability handling.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
---
 arch/x86/kernel/cpu/bugs.c | 94 ++++++++++++++++++++++++--------------
 1 file changed, 59 insertions(+), 35 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 4295502ea082..c0ba034ae1f9 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -65,6 +65,8 @@ static void __init mds_apply_mitigation(void);
 static void __init md_clear_update_mitigation(void);
 static void __init md_clear_select_mitigation(void);
 static void __init taa_select_mitigation(void);
+static void __init taa_update_mitigation(void);
+static void __init taa_apply_mitigation(void);
 static void __init mmio_select_mitigation(void);
 static void __init srbds_select_mitigation(void);
 static void __init l1d_flush_select_mitigation(void);
@@ -194,6 +196,7 @@ void __init cpu_select_mitigations(void)
 	ssb_select_mitigation();
 	l1tf_select_mitigation();
 	mds_select_mitigation();
+	taa_select_mitigation();
 	md_clear_select_mitigation();
 	srbds_select_mitigation();
 	l1d_flush_select_mitigation();
@@ -210,8 +213,10 @@ void __init cpu_select_mitigations(void)
 	 * choices.
 	 */
 	mds_update_mitigation();
+	taa_update_mitigation();
 
 	mds_apply_mitigation();
+	taa_apply_mitigation();
 }
 
 /*
@@ -394,6 +399,11 @@ static const char * const taa_strings[] = {
 	[TAA_MITIGATION_TSX_DISABLED]	= "Mitigation: TSX disabled",
 };
 
+static bool __init taa_vulnerable(void)
+{
+	return boot_cpu_has_bug(X86_BUG_TAA) && boot_cpu_has(X86_FEATURE_RTM);
+}
+
 static void __init taa_select_mitigation(void)
 {
 	if (!boot_cpu_has_bug(X86_BUG_TAA)) {
@@ -407,48 +417,63 @@ static void __init taa_select_mitigation(void)
 		return;
 	}
 
-	if (cpu_mitigations_off()) {
+	if (cpu_mitigations_off())
 		taa_mitigation = TAA_MITIGATION_OFF;
-		return;
-	}
 
-	/*
-	 * TAA mitigation via VERW is turned off if both
-	 * tsx_async_abort=off and mds=off are specified.
-	 */
-	if (taa_mitigation == TAA_MITIGATION_OFF &&
-	    mds_mitigation == MDS_MITIGATION_OFF)
+	/* Microcode will be checked in taa_update_mitigation(). */
+	if (taa_mitigation == TAA_MITIGATION_AUTO)
+		taa_mitigation = TAA_MITIGATION_VERW;
+
+	if (taa_mitigation != TAA_MITIGATION_OFF)
+		verw_mitigation_selected = true;
+}
+
+static void __init taa_update_mitigation(void)
+{
+	if (!taa_vulnerable() || cpu_mitigations_off())
 		return;
 
-	if (boot_cpu_has(X86_FEATURE_MD_CLEAR))
+	if (verw_mitigation_selected)
 		taa_mitigation = TAA_MITIGATION_VERW;
-	else
-		taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;
 
-	/*
-	 * VERW doesn't clear the CPU buffers when MD_CLEAR=1 and MDS_NO=1.
-	 * A microcode update fixes this behavior to clear CPU buffers. It also
-	 * adds support for MSR_IA32_TSX_CTRL which is enumerated by the
-	 * ARCH_CAP_TSX_CTRL_MSR bit.
-	 *
-	 * On MDS_NO=1 CPUs if ARCH_CAP_TSX_CTRL_MSR is not set, microcode
-	 * update is required.
-	 */
-	if ( (x86_arch_cap_msr & ARCH_CAP_MDS_NO) &&
-	    !(x86_arch_cap_msr & ARCH_CAP_TSX_CTRL_MSR))
-		taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;
+	if (taa_mitigation == TAA_MITIGATION_VERW) {
+		/* Check if the requisite ucode is available. */
+		if (!boot_cpu_has(X86_FEATURE_MD_CLEAR))
+			taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;
 
-	/*
-	 * TSX is enabled, select alternate mitigation for TAA which is
-	 * the same as MDS. Enable MDS static branch to clear CPU buffers.
-	 *
-	 * For guests that can't determine whether the correct microcode is
-	 * present on host, enable the mitigation for UCODE_NEEDED as well.
-	 */
-	setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+		/*
+		 * VERW doesn't clear the CPU buffers when MD_CLEAR=1 and MDS_NO=1.
+		 * A microcode update fixes this behavior to clear CPU buffers. It also
+		 * adds support for MSR_IA32_TSX_CTRL which is enumerated by the
+		 * ARCH_CAP_TSX_CTRL_MSR bit.
+		 *
+		 * On MDS_NO=1 CPUs if ARCH_CAP_TSX_CTRL_MSR is not set, microcode
+		 * update is required.
+		 */
+		if ((x86_arch_cap_msr & ARCH_CAP_MDS_NO) &&
+		   !(x86_arch_cap_msr & ARCH_CAP_TSX_CTRL_MSR))
+			taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;
+	}
 
-	if (taa_nosmt || cpu_mitigations_auto_nosmt())
-		cpu_smt_disable(false);
+	pr_info("%s\n", taa_strings[taa_mitigation]);
+}
+
+static void __init taa_apply_mitigation(void)
+{
+	if (taa_mitigation == TAA_MITIGATION_VERW ||
+	    taa_mitigation == TAA_MITIGATION_UCODE_NEEDED) {
+		/*
+		 * TSX is enabled, select alternate mitigation for TAA which is
+		 * the same as MDS. Enable MDS static branch to clear CPU buffers.
+		 *
+		 * For guests that can't determine whether the correct microcode is
+		 * present on host, enable the mitigation for UCODE_NEEDED as well.
+		 */
+		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+
+		if (taa_nosmt || cpu_mitigations_auto_nosmt())
+			cpu_smt_disable(false);
+	}
 }
 
 static int __init tsx_async_abort_parse_cmdline(char *str)
@@ -654,7 +679,6 @@ static void __init md_clear_update_mitigation(void)
 
 static void __init md_clear_select_mitigation(void)
 {
-	taa_select_mitigation();
 	mmio_select_mitigation();
 	rfds_select_mitigation();
 
-- 
2.34.1

Re: [PATCH v5 02/16] x86/bugs: Restructure TAA mitigation

Posted by Borislav Petkov 7 months, 4 weeks ago

On Fri, Apr 18, 2025 at 11:17:07AM -0500, David Kaplan wrote:
> @@ -394,6 +399,11 @@ static const char * const taa_strings[] = {
>  	[TAA_MITIGATION_TSX_DISABLED]	= "Mitigation: TSX disabled",
>  };
>  
> +static bool __init taa_vulnerable(void)
> +{
> +	return boot_cpu_has_bug(X86_BUG_TAA) && boot_cpu_has(X86_FEATURE_RTM);
> +}
> +
>  static void __init taa_select_mitigation(void)
>  {
>  	if (!boot_cpu_has_bug(X86_BUG_TAA)) {

Shouldn't you use !taa_vulnerable() here directly too, since we're introducing
it as a helper?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

[tip: x86/bugs] x86/bugs: Restructure TAA mitigation

Posted by tip-bot2 for David Kaplan 7 months, 2 weeks ago

The following commit has been merged into the x86/bugs branch of tip:

Commit-ID:     bdd7fce7a8168cebd400926d6352d2fbc1ac9f79
Gitweb:        https://git.kernel.org/tip/bdd7fce7a8168cebd400926d6352d2fbc1ac9f79
Author:        David Kaplan <david.kaplan@amd.com>
AuthorDate:    Fri, 18 Apr 2025 11:17:07 -05:00
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Mon, 28 Apr 2025 13:02:04 +02:00

x86/bugs: Restructure TAA mitigation

Restructure TAA mitigation to use select/update/apply functions to
create consistent vulnerability handling.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/20250418161721.1855190-3-david.kaplan@amd.com
---
 arch/x86/kernel/cpu/bugs.c | 94 +++++++++++++++++++++++--------------
 1 file changed, 59 insertions(+), 35 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index f697e6b..5db21d2 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -65,6 +65,8 @@ static void __init mds_apply_mitigation(void);
 static void __init md_clear_update_mitigation(void);
 static void __init md_clear_select_mitigation(void);
 static void __init taa_select_mitigation(void);
+static void __init taa_update_mitigation(void);
+static void __init taa_apply_mitigation(void);
 static void __init mmio_select_mitigation(void);
 static void __init srbds_select_mitigation(void);
 static void __init l1d_flush_select_mitigation(void);
@@ -194,6 +196,7 @@ void __init cpu_select_mitigations(void)
 	ssb_select_mitigation();
 	l1tf_select_mitigation();
 	mds_select_mitigation();
+	taa_select_mitigation();
 	md_clear_select_mitigation();
 	srbds_select_mitigation();
 	l1d_flush_select_mitigation();
@@ -210,8 +213,10 @@ void __init cpu_select_mitigations(void)
 	 * choices.
 	 */
 	mds_update_mitigation();
+	taa_update_mitigation();
 
 	mds_apply_mitigation();
+	taa_apply_mitigation();
 }
 
 /*
@@ -397,6 +402,11 @@ static const char * const taa_strings[] = {
 	[TAA_MITIGATION_TSX_DISABLED]	= "Mitigation: TSX disabled",
 };
 
+static bool __init taa_vulnerable(void)
+{
+	return boot_cpu_has_bug(X86_BUG_TAA) && boot_cpu_has(X86_FEATURE_RTM);
+}
+
 static void __init taa_select_mitigation(void)
 {
 	if (!boot_cpu_has_bug(X86_BUG_TAA)) {
@@ -410,48 +420,63 @@ static void __init taa_select_mitigation(void)
 		return;
 	}
 
-	if (cpu_mitigations_off()) {
+	if (cpu_mitigations_off())
 		taa_mitigation = TAA_MITIGATION_OFF;
-		return;
-	}
 
-	/*
-	 * TAA mitigation via VERW is turned off if both
-	 * tsx_async_abort=off and mds=off are specified.
-	 */
-	if (taa_mitigation == TAA_MITIGATION_OFF &&
-	    mds_mitigation == MDS_MITIGATION_OFF)
+	/* Microcode will be checked in taa_update_mitigation(). */
+	if (taa_mitigation == TAA_MITIGATION_AUTO)
+		taa_mitigation = TAA_MITIGATION_VERW;
+
+	if (taa_mitigation != TAA_MITIGATION_OFF)
+		verw_clear_cpu_buf_mitigation_selected = true;
+}
+
+static void __init taa_update_mitigation(void)
+{
+	if (!taa_vulnerable() || cpu_mitigations_off())
 		return;
 
-	if (boot_cpu_has(X86_FEATURE_MD_CLEAR))
+	if (verw_clear_cpu_buf_mitigation_selected)
 		taa_mitigation = TAA_MITIGATION_VERW;
-	else
-		taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;
 
-	/*
-	 * VERW doesn't clear the CPU buffers when MD_CLEAR=1 and MDS_NO=1.
-	 * A microcode update fixes this behavior to clear CPU buffers. It also
-	 * adds support for MSR_IA32_TSX_CTRL which is enumerated by the
-	 * ARCH_CAP_TSX_CTRL_MSR bit.
-	 *
-	 * On MDS_NO=1 CPUs if ARCH_CAP_TSX_CTRL_MSR is not set, microcode
-	 * update is required.
-	 */
-	if ( (x86_arch_cap_msr & ARCH_CAP_MDS_NO) &&
-	    !(x86_arch_cap_msr & ARCH_CAP_TSX_CTRL_MSR))
-		taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;
+	if (taa_mitigation == TAA_MITIGATION_VERW) {
+		/* Check if the requisite ucode is available. */
+		if (!boot_cpu_has(X86_FEATURE_MD_CLEAR))
+			taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;
 
-	/*
-	 * TSX is enabled, select alternate mitigation for TAA which is
-	 * the same as MDS. Enable MDS static branch to clear CPU buffers.
-	 *
-	 * For guests that can't determine whether the correct microcode is
-	 * present on host, enable the mitigation for UCODE_NEEDED as well.
-	 */
-	setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+		/*
+		 * VERW doesn't clear the CPU buffers when MD_CLEAR=1 and MDS_NO=1.
+		 * A microcode update fixes this behavior to clear CPU buffers. It also
+		 * adds support for MSR_IA32_TSX_CTRL which is enumerated by the
+		 * ARCH_CAP_TSX_CTRL_MSR bit.
+		 *
+		 * On MDS_NO=1 CPUs if ARCH_CAP_TSX_CTRL_MSR is not set, microcode
+		 * update is required.
+		 */
+		if ((x86_arch_cap_msr & ARCH_CAP_MDS_NO) &&
+		   !(x86_arch_cap_msr & ARCH_CAP_TSX_CTRL_MSR))
+			taa_mitigation = TAA_MITIGATION_UCODE_NEEDED;
+	}
 
-	if (taa_nosmt || cpu_mitigations_auto_nosmt())
-		cpu_smt_disable(false);
+	pr_info("%s\n", taa_strings[taa_mitigation]);
+}
+
+static void __init taa_apply_mitigation(void)
+{
+	if (taa_mitigation == TAA_MITIGATION_VERW ||
+	    taa_mitigation == TAA_MITIGATION_UCODE_NEEDED) {
+		/*
+		 * TSX is enabled, select alternate mitigation for TAA which is
+		 * the same as MDS. Enable MDS static branch to clear CPU buffers.
+		 *
+		 * For guests that can't determine whether the correct microcode is
+		 * present on host, enable the mitigation for UCODE_NEEDED as well.
+		 */
+		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+
+		if (taa_nosmt || cpu_mitigations_auto_nosmt())
+			cpu_smt_disable(false);
+	}
 }
 
 static int __init tsx_async_abort_parse_cmdline(char *str)
@@ -660,7 +685,6 @@ out:
 
 static void __init md_clear_select_mitigation(void)
 {
-	taa_select_mitigation();
 	mmio_select_mitigation();
 	rfds_select_mitigation();