x86/speculation: Fix incorrect MDS/TAA mitigation status

For MDS vulnerable processors with TSX support, enabling either MDS or
TAA mitigations will enable the use of VERW to flush internal processor
buffers at the right code path. IOW, they are either both mitigated
or both not. However, if the command line options are inconsistent,
the vulnerabilites sysfs files may not report the mitigation status
correctly.

For example, with only the "mds=off" option:

  vulnerabilities/mds:Vulnerable; SMT vulnerable
  vulnerabilities/tsx_async_abort:Mitigation: Clear CPU buffers; SMT vulnerable

The mds vulnerabilities file has wrong status in this case. Similarly,
the taa vulnerability file will be wrong with mds mitigation on, but
taa off.

Change taa_select_mitigation() to sync up the two mitigation status
and have them turned off if both "mds=off" and "tsx_async_abort=off"
are present.

Update documentation to emphasize the fact that both "mds=off" and
"tsx_async_abort=off" have to be specified together for processors that
are affected by both TAA and MDS to be effective.

 [ bp: Massage and add kernel-parameters.txt change too. ]

Fixes: 1b42f017415b ("x86/speculation/taa: Add mitigation for TSX Async Abort")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: linux-doc@vger.kernel.org
Cc: Mark Gross <mgross@linux.intel.com>
Cc: <stable@vger.kernel.org>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Tyler Hicks <tyhicks@canonical.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20191115161445.30809-2-longman@redhat.com

1 files changed, 15 insertions, 2 deletions

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 4c7b0fa15a19..cb513eaa0df1 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -304,8 +304,12 @@ static void __init taa_select_mitigation(void)
 		return;
 	}
 
-	/* TAA mitigation is turned off on the cmdline (tsx_async_abort=off) */
-	if (taa_mitigation == TAA_MITIGATION_OFF)
+	/*
+	 * 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)
 		goto out;
 
 	if (boot_cpu_has(X86_FEATURE_MD_CLEAR))
@@ -339,6 +343,15 @@ static void __init taa_select_mitigation(void)
 	if (taa_nosmt || cpu_mitigations_auto_nosmt())
 		cpu_smt_disable(false);
 
+	/*
+	 * Update MDS mitigation, if necessary, as the mds_user_clear is
+	 * now enabled for TAA mitigation.
+	 */
+	if (mds_mitigation == MDS_MITIGATION_OFF &&
+	    boot_cpu_has_bug(X86_BUG_MDS)) {
+		mds_mitigation = MDS_MITIGATION_FULL;
+		mds_select_mitigation();
+	}
 out:
 	pr_info("%s\n", taa_strings[taa_mitigation]);
 }