x86/speculation: Fix incorrect MDS/TAA mitigation status

commit 64870ed1b12e235cfca3f6c6da75b542c973ff78 upstream.

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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/admin-guide/hw-vuln/mds.rst b/Documentation/admin-guide/hw-vuln/mds.rst
index e3a796c0d3a24d5e7e91b694103e47ae3a48ecbb..2d19c9f4c1fec13752884021be630bd0423708c5 100644 (file)
--- a/Documentation/admin-guide/hw-vuln/mds.rst
+++ b/Documentation/admin-guide/hw-vuln/mds.rst
@@ -265,8 +265,11 @@ time with the option "mds=". The valid arguments for this option are:
 
   ============  =============================================================
 
-Not specifying this option is equivalent to "mds=full".
-
+Not specifying this option is equivalent to "mds=full". For processors
+that are affected by both TAA (TSX Asynchronous Abort) and MDS,
+specifying just "mds=off" without an accompanying "tsx_async_abort=off"
+will have no effect as the same mitigation is used for both
+vulnerabilities.
 
 Mitigation selection guide
 --------------------------

diff --git a/Documentation/admin-guide/hw-vuln/tsx_async_abort.rst b/Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
index fddbd7579c5351368a118a7cfb95cccc3c8c9a00..af6865b822d21d18946016c1458a4dc3567050e9 100644 (file)
--- a/Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
+++ b/Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
@@ -174,7 +174,10 @@ the option "tsx_async_abort=". The valid arguments for this option are:
                 CPU is not vulnerable to cross-thread TAA attacks.
   ============  =============================================================
 
-Not specifying this option is equivalent to "tsx_async_abort=full".
+Not specifying this option is equivalent to "tsx_async_abort=full". For
+processors that are affected by both TAA and MDS, specifying just
+"tsx_async_abort=off" without an accompanying "mds=off" will have no
+effect as the same mitigation is used for both vulnerabilities.
 
 The kernel command line also allows to control the TSX feature using the
 parameter "tsx=" on CPUs which support TSX control. MSR_IA32_TSX_CTRL is used

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 8dee8f68fe15cfd99278ac91ed8775462b828060..9983ac73b66df31302016c72a145e8f397881edb 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2473,6 +2473,12 @@
                                     SMT on vulnerable CPUs
                        off        - Unconditionally disable MDS mitigation
 
+                       On TAA-affected machines, mds=off can be prevented by
+                       an active TAA mitigation as both vulnerabilities are
+                       mitigated with the same mechanism so in order to disable
+                       this mitigation, you need to specify tsx_async_abort=off
+                       too.
+
                        Not specifying this option is equivalent to
                        mds=full.
 
@@ -4931,6 +4937,11 @@
                                     vulnerable to cross-thread TAA attacks.
                        off        - Unconditionally disable TAA mitigation
 
+                       On MDS-affected machines, tsx_async_abort=off can be
+                       prevented by an active MDS mitigation as both vulnerabilities
+                       are mitigated with the same mechanism so in order to disable
+                       this mitigation, you need to specify mds=off too.
+
                        Not specifying this option is equivalent to
                        tsx_async_abort=full.  On CPUs which are MDS affected
                        and deploy MDS mitigation, TAA mitigation is not

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 4c7b0fa15a19403019f7366811655a5174dfc812..cb513eaa0df1838d592a8a1a3a50fb2ffc3a4aed 100644 (file)
--- 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]);
 }