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: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Jon Masters <jcm@redhat.com>
Tested-by: Jon Masters <jcm@redhat.com>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 858b6c0b9a15ae71853645013c718dc0398804d4..dddb024eb52326fd0b40a6171398739e5c3a97a3 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -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.

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 33051436c86459131a23005b67ecfdaa952cc123..1f0295783325c6873991ef423d99180d15339c55 100644 (file)
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -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 */

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9169951673010b3e086b6b18d4954110d69db933..c7b29d200d27fc163bc7a86988033beca13e028e 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -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);
 }