x86/bugs: Keep a per-CPU IA32_SPEC_CTRL value

Due to TIF_SSBD and TIF_SPEC_IB the actual IA32_SPEC_CTRL value can
differ from x86_spec_ctrl_base. As such, keep a per-CPU value
reflecting the current task's MSR content.

  [jpoimboe: rename]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 5ca60ae..bac243d 100644 (file)
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -253,6 +253,7 @@ static inline void indirect_branch_prediction_barrier(void)
 
 /* The Intel SPEC CTRL MSR base value cache */
 extern u64 x86_spec_ctrl_base;
+extern void write_spec_ctrl_current(u64 val);
 
 /*
  * With retpoline, we must use IBRS to restrict branch prediction

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index fb249b2..ce74355 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -49,12 +49,30 @@ static void __init mmio_select_mitigation(void);
 static void __init srbds_select_mitigation(void);
 static void __init l1d_flush_select_mitigation(void);
 
-/* The base value of the SPEC_CTRL MSR that always has to be preserved. */
+/* The base value of the SPEC_CTRL MSR without task-specific bits set */
 u64 x86_spec_ctrl_base;
 EXPORT_SYMBOL_GPL(x86_spec_ctrl_base);
+
+/* The current value of the SPEC_CTRL MSR with task-specific bits set */
+DEFINE_PER_CPU(u64, x86_spec_ctrl_current);
+EXPORT_SYMBOL_GPL(x86_spec_ctrl_current);
+
 static DEFINE_MUTEX(spec_ctrl_mutex);
 
 /*
+ * Keep track of the SPEC_CTRL MSR value for the current task, which may differ
+ * from x86_spec_ctrl_base due to STIBP/SSB in __speculation_ctrl_update().
+ */
+void write_spec_ctrl_current(u64 val)
+{
+       if (this_cpu_read(x86_spec_ctrl_current) == val)
+               return;
+
+       this_cpu_write(x86_spec_ctrl_current, val);
+       wrmsrl(MSR_IA32_SPEC_CTRL, val);
+}
+
+/*
  * The vendor and possibly platform specific bits which can be modified in
  * x86_spec_ctrl_base.
  */
@@ -1279,7 +1297,7 @@ static void __init spectre_v2_select_mitigation(void)
        if (spectre_v2_in_eibrs_mode(mode)) {
                /* Force it so VMEXIT will restore correctly */
                x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
-               wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+               write_spec_ctrl_current(x86_spec_ctrl_base);
        }
 
        switch (mode) {
@@ -1334,7 +1352,7 @@ static void __init spectre_v2_select_mitigation(void)
 
 static void update_stibp_msr(void * __unused)
 {
-       wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+       write_spec_ctrl_current(x86_spec_ctrl_base);
 }
 
 /* Update x86_spec_ctrl_base in case SMT state changed. */
@@ -1577,7 +1595,7 @@ static enum ssb_mitigation __init __ssb_select_mitigation(void)
                        x86_amd_ssb_disable();
                } else {
                        x86_spec_ctrl_base |= SPEC_CTRL_SSBD;
-                       wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+                       write_spec_ctrl_current(x86_spec_ctrl_base);
                }
        }
 
@@ -1828,7 +1846,7 @@ int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)
 void x86_spec_ctrl_setup_ap(void)
 {
        if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
-               wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
+               write_spec_ctrl_current(x86_spec_ctrl_base);
 
        if (ssb_mode == SPEC_STORE_BYPASS_DISABLE)
                x86_amd_ssb_disable();

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 9b2772b..05611f6 100644 (file)
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -600,7 +600,7 @@ static __always_inline void __speculation_ctrl_update(unsigned long tifp,
        }
 
        if (updmsr)
-               wrmsrl(MSR_IA32_SPEC_CTRL, msr);
+               write_spec_ctrl_current(msr);
 }
 
 static unsigned long speculation_ctrl_update_tif(struct task_struct *tsk)