x86/mm: Prepare for opt-in based L1D flush in switch_mm()

The goal of this is to allow tasks that want to protect sensitive
information, against e.g. the recently found snoop assisted data sampling
vulnerabilites, to flush their L1D on being switched out.  This protects
their data from being snooped or leaked via side channels after the task
has context switched out.

This could also be used to wipe L1D when an untrusted task is switched in,
but that's not a really well defined scenario while the opt-in variant is
clearly defined.

The mechanism is default disabled and can be enabled on the kernel command
line.

Prepare for the actual prctl based opt-in:

  1) Provide the necessary setup functionality similar to the other
     mitigations and enable the static branch when the command line option
     is set and the CPU provides support for hardware assisted L1D
     flushing. Software based L1D flush is not supported because it's CPU
     model specific and not really well defined.

     This does not come with a sysfs file like the other mitigations
     because it is not bound to any specific vulnerability.

     Support has to be queried via the prctl(2) interface.

  2) Add TIF_SPEC_L1D_FLUSH next to L1D_SPEC_IB so the two bits can be
     mangled into the mm pointer in one go which allows to reuse the
     existing mechanism in switch_mm() for the conditional IBPB speculation
     barrier efficiently.

  3) Add the L1D flush specific functionality which flushes L1D when the
     outgoing task opted in.

     Also check whether the incoming task has requested L1D flush and if so
     validate that it is not accidentaly running on an SMT sibling as this
     makes the whole excercise moot because SMT siblings share L1D which
     opens tons of other attack vectors. If that happens schedule task work
     which signals the incoming task on return to user/guest with SIGBUS as
     this is part of the paranoid L1D flush contract.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Balbir Singh <sblbir@amazon.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210108121056.21940-1-sblbir@amazon.com

1 files changed, 56 insertions, 2 deletions

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index c98bc84a82f6..59ba2968af1b 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -8,11 +8,13 @@
 #include <linux/export.h>
 #include <linux/cpu.h>
 #include <linux/debugfs.h>
+#include <linux/sched/smt.h>
 
 #include <asm/tlbflush.h>
 #include <asm/mmu_context.h>
 #include <asm/nospec-branch.h>
 #include <asm/cache.h>
+#include <asm/cacheflush.h>
 #include <asm/apic.h>
 #include <asm/perf_event.h>
 
@@ -43,11 +45,12 @@
  */
 
 /*
- * Bits to mangle the TIF_SPEC_IB state into the mm pointer which is
+ * Bits to mangle the TIF_SPEC_* state into the mm pointer which is
  * stored in cpu_tlb_state.last_user_mm_spec.
  */
 #define LAST_USER_MM_IBPB	0x1UL
-#define LAST_USER_MM_SPEC_MASK	(LAST_USER_MM_IBPB)
+#define LAST_USER_MM_L1D_FLUSH	0x2UL
+#define LAST_USER_MM_SPEC_MASK	(LAST_USER_MM_IBPB | LAST_USER_MM_L1D_FLUSH)
 
 /* Bits to set when tlbstate and flush is (re)initialized */
 #define LAST_USER_MM_INIT	LAST_USER_MM_IBPB
@@ -321,11 +324,52 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 	local_irq_restore(flags);
 }
 
+/*
+ * Invoked from return to user/guest by a task that opted-in to L1D
+ * flushing but ended up running on an SMT enabled core due to wrong
+ * affinity settings or CPU hotplug. This is part of the paranoid L1D flush
+ * contract which this task requested.
+ */
+static void l1d_flush_force_sigbus(struct callback_head *ch)
+{
+	force_sig(SIGBUS);
+}
+
+static void l1d_flush_evaluate(unsigned long prev_mm, unsigned long next_mm,
+				struct task_struct *next)
+{
+	/* Flush L1D if the outgoing task requests it */
+	if (prev_mm & LAST_USER_MM_L1D_FLUSH)
+		wrmsrl(MSR_IA32_FLUSH_CMD, L1D_FLUSH);
+
+	/* Check whether the incoming task opted in for L1D flush */
+	if (likely(!(next_mm & LAST_USER_MM_L1D_FLUSH)))
+		return;
+
+	/*
+	 * Validate that it is not running on an SMT sibling as this would
+	 * make the excercise pointless because the siblings share L1D. If
+	 * it runs on a SMT sibling, notify it with SIGBUS on return to
+	 * user/guest
+	 */
+	if (this_cpu_read(cpu_info.smt_active)) {
+		clear_ti_thread_flag(&next->thread_info, TIF_SPEC_L1D_FLUSH);
+		next->l1d_flush_kill.func = l1d_flush_force_sigbus;
+		task_work_add(next, &next->l1d_flush_kill, TWA_RESUME);
+	}
+}
+
 static unsigned long mm_mangle_tif_spec_bits(struct task_struct *next)
 {
 	unsigned long next_tif = task_thread_info(next)->flags;
 	unsigned long spec_bits = (next_tif >> TIF_SPEC_IB) & LAST_USER_MM_SPEC_MASK;
 
+	/*
+	 * Ensure that the bit shift above works as expected and the two flags
+	 * end up in bit 0 and 1.
+	 */
+	BUILD_BUG_ON(TIF_SPEC_L1D_FLUSH != TIF_SPEC_IB + 1);
+
 	return (unsigned long)next->mm | spec_bits;
 }
 
@@ -403,6 +447,16 @@ static void cond_mitigation(struct task_struct *next)
 			indirect_branch_prediction_barrier();
 	}
 
+	if (static_branch_unlikely(&switch_mm_cond_l1d_flush)) {
+		/*
+		 * Flush L1D when the outgoing task requested it and/or
+		 * check whether the incoming task requested L1D flushing
+		 * and ended up on an SMT sibling.
+		 */
+		if (unlikely((prev_mm | next_mm) & LAST_USER_MM_L1D_FLUSH))
+			l1d_flush_evaluate(prev_mm, next_mm, next);
+	}
+
 	this_cpu_write(cpu_tlbstate.last_user_mm_spec, next_mm);
 }