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Subject[PATCH 2/7] x86/mm/tlb: Restructure switch_mm_irqs_off()
Date
Move some code that will be needed for the lazy -> !lazy state
transition when a lazy TLB CPU has gotten out of date.

No functional changes, since the if (real_prev == next) branch
always returns.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Rik van Riel <riel@surriel.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: efault@gmx.de
Cc: kernel-team@fb.com
Link: http://lkml.kernel.org/r/20180716190337.26133-4-riel@surriel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit 61d0beb5796ab11f7f3bf38cb2eccc6579aaa70b)
---
arch/x86/mm/tlb.c | 87 +++++++++++++++++++++++++++++------------------
1 file changed, 54 insertions(+), 33 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index d19f424073d9..ac05d61cc90e 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -158,6 +158,8 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
u16 prev_asid = this_cpu_read(cpu_tlbstate.loaded_mm_asid);
unsigned cpu = smp_processor_id();
u64 next_tlb_gen;
+ bool need_flush;
+ u16 new_asid;

/*
* NB: The scheduler will call us with prev == next when switching
@@ -217,8 +219,27 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,

return;
} else {
- u16 new_asid;
- bool need_flush;
+ u64 last_ctx_id = this_cpu_read(cpu_tlbstate.last_ctx_id);
+
+ /*
+ * Avoid user/user BTB poisoning by flushing the branch
+ * predictor when switching between processes. This stops
+ * one process from doing Spectre-v2 attacks on another.
+ *
+ * As an optimization, flush indirect branches only when
+ * switching into processes that disable dumping. This
+ * protects high value processes like gpg, without having
+ * too high performance overhead. IBPB is *expensive*!
+ *
+ * This will not flush branches when switching into kernel
+ * threads. It will also not flush if we switch to idle
+ * thread and back to the same process. It will flush if we
+ * switch to a different non-dumpable process.
+ */
+ if (tsk && tsk->mm &&
+ tsk->mm->context.ctx_id != last_ctx_id &&
+ get_dumpable(tsk->mm) != SUID_DUMP_USER)
+ indirect_branch_prediction_barrier();

if (IS_ENABLED(CONFIG_VMAP_STACK)) {
/*
@@ -249,44 +270,44 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
/* Let nmi_uaccess_okay() know that we're changing CR3. */
this_cpu_write(cpu_tlbstate.loaded_mm, LOADED_MM_SWITCHING);
barrier();
+ }

- if (need_flush) {
- this_cpu_write(cpu_tlbstate.ctxs[new_asid].ctx_id, next->context.ctx_id);
- this_cpu_write(cpu_tlbstate.ctxs[new_asid].tlb_gen, next_tlb_gen);
- load_new_mm_cr3(next->pgd, new_asid, true);
-
- /*
- * NB: This gets called via leave_mm() in the idle path
- * where RCU functions differently. Tracing normally
- * uses RCU, so we need to use the _rcuidle variant.
- *
- * (There is no good reason for this. The idle code should
- * be rearranged to call this before rcu_idle_enter().)
- */
- trace_tlb_flush_rcuidle(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
- } else {
- /* The new ASID is already up to date. */
- load_new_mm_cr3(next->pgd, new_asid, false);
-
- /* See above wrt _rcuidle. */
- trace_tlb_flush_rcuidle(TLB_FLUSH_ON_TASK_SWITCH, 0);
- }
+ if (need_flush) {
+ this_cpu_write(cpu_tlbstate.ctxs[new_asid].ctx_id, next->context.ctx_id);
+ this_cpu_write(cpu_tlbstate.ctxs[new_asid].tlb_gen, next_tlb_gen);
+ load_new_mm_cr3(next->pgd, new_asid, true);

/*
- * Record last user mm's context id, so we can avoid
- * flushing branch buffer with IBPB if we switch back
- * to the same user.
+ * NB: This gets called via leave_mm() in the idle path
+ * where RCU functions differently. Tracing normally
+ * uses RCU, so we need to use the _rcuidle variant.
+ *
+ * (There is no good reason for this. The idle code should
+ * be rearranged to call this before rcu_idle_enter().)
*/
- if (next != &init_mm)
- this_cpu_write(cpu_tlbstate.last_ctx_id, next->context.ctx_id);
-
- /* Make sure we write CR3 before loaded_mm. */
- barrier();
+ trace_tlb_flush_rcuidle(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
+ } else {
+ /* The new ASID is already up to date. */
+ load_new_mm_cr3(next->pgd, new_asid, false);

- this_cpu_write(cpu_tlbstate.loaded_mm, next);
- this_cpu_write(cpu_tlbstate.loaded_mm_asid, new_asid);
+ /* See above wrt _rcuidle. */
+ trace_tlb_flush_rcuidle(TLB_FLUSH_ON_TASK_SWITCH, 0);
}

+ /*
+ * Record last user mm's context id, so we can avoid
+ * flushing branch buffer with IBPB if we switch back
+ * to the same user.
+ */
+ if (next != &init_mm)
+ this_cpu_write(cpu_tlbstate.last_ctx_id, next->context.ctx_id);
+
+ /* Make sure we write CR3 before loaded_mm. */
+ barrier();
+
+ this_cpu_write(cpu_tlbstate.loaded_mm, next);
+ this_cpu_write(cpu_tlbstate.loaded_mm_asid, new_asid);
+
load_mm_cr4(next);
switch_ldt(real_prev, next);
}
--
2.17.1
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