sparc64: Prevent perf from running during super critical sections

This fixes another cause of random segfaults and bus errors that may
occur while running perf with the callgraph option.

Critical sections beginning with spin_lock_irqsave() raise the interrupt
level to PIL_NORMAL_MAX (14) and intentionally do not block performance
counter interrupts, which arrive at PIL_NMI (15).

But some sections of code are "super critical" with respect to perf
because the perf_callchain_user() path accesses user space and may cause
TLB activity as well as faults as it unwinds the user stack.

One particular critical section occurs in switch_mm:

        spin_lock_irqsave(&mm->context.lock, flags);
        ...
        load_secondary_context(mm);
        tsb_context_switch(mm);
        ...
        spin_unlock_irqrestore(&mm->context.lock, flags);

If a perf interrupt arrives in between load_secondary_context() and
tsb_context_switch(), then perf_callchain_user() could execute with
the context ID of one process, but with an active TSB for a different
process. When the user stack is accessed, it is very likely to
incur a TLB miss, since the h/w context ID has been changed. The TLB
will then be reloaded with a translation from the TSB for one process,
but using a context ID for another process. This exposes memory from
one process to another, and since it is a mapping for stack memory,
this usually causes the new process to crash quickly.

This super critical section needs more protection than is provided
by spin_lock_irqsave() since perf interrupts must not be allowed in.

Since __tsb_context_switch already goes through the trouble of
disabling interrupts completely, we fix this by moving the secondary
context load down into this better protected region.

Orabug: 25577560

Signed-off-by: Dave Aldridge <david.j.aldridge@oracle.com>
Signed-off-by: Rob Gardner <rob.gardner@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 5 deletions

diff --git a/arch/sparc/include/asm/mmu_context_64.h b/arch/sparc/include/asm/mmu_context_64.h
index 2cddcda4f85f..87841d687f8d 100644
--- a/arch/sparc/include/asm/mmu_context_64.h
+++ b/arch/sparc/include/asm/mmu_context_64.h
@@ -27,9 +27,11 @@ void destroy_context(struct mm_struct *mm);
 void __tsb_context_switch(unsigned long pgd_pa,
 			  struct tsb_config *tsb_base,
 			  struct tsb_config *tsb_huge,
-			  unsigned long tsb_descr_pa);
+			  unsigned long tsb_descr_pa,
+			  unsigned long secondary_ctx);
 
-static inline void tsb_context_switch(struct mm_struct *mm)
+static inline void tsb_context_switch_ctx(struct mm_struct *mm,
+					  unsigned long ctx)
 {
 	__tsb_context_switch(__pa(mm->pgd),
 			     &mm->context.tsb_block[MM_TSB_BASE],
@@ -40,9 +42,12 @@ static inline void tsb_context_switch(struct mm_struct *mm)
 #else
 			     NULL
 #endif
-			     , __pa(&mm->context.tsb_descr[MM_TSB_BASE]));
+			     , __pa(&mm->context.tsb_descr[MM_TSB_BASE]),
+			     ctx);
 }
 
+#define tsb_context_switch(X) tsb_context_switch_ctx(X, 0)
+
 void tsb_grow(struct mm_struct *mm,
 	      unsigned long tsb_index,
 	      unsigned long mm_rss);
@@ -112,8 +117,7 @@ static inline void switch_mm(struct mm_struct *old_mm, struct mm_struct *mm, str
 	 * cpu0 to update it's TSB because at that point the cpu_vm_mask
 	 * only had cpu1 set in it.
 	 */
-	load_secondary_context(mm);
-	tsb_context_switch(mm);
+	tsb_context_switch_ctx(mm, CTX_HWBITS(mm->context));
 
 	/* Any time a processor runs a context on an address space
 	 * for the first time, we must flush that context out of the