[PATCH 4.9 15/16] sparc64: Prevent perf from running during super critical sections

Greg Kroah-Hartman
2017-08-11T22:02:02Z

From: Rob Gardner <rob.gardner@oracle.com>

commit fc290a114fc6034b0f6a5a46e2fb7d54976cf87a upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/sparc/include/asm/mmu_context_64.h | 14 +++++++++-----
arch/sparc/kernel/tsb.S | 12 ++++++++++++
arch/sparc/power/hibernate.c | 3 +--
3 files changed, 22 insertions(+), 7 deletions(-)

--- a/arch/sparc/include/asm/mmu_context_64.h
+++ b/arch/sparc/include/asm/mmu_context_64.h
@@ -25,9 +25,11 @@ void destroy_context(struct mm_struct *m
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[0],
@@ -38,9 +40,12 @@ static inline void tsb_context_switch(st
#else
NULL
#endif
- , __pa(&mm->context.tsb_descr[0]));
+ , __pa(&mm->context.tsb_descr[0]),
+ 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);
@@ -110,8 +115,7 @@ static inline void switch_mm(struct mm_s
* 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
--- a/arch/sparc/kernel/tsb.S
+++ b/arch/sparc/kernel/tsb.S
@@ -375,6 +375,7 @@ tsb_flush:
* %o1: TSB base config pointer
* %o2: TSB huge config pointer, or NULL if none
* %o3: Hypervisor TSB descriptor physical address
+ * %o4: Secondary context to load, if non-zero
*
* We have to run this whole thing with interrupts
* disabled so that the current cpu doesn't change
@@ -387,6 +388,17 @@ __tsb_context_switch:
rdpr %pstate, %g1
wrpr %g1, PSTATE_IE, %pstate

+ brz,pn %o4, 1f
+ mov SECONDARY_CONTEXT, %o5
+
+661: stxa %o4, [%o5] ASI_DMMU
+ .section .sun4v_1insn_patch, "ax"
+ .word 661b
+ stxa %o4, [%o5] ASI_MMU
+ .previous
+ flush %g6
+
+1:
TRAP_LOAD_TRAP_BLOCK(%g2, %g3)

stx %o0, [%g2 + TRAP_PER_CPU_PGD_PADDR]
--- a/arch/sparc/power/hibernate.c
+++ b/arch/sparc/power/hibernate.c
@@ -35,6 +35,5 @@ void restore_processor_state(void)
{
struct mm_struct *mm = current->active_mm;

- load_secondary_context(mm);
- tsb_context_switch(mm);
+ tsb_context_switch_ctx(mm, CTX_HWBITS(mm->context));
}