commit fc290a11 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:
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.
Signed-off-by: Dave Aldridge <firstname.lastname@example.org>
Signed-off-by: Rob Gardner <email@example.com>
Signed-off-by: David S. Miller <firstname.lastname@example.org>
Signed-off-by: Greg Kroah-Hartman <email@example.com>