Commit c9484b98 authored by Mark Rutland's avatar Mark Rutland Committed by Linus Torvalds
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kcov: ensure irq code sees a valid area

Patch series "kcov: fix unexpected faults".

These patches fix a few issues where KCOV code could trigger recursive
faults, discovered while debugging a patch enabling KCOV for arch/arm:

* On CONFIG_PREEMPT kernels, there's a small race window where
  __sanitizer_cov_trace_pc() can see a bogus kcov_area.

* Lazy faulting of the vmalloc area can cause mutual recursion between
  fault handling code and __sanitizer_cov_trace_pc().

* During the context switch, switching the mm can cause the kcov_area to
  be transiently unmapped.

These are prerequisites for enabling KCOV on arm, but the issues
themsevles are generic -- we just happen to avoid them by chance rather
than design on x86-64 and arm64.

This patch (of 3):

For kernels built with CONFIG_PREEMPT, some C code may execute before or
after the interrupt handler, while the hardirq count is zero.  In these
cases, in_task() can return true.

A task can be interrupted in the middle of a KCOV_DISABLE ioctl while it
resets the task's kcov data via kcov_task_init().  Instrumented code
executed during this period will call __sanitizer_cov_trace_pc(), and as
in_task() returns true, will inspect t->kcov_mode before trying to write
to t->kcov_area.

In kcov_init_task() we update t->kcov_{mode,area,size} with plain stores,
which may be re-ordered, torn, etc.  Thus __sanitizer_cov_trace_pc() may
see bogus values for any of these fields, and may attempt to write to
memory which is not mapped.

Let's avoid this by using WRITE_ONCE() to set t->kcov_mode, with a
barrier() to ensure this is ordered before we clear t->kov_{area,size}.
This ensures that any code execute while kcov_init_task() is preempted
will either see valid values for t->kcov_{area,size}, or will see that
t->kcov_mode is KCOV_MODE_DISABLED, and bail out without touching


Signed-off-by: Mark Rutland's avatarMark Rutland <>
Acked-by: default avatarAndrey Ryabinin <>
Cc: Dmitry Vyukov <>
Cc: Ingo Molnar <>
Cc: Peter Zijlstra <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 3fb3894b
......@@ -241,7 +241,8 @@ static void kcov_put(struct kcov *kcov)
void kcov_task_init(struct task_struct *t)
t->kcov_mode = KCOV_MODE_DISABLED;
t->kcov_size = 0;
t->kcov_area = NULL;
t->kcov = NULL;
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