4.14-stable review patch. If anyone has any objections, please let me know.
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From: Mark Rutland mark.rutland@arm.com
[ Upstream commit c9484b986ef03492357fddd50afbdd02929cfa72 ]
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 t->kcov_area.
Link: http://lkml.kernel.org/r/20180504135535.53744-2-mark.rutland@arm.com Signed-off-by: Mark Rutland mark.rutland@arm.com Acked-by: Andrey Ryabinin aryabinin@virtuozzo.com Cc: Dmitry Vyukov dvyukov@google.com Cc: Ingo Molnar mingo@redhat.com Cc: Peter Zijlstra peterz@infradead.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/kcov.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
--- a/kernel/kcov.c +++ b/kernel/kcov.c @@ -108,7 +108,8 @@ static void kcov_put(struct kcov *kcov)
void kcov_task_init(struct task_struct *t) { - t->kcov_mode = KCOV_MODE_DISABLED; + WRITE_ONCE(t->kcov_mode, KCOV_MODE_DISABLED); + barrier(); t->kcov_size = 0; t->kcov_area = NULL; t->kcov = NULL;