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Subject[PATCH 2/3] kcov: prefault the kcov_area
Date
On many architectures the vmalloc area is lazily faulted in upon first
access. This is problematic for KCOV, as __sanitizer_cov_trace_pc
accesses the (vmalloc'd) kcov_area, and fault handling code may be
instrumented. If an access to kcov_area faults, this will result in
mutual recursion through the fault handling code and
__sanitizer_cov_trace_pc(), eventually leading to stack corruption
and/or overflow.

We can avoid this by faulting in the kcov_area before
__sanitizer_cov_trace_pc() is permitted to access it. Once it has been
faulted in, it will remain present in the process page tables, and will
not fault again.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
---
kernel/kcov.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/kernel/kcov.c b/kernel/kcov.c
index 5be9a60a959f..3b82f8e258da 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -324,6 +324,17 @@ static int kcov_close(struct inode *inode, struct file *filep)
return 0;
}

+static void kcov_fault_in_area(struct kcov *kcov)
+{
+ unsigned long stride = PAGE_SIZE / sizeof(unsigned long);
+ unsigned long *area = kcov->area;
+ unsigned long offset;
+
+ for (offset = 0; offset < kcov->size; offset += stride) {
+ READ_ONCE(area[offset]);
+ }
+}
+
static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
unsigned long arg)
{
@@ -372,6 +383,7 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
#endif
else
return -EINVAL;
+ kcov_fault_in_area(kcov);
/* Cache in task struct for performance. */
t->kcov_size = kcov->size;
t->kcov_area = kcov->area;
--
2.11.0
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