Commit dc55daff authored Jun 14, 2018 by Mark Rutland Committed by Linus Torvalds Jun 15, 2018

kcov: prefault the kcov_area

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.

[akpm@linux-foundation.org: code cleanup]
[akpm@linux-foundation.org: add comment explaining kcov_fault_in_area()]
[akpm@linux-foundation.org: fancier code comment from Mark]
Link: http://lkml.kernel.org/r/20180504135535.53744-3-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>

parent c9484b98

kernel/kcov.c

+16 −0

Original line number	Diff line number	Diff line
		@@ -324,6 +324,21 @@ static int kcov_close(struct inode inode, struct file filep)
		return 0;
		}

		/*
		* Fault in a lazily-faulted vmalloc area before it can be used by
		* __santizer_cov_trace_pc(), to avoid recursion issues if any code on the
		* vmalloc fault handling path is instrumented.
		*/
		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 +387,7 @@ 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;

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