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Commit dc55daff authored by Mark Rutland's avatar Mark Rutland Committed by Linus Torvalds
Browse files

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: default avatarMark Rutland <mark.rutland@arm.com>
Acked-by: default avatarAndrey 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus 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;