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: Mark Rutland's 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
......@@ -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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