[PATCH v4 2/7] KVM: s390: Fix dat_peek_cmma() overflow

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

If userspace passes a start address that is out of bounds,
_dat_walk_gfn_range() will fail with -EFAULT, but state.end will not be
touched and will stay 0. This will cause *count to underflow and report
a very high number, and the function will end up erroneously reporting
success.

Fix by only setting *count if the end address is not smaller than the
starting address. This way invalid starting addresses will correctly
return -EFAULT and *count will correctly indicate that no values have
been returned.

Fixes: 7b368470e1a4 ("KVM: s390: KVM page table management functions: CMMA")
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 arch/s390/kvm/dat.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/kvm/dat.c b/arch/s390/kvm/dat.c
index 4a41c0247ffa..cffac7782c4b 100644
--- a/arch/s390/kvm/dat.c
+++ b/arch/s390/kvm/dat.c
@@ -1209,7 +1209,7 @@ int dat_peek_cmma(gfn_t start, union asce asce, unsigned int *count, u8 *values)
 	int rc;
 
 	rc = _dat_walk_gfn_range(start, start + *count, asce, &ops, DAT_WALK_DEFAULT, &state);
-	*count = state.end - start;
+	*count = state.end >= start ? state.end - start : 0;
 	/* Return success if at least one value was saved, otherwise an error. */
 	return (rc == -EFAULT && *count > 0) ? 0 : rc;
 }
-- 
2.54.0