[PATCH v1 6/6] KVM: s390: Fix potential race in dat_cond_set_storage_key()

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

When dat_cond_set_storage_key() finds a large page, it will
conditionally set the storage key in absolute memory using
large_crste_to_phys() to get the absolute address.

There is a race window between dat_entry_walk() and
large_crste_to_phys(): the large page could have been split
concurrently, and large_crste_to_phys() might be called with a crste
that does not designate a large page, leading to crashes.

Fix by performing a READ_ONCE on the crste pointer, checking and using
the result, instead of dereferencing the pointer again. In case a race
is detacted, try dat_entry_walk() again.

Fixes: 8e03e8316eb2 ("KVM: s390: KVM page table management functions: storage keys")
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 arch/s390/kvm/dat.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kvm/dat.c b/arch/s390/kvm/dat.c
index 7e5dd5a1eb1e..b467b95689b2 100644
--- a/arch/s390/kvm/dat.c
+++ b/arch/s390/kvm/dat.c
@@ -713,10 +713,12 @@ int dat_cond_set_storage_key(struct kvm_s390_mmu_cache *mmc, union asce asce, gf
 {
 	union pgste pgste, old;
 	union crste *crstep;
+	union crste crste;
 	union skey prev;
 	union pte *ptep;
 	int rc;
 
+again:
 	rc = dat_entry_walk(mmc, gfn, asce, DAT_WALK_LEAF_ALLOC, TABLE_TYPE_PAGE_TABLE,
 			    &crstep, &ptep);
 	if (rc)
@@ -725,7 +727,11 @@ int dat_cond_set_storage_key(struct kvm_s390_mmu_cache *mmc, union asce asce, gf
 	if (!ptep) {
 		if (!oldkey)
 			oldkey = &prev;
-		return page_cond_set_storage_key(large_crste_to_phys(*crstep, gfn), skey, oldkey,
+		crste = READ_ONCE(*crstep);
+		/* A large page has been split concurrently, try again */
+		if (!crste_leaf(crste))
+			goto again;
+		return page_cond_set_storage_key(large_crste_to_phys(crste, gfn), skey, oldkey,
 						 nq, mr, mc);
 	}
 
-- 
2.55.0