[PATCH v2 6/6] KVM: s390: Fix potential races in dat skey functions

[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.

Similar issues were also present in dat_set_storage_key().

dat_get_storage_key() and dat_reset_reference_bit() did instead check
for a potential concurrent splitting of the large page, but then
handled it incorrectly.

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 | 36 ++++++++++++++++++++++++++++--------
 1 file changed, 28 insertions(+), 8 deletions(-)

diff --git a/arch/s390/kvm/dat.c b/arch/s390/kvm/dat.c
index 7e5dd5a1eb1e..b4c318ebd91e 100644
--- a/arch/s390/kvm/dat.c
+++ b/arch/s390/kvm/dat.c
@@ -620,17 +620,20 @@ int dat_get_storage_key(union asce asce, gfn_t gfn, union skey *skey)
 	union pte *ptep;
 	int rc;
 
+again:
 	skey->skey = 0;
 	rc = dat_entry_walk(NULL, gfn, asce, DAT_WALK_ANY, TABLE_TYPE_PAGE_TABLE, &crstep, &ptep);
 	if (rc)
 		return rc;
 
 	if (!ptep) {
-		union crste crste;
+		union crste crste = READ_ONCE(*crstep);
 
-		crste = READ_ONCE(*crstep);
-		if (!crste.h.fc || !crste.s.fc1.pr)
+		if (!crste_leaf(crste))
+			goto again;
+		if (!crste.s.fc1.pr)
 			return 0;
+
 		skey->skey = page_get_storage_key(large_crste_to_phys(crste, gfn));
 		return 0;
 	}
@@ -661,13 +664,20 @@ int dat_set_storage_key(struct kvm_s390_mmu_cache *mc, union asce asce, gfn_t gf
 	union pte *ptep;
 	int rc;
 
+again:
 	rc = dat_entry_walk(mc, gfn, asce, DAT_WALK_LEAF_ALLOC, TABLE_TYPE_PAGE_TABLE,
 			    &crstep, &ptep);
 	if (rc)
 		return rc;
 
 	if (!ptep) {
-		page_set_storage_key(large_crste_to_phys(*crstep, gfn), skey.skey, !nq);
+		union crste crste = READ_ONCE(*crstep);
+
+		/* A large page has been split concurrently, try again */
+		if (!crste_leaf(crste))
+			goto again;
+
+		page_set_storage_key(large_crste_to_phys(crste, gfn), skey.skey, !nq);
 		return 0;
 	}
 
@@ -717,15 +727,22 @@ int dat_cond_set_storage_key(struct kvm_s390_mmu_cache *mmc, union asce asce, gf
 	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)
 		return rc;
 
 	if (!ptep) {
+		union crste crste = READ_ONCE(*crstep);
+
+		/* A large page has been split concurrently, try again */
+		if (!crste_leaf(crste))
+			goto again;
 		if (!oldkey)
 			oldkey = &prev;
-		return page_cond_set_storage_key(large_crste_to_phys(*crstep, gfn), skey, oldkey,
+
+		return page_cond_set_storage_key(large_crste_to_phys(crste, gfn), skey, oldkey,
 						 nq, mr, mc);
 	}
 
@@ -767,7 +784,7 @@ int dat_reset_reference_bit(union asce asce, gfn_t gfn, union skey *skey)
 	int rc;
 
 	skey->skey = 0;
-
+again:
 	rc = dat_entry_walk(NULL, gfn, asce, DAT_WALK_ANY, TABLE_TYPE_PAGE_TABLE, &crstep, &ptep);
 	if (rc)
 		return rc;
@@ -775,9 +792,12 @@ int dat_reset_reference_bit(union asce asce, gfn_t gfn, union skey *skey)
 	if (!ptep) {
 		union crste crste = READ_ONCE(*crstep);
 
-		if (!crste.h.fc || !crste.s.fc1.pr)
+		/* A large page has been split concurrently, try again */
+		if (!crste_leaf(crste))
+			goto again;
+		if (!crste.s.fc1.pr)
 			return 0;
-		skey->skey = page_reset_referenced(large_crste_to_phys(*crstep, gfn)) << 1;
+		skey->skey = page_reset_referenced(large_crste_to_phys(crste, gfn)) << 1;
 		return 0;
 	}
 	old = pgste_get_lock(ptep);
-- 
2.55.0