[PATCH v2] KVM: SEV: drop FOLL_WRITE for encrypted region registration

[Pankaj Gupta]

Commit 7e066cb9b71a ("KVM: SEV: Use long-term pin when registering
encrypted memory regions") added FOLL_LONGTERM to
sev_mem_enc_register_region() so anonymous guest RAM is migrated out of
MIGRATE_CMA/ZONE_MOVABLE before a long term pin. It also kept
FOLL_WRITE on the pin.

Combining FOLL_WRITE with FOLL_LONGTERM breaks registration of file-backed
guest memory, such as virtio-pmem host memory-backend-file mappings
(MAP_SHARED). GUP rejects long-term writable pins on dirty tracked file
mappings since:

commit 8ac268436e6d ("mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings")
commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings").

Region registration only requires long-term pin to prevent page migration and
does not write through this GUP pin.

Drop FOLL_WRITE and pin guest memory only with FOLL_LONGTERM.

Fixes: 7e066cb9b71a ("KVM: SEV: Use long-term pin when registering encrypted memory regions")
Cc: stable@xxxxxxxxxxxxxxx
Suggested-by: "David Hildenbrand (Arm)" <david@xxxxxxxxxx>
Link: https://lore.kernel.org/all/ad784f05-b36c-4e91-9f17-4c5b826735d0@xxxxxxxxxx/
Signed-off-by: Pankaj Gupta <pankaj.gupta@xxxxxxx>
---
v1 -> v2
- Remove FOLL_WRITE when the pin is not used for host writes

v1: https://lore.kernel.org/all/20260701144543.39582-1-pankaj.gupta@xxxxxxx/

 arch/x86/kvm/svm/sev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 427229347876..5f2998761462 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2752,7 +2752,7 @@ int sev_mem_enc_register_region(struct kvm *kvm,
 		return -ENOMEM;
 
 	region->pages = sev_pin_memory(kvm, range->addr, range->size, &region->npages,
-				       FOLL_WRITE | FOLL_LONGTERM);
+				       FOLL_LONGTERM);
 	if (IS_ERR(region->pages)) {
 		ret = PTR_ERR(region->pages);
 		goto e_free;
-- 
2.34.1