[PATCH v2 1/5] KVM: guest_memfd: Use write permissions when GUP-ing source pages

[Ackerley Tng via B4 Relay]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

sev_gmem_post_populate() may write to the source page if there was an error
while performing SNP_LAUNCH_UPDATE.

Since GUP requested only reads, there is a chance sev_gmem_post_populate()
could be writing to some read-only page.

sev_gmem_post_populate() will only ever write the source page if the type
of page being LAUNCH_UPDATEd is a CPUID page. Hence, request a writable
page only when loading the CPUID page.

Since TDX never writes to the source page, always pass false to
kvm_gmem_populate().

With this, even if a read-only mapping or the global zero page was provided
as the source page, GUP will do a copy-on-write, making it writable before
the write happens in gvm_post_populate.

Fixes: 2a62345b30529 ("KVM: guest_memfd: GUP source pages prior to populating guest memory")
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
 arch/x86/kvm/svm/sev.c   | 1 +
 arch/x86/kvm/vmx/tdx.c   | 2 +-
 include/linux/kvm_host.h | 3 ++-
 virt/kvm/guest_memfd.c   | 6 ++++--
 4 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 940b97d4a8523..2f254c447923e 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2469,6 +2469,7 @@ static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	sev_populate_args.type = params.type;
 
 	count = kvm_gmem_populate(kvm, params.gfn_start, src, npages,
+				  params.type == KVM_SEV_SNP_PAGE_TYPE_CPUID,
 				  sev_gmem_post_populate, &sev_populate_args);
 	if (count < 0) {
 		argp->error = sev_populate_args.fw_error;
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index b8c3d3d8bbfe5..00dcfcbc47f68 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -3185,7 +3185,7 @@ static int tdx_vcpu_init_mem_region(struct kvm_vcpu *vcpu, struct kvm_tdx_cmd *c
 		};
 		gmem_ret = kvm_gmem_populate(kvm, gpa_to_gfn(region.gpa),
 					     u64_to_user_ptr(region.source_addr),
-					     1, tdx_gmem_post_populate, &arg);
+					     1, false, tdx_gmem_post_populate, &arg);
 		if (gmem_ret < 0) {
 			ret = gmem_ret;
 			break;
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 4c14aee1fb063..2c5ad9a6d5ce8 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2596,7 +2596,8 @@ int kvm_arch_gmem_prepare(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_ord
 typedef int (*kvm_gmem_populate_cb)(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
 				    struct page *page, void *opaque);
 
-long kvm_gmem_populate(struct kvm *kvm, gfn_t gfn, void __user *src, long npages,
+long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src,
+		       long npages, bool may_writeback_src,
 		       kvm_gmem_populate_cb post_populate, void *opaque);
 #endif
 
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 69c9d6d546b28..07d8db344872b 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -858,7 +858,8 @@ static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
 	return ret;
 }
 
-long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long npages,
+long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src,
+		       long npages, bool may_writeback_src,
 		       kvm_gmem_populate_cb post_populate, void *opaque)
 {
 	struct kvm_memory_slot *slot;
@@ -892,8 +893,9 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long
 
 		if (src) {
 			unsigned long uaddr = (unsigned long)src + i * PAGE_SIZE;
+			unsigned int flags = may_writeback_src ? FOLL_WRITE : 0;
 
-			ret = get_user_pages_fast(uaddr, 1, 0, &src_page);
+			ret = get_user_pages_fast(uaddr, 1, flags, &src_page);
 			if (ret < 0)
 				break;
 			if (ret != 1) {

-- 
2.54.0.794.g4f17f83d09-goog