[PATCH v2 1/4] KVM: guest_memfd: Gracefully handle xarray errors when binding a memslot

[Sean Christopherson <seanjc@google.com>]

If inserting a memslot into a guest_memfd's bindings xarray fails,
propagate the error back to the caller, i.e. fail memslot creation as well.
Signalling success and continuing on with memslot creation results in
use-after-free, as the guest_memfd instance will remain reachable via the
memslot after the file is freed (kvm_gmem_release() won't nullify the file
pointer due to lack of a valid binding).

Opportunistically WARN and reject binding if KVM_MEMSLOT_GMEM_ONLY is
already set, partly to guard against goofs elsewhere, but mostly so that
KVM doesn't need to worry about clobbering flags when unwinding on failure.

Fixes: a7800aa80ea4 ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory")
Cc: stable@vger.kernel.org
Reported-by: Stefan Teodorescu <fane@google.com>
Reported-by: Dennis Tighe <dtighe@google.com>
Reported-by: Sashiko Bot <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260823135031.4F6DC1F000E9%40smtp.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/guest_memfd.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index b596486d184c..2c8d8735de5f 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -612,10 +612,14 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 	struct inode *inode;
 	struct file *file;
 	int r = -EINVAL;
+	void *xar;
 
 	BUILD_BUG_ON(sizeof(gpa_t) != sizeof(offset));
 	BUILD_BUG_ON(sizeof(gfn_t) != sizeof(slot->gmem.pgoff));
 
+	if (WARN_ON_ONCE(slot->flags & KVM_MEMSLOT_GMEM_ONLY))
+		return -EINVAL;
+
 	file = fget(fd);
 	if (!file)
 		return -EBADF;
@@ -654,7 +658,7 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 	if (kvm_gmem_supports_mmap(inode))
 		slot->flags |= KVM_MEMSLOT_GMEM_ONLY;
 
-	xa_store_range(&f->bindings, start, end - 1, slot, GFP_KERNEL);
+	xar = xa_store_range(&f->bindings, start, end - 1, slot, GFP_KERNEL);
 	filemap_invalidate_unlock(inode->i_mapping);
 
 	/*
@@ -662,7 +666,13 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 	 * not the other way 'round.  Active bindings are invalidated if the
 	 * file is closed before memslots are destroyed.
 	 */
-	r = 0;
+	r = xa_is_err(xar) ? xa_err(xar) : 0;
+	if (r) {
+		xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL);
+		slot->gmem.file = NULL;
+		slot->gmem.pgoff = 0;
+		slot->flags &= ~KVM_MEMSLOT_GMEM_ONLY;
+	}
 err:
 	fput(file);
 	return r;
-- 
2.55.0.970.g62bdec98f9-goog