nitro_enclaves: fix use-after-free on SLOT_ALLOC failure

When ne_create_vm_ioctl() fails the SLOT_ALLOC request after
anon_inode_getfile() has succeeded, the error path calls
fput(enclave_file) and then frees ne_enclave.

In normal userspace context, fput() defers the final __fput() via
task_work. ne_enclave_release() therefore runs after ne_enclave has
already been freed and dereferences ne_enclave->slot_uid, causing a
use-after-free: KASAN: slab-use-after-free in ne_enclave_release.

The enclave has no slot allocated and is not yet linked into the
enclaves list on this error path, so ne_enclave_release() is expected
to return early when slot_uid is zero. However, reading slot_uid
already accesses the freed object.

Clear enclave_file->private_data before fput() on the error path.
ne_enclave_release() then returns immediately when private_data is
NULL, leaving the ioctl error path as the sole owner of ne_enclave.
This is safe because the file has not been fd_install()'d yet.

Tested on an AWS EC2 m5.2xlarge with CONFIG_KASAN=y. Without the
patch, the reproducer triggers a KASAN slab-use-after-free on every
SLOT_ALLOC failure. With the patch, no KASAN report is produced and
the SLOT_ALLOC error is still returned. Normal enclave creation and
teardown are unaffected.

Fixes: 9c8eb50fe9 ("nitro_enclaves: Add logic for terminating an enclave")
Cc: [email protected]
Co-developed-by: Zhaofeng Chen <[email protected]>
Signed-off-by: Zhaofeng Chen <[email protected]>
Signed-off-by: Yuxiao Wang <[email protected]>
Reviewed-by: Alexander Graf <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/virt/nitro_enclaves/ne_misc_dev.c

@@ -1706,6 +1706,7 @@ static int ne_create_vm_ioctl(struct ne_pci_dev *ne_pci_dev, u64 __user *slot_ui
 	return enclave_fd;
 
 put_file:
+	enclave_file->private_data = NULL;
 	fput(enclave_file);
 put_fd:
 	put_unused_fd(enclave_fd);