crypto: ccp - Fix the INIT_EX data file open failure

There are 2 common cases when INIT_EX data file might not be
opened successfully and fail the sev initialization:

1. In user namespaces, normal user tasks (e.g. VMM) can change their
current->fs->root to point to arbitrary directories. While
init_ex_path is provided as a module param related to root file
system. Solution: use the root directory of init_task to avoid
accessing the wrong file.

2. Normal user tasks (e.g. VMM) don't have the privilege to access
the INIT_EX data file. Solution: open the file as root and
restore permissions immediately.

Fixes: 3d725965f836 ("crypto: ccp - Add SEV_INIT_EX support")
Signed-off-by: Jacky Li <[e-mail]>
Reviewed-by: Peter Gonda <[e-mail]>
Acked-by: Tom Lendacky <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

drivers/crypto/ccp/sev-dev.c

···
 #include <linux/gfp.h>
 #include <linux/cpufeature.h>
 #include <linux/fs.h>
+#include <linux/fs_struct.h>
 
 #include <asm/smp.h>
 
···
 	return page_address(page);
 }
 
+static struct file *open_file_as_root(const char *filename, int flags, umode_t mode)
+{
+	struct file *fp;
+	struct path root;
+	struct cred *cred;
+	const struct cred *old_cred;
+
+	task_lock(&init_task);
+	get_fs_root(init_task.fs, &root);
+	task_unlock(&init_task);
+
+	cred = prepare_creds();
+	if (!cred)
+		return ERR_PTR(-ENOMEM);
+	cred->fsuid = GLOBAL_ROOT_UID;
+	old_cred = override_creds(cred);
+
+	fp = file_open_root(&root, filename, flags, mode);
+	path_put(&root);
+
+	revert_creds(old_cred);
+
+	return fp;
+}
+
 static int sev_read_init_ex_file(void)
 {
 	struct sev_device *sev = psp_master->sev_data;
···
 	if (!sev_init_ex_buffer)
 		return -EOPNOTSUPP;
 
-	fp = filp_open(init_ex_path, O_RDONLY, 0);
+	fp = open_file_as_root(init_ex_path, O_RDONLY, 0);
 	if (IS_ERR(fp)) {
 		int ret = PTR_ERR(fp);
 
···
 	if (!sev_init_ex_buffer)
 		return;
 
-	fp = filp_open(init_ex_path, O_CREAT | O_WRONLY, 0600);
+	fp = open_file_as_root(init_ex_path, O_CREAT | O_WRONLY, 0600);
 	if (IS_ERR(fp)) {
 		dev_err(sev->dev,
 			"SEV: could not open file for write, error %ld\n",