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 <jackyli@google.com>
Reviewed-by: Peter Gonda <pgonda@google.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
index fd92819..799b476 100644 (file)
--- a/drivers/crypto/ccp/sev-dev.c
+++ b/drivers/crypto/ccp/sev-dev.c
@@ -23,6 +23,7 @@
 #include <linux/gfp.h>
 #include <linux/cpufeature.h>
 #include <linux/fs.h>
+#include <linux/fs_struct.h>
 
 #include <asm/smp.h>
 
@@ -170,6 +171,31 @@ static void *sev_fw_alloc(unsigned long len)
        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;
@@ -181,7 +207,7 @@ static int sev_read_init_ex_file(void)
        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);
 
@@ -217,7 +243,7 @@ static void sev_write_init_ex_file(void)
        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",