Re: [PATCH] crypto: ccp - Fix the INIT_EX data file open failure

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On 4/11/22 13:00, Jacky Li wrote:
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.

Signed-off-by: Jacky Li <jackyli@xxxxxxxxxx>

Should this have a Fixes: tag?

Thanks,
Tom

---
  drivers/crypto/ccp/sev-dev.c | 30 ++++++++++++++++++++++++++++--
  1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
index 6ab93dfd478a..3aefb177715e 100644
--- 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",



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