[PATCH v2] ima: Fix NULL pointer dereference in ima_file_hash

[KP Singh <kpsingh@chromium.org>]

From: KP Singh <kpsingh@google.com>

ima_file_hash can be called when there is no iint->ima_hash available
even though the inode exists in the integrity cache.

An example where this can happen (suggested by Jann Horn):

Process A does:

	while(1) {
		unlink("/tmp/imafoo");
		fd = open("/tmp/imafoo", O_RDWR|O_CREAT|O_TRUNC, 0700);
		if (fd == -1) {
			perror("open");
			continue;
		}
		write(fd, "A", 1);
		close(fd);
	}

and Process B does:

	while (1) {
		int fd = open("/tmp/imafoo", O_RDONLY);
		if (fd == -1)
			continue;
    		char *mapping = mmap(NULL, 0x1000, PROT_READ|PROT_EXEC,
			 	     MAP_PRIVATE, fd, 0);
		if (mapping != MAP_FAILED)
			munmap(mapping, 0x1000);
		close(fd);
  	}

Due to the race to get the iint->mutex between ima_file_hash and
process_measurement iint->ima_hash could still be NULL.

Fixes: 6beea7afcc72 ("ima: add the ability to query the cached hash of a given file")
Signed-off-by: KP Singh <kpsingh@google.com>
Reviewed-by: Florent Revest <revest@chromium.org>
---
 security/integrity/ima/ima_main.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 8a91711ca79b..4c86cd4eece0 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -531,6 +531,16 @@ int ima_file_hash(struct file *file, char *buf, size_t buf_size)
 		return -EOPNOTSUPP;
 
 	mutex_lock(&iint->mutex);
+
+	/*
+	 * ima_file_hash can be called when ima_collect_measurement has still
+	 * not been called, we might not always have a hash.
+	 */
+	if (!iint->ima_hash) {
+		mutex_unlock(&iint->mutex);
+		return -EOPNOTSUPP;
+	}
+
 	if (buf) {
 		size_t copied_size;
 
-- 
2.28.0.526.ge36021eeef-goog