[PATCH] ima: Fix NULL pointer dereference in ima_file_hash

[KP Singh]

From: KP Singh <kpsi...@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 <kpsi...@google.com>
---
 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;
 
-- 
2.28.0.618.gf4bc123cb7-goog