2020-09-16 20:52:00

by KP Singh

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

From: KP Singh <[email protected]>

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 <[email protected]>
Reviewed-by: Florent Revest <[email protected]>
---
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

2020-09-16 21:00:50

by Mimi Zohar

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

On Wed, 2020-09-16 at 14:49 +0200, KP Singh wrote:
> From: KP Singh <[email protected]>
>
> 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 <[email protected]>
> Reviewed-by: Florent Revest <[email protected]>
> ---
> 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;
> + }
> +

Not having a file hash is rather common (e.g. mknodat, prior to the
file being closed). Before appraising the integrity of a file, it
checks whether it is a new file (eg. IMA_NEW_FILE), but, unfortunately,
the flag is only set for those files in the appraise policy.

The patch looks fine, but you might want to reflect not having a file
hash is common in the patch description.

Mimi

> if (buf) {
> size_t copied_size;
>