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Subject: Re: [PATCH v2] ima: Fix NULL pointer dereference in ima_file_hash
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     KP Singh <kpsingh@chromium.org>, linux-kernel@vger.kernel.org,
        linux-security-module@vger.kernel.org
Cc:     Florent Revest <revest@chromium.org>,
        James Morris <jmorris@namei.org>,
        Kees Cook <keescook@chromium.org>,
        Lakshmi Ramasubramanian <nramas@linux.microsoft.com>,
        Jann Horn <jannh@google.com>
Date:   Wed, 16 Sep 2020 12:00:10 -0400
In-Reply-To: <20200916124931.1254990-1-kpsingh@chromium.org>
References: <20200916124931.1254990-1-kpsingh@chromium.org>
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On Wed, 2020-09-16 at 14:49 +0200, KP Singh wrote:
> 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;
> +	}
> +

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;
>