[PATCH v2] exec: Set file unwritable before LSM check

From: Kees Cook <keescook@chromium.org>
To: James Morris <jmorris@namei.org>
Cc: linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Mimi Zohar <zohar@linux.vnet.ibm.com>,
	linux-integrity <linux-integrity@vger.kernel.org>,
	Paul Moore <paul@paul-moore.com>,
	Stephen Smalley <sds@tycho.nsa.gov>
Subject: [PATCH v2] exec: Set file unwritable before LSM check
Date: Fri, 9 Mar 2018 11:30:20 -0800	[thread overview]
Message-ID: <20180309193020.GA5149@beast> (raw)

The LSM check should happen after the file has been confirmed to be
unchanging. Without this, we could have a race between the Time of Check
(the call to security_kernel_read_file() which could read the file and
make access policy decisions) and the Time of Use (starting with
kernel_read_file()'s reading of the file contents). In theory, file
contents could change between the two.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
v2: Clarify the ToC/ToU race (Linus)

Only loadpin and SELinux currently implement this hook. From what
I can see, this won't change anything for either of them. IMA calls
kernel_read_file(), but looking there it seems those callers won't be
negatively impacted either. Can folks double-check this and send an
Ack please?
---
 fs/exec.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 7eb8d21bcab9..a919a827d181 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -895,13 +895,13 @@ int kernel_read_file(struct file *file, void **buf, loff_t *size,
 	if (!S_ISREG(file_inode(file)->i_mode) || max_size < 0)
 		return -EINVAL;
 
-	ret = security_kernel_read_file(file, id);
+	ret = deny_write_access(file);
 	if (ret)
 		return ret;
 
-	ret = deny_write_access(file);
+	ret = security_kernel_read_file(file, id);
 	if (ret)
-		return ret;
+		goto out;
 
 	i_size = i_size_read(file_inode(file));
 	if (max_size > 0 && i_size > max_size) {
-- 
2.7.4


-- 
Kees Cook
Pixel Security
