linux-kernel - [PATCH] exec: Set file unwritable before LSM check

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Message-ID: <20180309190736.GA4657@beast>
Date:   Fri, 9 Mar 2018 11:07:36 -0800
From:   Kees Cook <keescook@...omium.org>
To:     James Morris <jmorris@...ei.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        linux-integrity@...r.kernel.org, Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>
Subject: [PATCH] exec: Set file unwritable before LSM check

The LSM check should happen after the file has been confirmed to be
unchanging. Without this, we could have a ToCToU issue between the
LSM verification and the actual contents of the file later.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
Only loadpin and SELinux 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