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

[Kees Cook <keescook@chromium.org>]

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