From: John Wood <redacted>
Date: Fri, 2 Jul 2021 16:59:54 +0200

Hi,

On Thu, Jul 01, 2021 at 11:55:14PM +0000, Alexander Lobakin wrote:

quoted

Hi,

From: John Wood <redacted>
Date: Sat, 5 Jun 2021 17:04:00 +0200

quoted

+static int brute_task_execve(struct linux_binprm *bprm, struct file *file)
+{
+	struct dentry *dentry = file_dentry(bprm->file);
+	struct inode *inode = file_inode(bprm->file);
+	struct brute_stats stats;
+	int rc;
+
+	inode_lock(inode);
+	rc = brute_get_xattr_stats(dentry, inode, &stats);
+	if (WARN_ON_ONCE(rc && rc != -ENODATA))
+		goto unlock;

I think I caught a problem here. Have you tested this with
initramfs?

No, it has not been tested with initramfs :(

quoted

According to init/do_mount.c's
init_rootfs()/rootfs_init_fs_context(), when `root=` cmdline
parameter is not empty, kernel creates rootfs of type ramfs
(tmpfs otherwise).
The thing about ramfs is that it doesn't support xattrs.

It is a known issue that systems without xattr support are not
suitable for Brute (there are a note in the documentation).
However, the purpose is not to panic the system :(

quoted

I'm running this v8 on a regular PC with initramfs and having
`root=` in cmdline, and Brute doesn't allow the kernel to run
any init processes (/init, /sbin/init, ...) with err == -95
(-EOPNOTSUPP) -- I'm getting a

WARNING: CPU: 0 PID: 173 at brute_task_execve+0x15d/0x200
<snip>
Failed to execute /init (error -95)

and so on (and a panic at the end).

If I omit `root=` from cmdline, then the kernel runs init process
just fine -- I guess because initramfs is then placed inside tmpfs
with xattr support.

As for me, this ramfs/tmpfs selection based on `root=` presence
is ridiculous and I don't see or know any reasons behind that.
But that's another story, and ramfs might be not the only one
system without xattr support.
I think Brute should have a fallback here, e.g. it could simply
ignore files from xattr-incapable filesystems instead of such
WARNING splats and stuff.

Ok, it seems reasonable to me: if the file system doesn't support
xattr, but Brute is enabled, Brute will do nothing and the system
will work normally.

On the other hand, it leaves a potentional window for attackers to
perform brute force from xattr-incapable filesystems. So at the end
of the day I think that the current implementation (a strong
rejection of such filesystems) is way more secure than having
a fallback I proposed.

I'm planning to make a patch which will eliminate such weird rootfs
type selection and just always use more feature-rich tmpfs if it's
compiled in. So, as an alternative, you could add it to your series
as a preparatory change and just add a Kconfig dependency on
CONFIG_TMPFS && CONFIG_TMPFS_XATTR to CONFIG_SECURITY_FORK_BRUTE
without messing with any fallbacks at all.
What do you think?

I will work on it for the next version.
Thanks for the feedback.

John Wood

Thanks,
Al