linux-kernel - Re: [PATCH bpf-next 2/2] selftests/bpf: Extend test fs

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Message-ID: <Zw4O3cqC6tlr5Kty@infradead.org>
Date: Mon, 14 Oct 2024 23:42:37 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Song Liu <songliubraving@...a.com>
Cc: Christoph Hellwig <hch@...radead.org>, Song Liu <song@...nel.org>,
	bpf <bpf@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Kernel Team <kernel-team@...a.com>,
	Andrii Nakryiko <andrii@...nel.org>,
	Eduard Zingerman <eddyz87@...il.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	KP Singh <kpsingh@...nel.org>,
	Matt Bobrowski <mattbobrowski@...gle.com>
Subject: Re: [PATCH bpf-next 2/2] selftests/bpf: Extend test fs_kfuncs to
 cover security.bpf xattr names

On Tue, Oct 15, 2024 at 05:52:02AM +0000, Song Liu wrote:
> >> Do you mean user.* xattrs are untrusted (any user can set it), so we 
> >> should not allow BPF programs to read them? Or do you mean xattr 
> >> name "user.kfuncs" might be taken by some use space?
> > 
> > All of the above.
> 
> This is a selftest, "user.kfunc" is picked for this test. The kfuncs
> (bpf_get_[file|dentry]_xattr) can read any user.* xattrs. 
> 
> Reading untrusted xattrs from trust BPF LSM program can be useful. 
> For example, we can sign a binary with private key, and save the
> signature in the xattr. Then the kernel can verify the signature
> and the binary matches the public key.

I would expect that to be done through an actual privileged interface.
Taking an arbitrary name that was available for use by user space
programs for 20 years and now giving it a new meaning is not a good
idea.