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

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Message-ID: <20241016135155.otibqwcyqczxt26f@quack3>
Date: Wed, 16 Oct 2024 15:51:55 +0200
From: Jan Kara <jack@...e.cz>
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 15-10-24 05:52:02, Song Liu wrote:
> > On Oct 14, 2024, at 10:25 PM, Christoph Hellwig <hch@...radead.org> wrote:
> > On Tue, Oct 15, 2024 at 05:21:48AM +0000, Song Liu wrote:
> >>>> Extend test_progs fs_kfuncs to cover different xattr names. Specifically:
> >>>> xattr name "user.kfuncs", "security.bpf", and "security.bpf.xxx" can be
> >>>> read from BPF program with kfuncs bpf_get_[file|dentry]_xattr(); while
> >>>> "security.bpfxxx" and "security.selinux" cannot be read.
> >>> 
> >>> So you read code from untrusted user.* xattrs?  How can you carve out
> >>> that space and not known any pre-existing userspace cod uses kfuncs
> >>> for it's own purpose?
> >> 
> >> I don't quite follow the comment here. 
> >> 
> >> 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. If the xattr is modified by
> untrusted user space, the BPF program will just deny the access. 

So I tend to agree with Christoph that e.g. for the above LSM usecase you
mention, using user. xattr space is a poor design choice because you have
to very carefully validate any xattr contents (anybody can provide
malicious content) and more importantly as different similar usecases
proliferate the chances of name collisions and resulting funcionality
issues increase. It is similar as if you decided to store some information
in a specially named file in each directory. If you choose special enough
name, it will likely work but long-term someone is going to break you :)

I think that getting user.* xattrs from bpf hooks can still be useful for
introspection and other tasks so I'm not convinced we should revert that
functionality but maybe it is too easy to misuse? I'm not really decided.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR