Re: [PATCH bpf-next 2/2] selftests/bpf: Extend test fs_kfuncs to cover security.bpf xattr names

[Song Liu <songliubraving@meta.com>]

> On Oct 14, 2024, at 10:25 PM, Christoph Hellwig <hch@infradead.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. 

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Song