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Date:   Tue, 28 Jun 2022 18:23:05 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     KP Singh <kpsingh@kernel.org>
Cc:     bpf@vger.kernel.org, linux-security-module@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        Yosry Ahmed <yosryahmed@google.com>
Subject: Re: [PATCH v5 bpf-next 4/5] bpf: Add a bpf_getxattr kfunc
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References: <20220628161948.475097-1-kpsingh@kernel.org>
 <20220628161948.475097-5-kpsingh@kernel.org>
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On Tue, Jun 28, 2022 at 04:19:47PM +0000, KP Singh wrote:
> LSMs like SELinux store security state in xattrs. bpf_getxattr enables
> BPF LSM to implement similar functionality. In combination with
> bpf_local_storage, xattrs can be used to develop more complex security
> policies.
> 
> This kfunc wraps around __vfs_getxattr which can sleep and is,
> therefore, limited to sleepable programs using the newly added
> sleepable_set for kfuncs.

"Sleepable" is nowhere near enough - for a trivial example, consider
what e.g. ext2_xattr_get() does.
        down_read(&EXT2_I(inode)->xattr_sem);
in there means that having that thing executed in anything that happens
to hold ->xattr_sem is a deadlock fodder.

"Can't use that in BPF program executed in non-blocking context" is
*not* sufficient to make it safe.