linux-kernel - RE: [PATCH 1/2] libbpf: Retry map access with read-only permission

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Date:   Tue, 31 May 2022 08:47:59 +0000
From:   Roberto Sassu <roberto.sassu@...wei.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "andrii@...nel.org" <andrii@...nel.org>,
        "kpsingh@...nel.org" <kpsingh@...nel.org>
CC:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/2] libbpf: Retry map access with read-only permission

> From: Daniel Borkmann [mailto:daniel@...earbox.net]
> Sent: Monday, May 30, 2022 11:55 PM
> On 5/30/22 10:45 AM, Roberto Sassu wrote:
> > Retry map access with read-only permission, if access was denied when all
> > permissions were requested (open_flags is set to zero). Write access might
> > have been denied by the bpf_map security hook.
> >
> > Some operations, such as show and dump, don't need write permissions, so
> > there is a good chance of success with retrying.
> >
> > Prefer this solution to extending the API, as otherwise a new mechanism
> > would need to be implemented to determine the right permissions for an
> > operation.
> >
> > Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> > ---
> >   tools/lib/bpf/bpf.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> > index 240186aac8e6..b4eec39021a4 100644
> > --- a/tools/lib/bpf/bpf.c
> > +++ b/tools/lib/bpf/bpf.c
> > @@ -1056,6 +1056,11 @@ int bpf_map_get_fd_by_id(__u32 id)
> >   	attr.map_id = id;
> >
> >   	fd = sys_bpf_fd(BPF_MAP_GET_FD_BY_ID, &attr, sizeof(attr));
> > +	if (fd < 0) {
> > +		attr.open_flags = BPF_F_RDONLY;
> > +		fd = sys_bpf_fd(BPF_MAP_GET_FD_BY_ID, &attr, sizeof(attr));
> > +	}
> > +
> 
> But then what about bpf_obj_get() API in libbpf? attr.file_flags has similar
> purpose as attr.open_flags in this case.

Ok, I missed it.

> The other issue is that this could have upgrade implications, e.g. where an
> application bailed out before, it is now passing wrt bpf_map_get_fd_by_id(),
> but then suddenly failing during map update calls.

Good point.

> Imho, it might be better to be explicit about user intent w/o the lib doing
> guess work upon failure cases (... or have the BPF LSM set the attr.open_flags
> to BPF_F_RDONLY from within the BPF prog).

Uhm, I don't like that the users should be aware of permissions assigned
to maps that they don't own.

Maybe, better the original idea, request read-only permission for the
list and dump operations.

Roberto

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