linux-kernel - Re: [PATCH v1 1/2] io_uring,audit: audit IORING_OP_FADVISE but not IORING_OP

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Date:   Fri, 27 Jan 2023 18:05:48 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Richard Guy Briggs <rgb@...hat.com>
Cc:     Linux-Audit Mailing List <linux-audit@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, io-uring@...r.kernel.org,
        Eric Paris <eparis@...isplace.org>,
        Steve Grubb <sgrubb@...hat.com>, Stefan Roesch <shr@...com>,
        Christian Brauner <brauner@...nel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>
Subject: Re: [PATCH v1 1/2] io_uring,audit: audit IORING_OP_FADVISE but not IORING_OP_MADVISE

On Fri, Jan 27, 2023 at 5:55 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> On 2023-01-27 17:35, Paul Moore wrote:
> > On Fri, Jan 27, 2023 at 12:24 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> > >
> > > Since FADVISE can truncate files and MADVISE operates on memory, reverse
> > > the audit_skip tags.
> > >
> > > Fixes: 5bd2182d58e9 ("audit,io_uring,io-wq: add some basic audit support to io_uring")
> > > Signed-off-by: Richard Guy Briggs <rgb@...hat.com>
> > > ---
> > >  io_uring/opdef.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/io_uring/opdef.c b/io_uring/opdef.c
> > > index 3aa0d65c50e3..a2bf53b4a38a 100644
> > > --- a/io_uring/opdef.c
> > > +++ b/io_uring/opdef.c
> > > @@ -306,12 +306,12 @@ const struct io_op_def io_op_defs[] = {
> > >         },
> > >         [IORING_OP_FADVISE] = {
> > >                 .needs_file             = 1,
> > > -               .audit_skip             = 1,
> > >                 .name                   = "FADVISE",
> > >                 .prep                   = io_fadvise_prep,
> > >                 .issue                  = io_fadvise,
> > >         },
> >
> > I've never used posix_fadvise() or the associated fadvise64*()
> > syscalls, but from quickly reading the manpages and the
> > generic_fadvise() function in the kernel I'm missing where the fadvise
> > family of functions could be used to truncate a file, can you show me
> > where this happens?  The closest I can see is the manipulation of the
> > page cache, but that shouldn't actually modify the file ... right?
>
> I don't know.  I was going on the advice of Steve Grubb.  I'm looking
> for feedback, validation, correction, here.

Keep in mind it's your name on the patch, not Steve's, and I would
hope that you should be able to stand up and vouch for your own patch.
Something to keep in mind for the future.

As it stands, I think the audit_skip line should stay for
IORING_OP_FADVISE, if you feel otherwise please provide more
explanation as to why auditing is necessary for this operation.

> > >         [IORING_OP_MADVISE] = {
> > > +               .audit_skip             = 1,
> > >                 .name                   = "MADVISE",
> > >                 .prep                   = io_madvise_prep,
> > >                 .issue                  = io_madvise,
> >
> > I *think* this should be okay, what testing/verification have you done
> > on this?  One of the things I like to check is to see if any LSMs
> > might perform an access check and/or generate an audit record on an
> > operation, if there is a case where that could happen we should setup
> > audit properly.  I did a very quick check of do_madvise() and nothing
> > jumped out at me, but I would be interested in knowing what testing or
> > verification you did here.
>
> No testing other than build/boot/audit-testsuite.  You had a test you
> had developed that went through several iterations?

There is an io_uring test in the audit-testsuite that verifies basic
audit record generation, it is not exhaustive.

Patch 2/2 is a no-go from a security perspective (we want to see those
records), and I think skipping on IORING_OP_FADVISE is the correct
thing to do.  It may be that skipping on IORING_OP_MADVISE is the
correct thing, but given that it doesn't appear a lot of in-depth
investigation has gone into these patches I would really like to see
some more reasoning on this before we change the current behavior.

-- 
paul-moore.com