linux-kernel - Re: [PATCH v2] fs: don't audit the capability check in simple_xattr

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Date:   Sun, 6 Nov 2022 17:50:11 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Ondrej Mosnacek <omosnace@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kernel@...r.kernel.org, Martin Pitt <mpitt@...hat.com>
Subject: Re: [PATCH v2] fs: don't audit the capability check in simple_xattr_list()

On Sat, Nov 5, 2022 at 7:34 AM Christian Brauner <brauner@...nel.org> wrote:
>
> On Sat, Nov 05, 2022 at 12:38:57AM -0400, Paul Moore wrote:
> > On Thu, Nov 3, 2022 at 11:13 AM Ondrej Mosnacek <omosnace@...hat.com> wrote:
> > >
> > > The check being unconditional may lead to unwanted denials reported by
> > > LSMs when a process has the capability granted by DAC, but denied by an
> > > LSM. In the case of SELinux such denials are a problem, since they can't
> > > be effectively filtered out via the policy and when not silenced, they
> > > produce noise that may hide a true problem or an attack.
> > >
> > > Checking for the capability only if any trusted xattr is actually
> > > present wouldn't really address the issue, since calling listxattr(2) on
> > > such node on its own doesn't indicate an explicit attempt to see the
> > > trusted xattrs. Additionally, it could potentially leak the presence of
> > > trusted xattrs to an unprivileged user if they can check for the denials
> > > (e.g. through dmesg).
> > >
> > > Therefore, it's best (and simplest) to keep the check unconditional and
> > > instead use ns_capable_noaudit() that will silence any associated LSM
> > > denials.
> > >
> > > Fixes: 38f38657444d ("xattr: extract simple_xattr code from tmpfs")
> > > Reported-by: Martin Pitt <mpitt@...hat.com>
> > > Suggested-by: Christian Brauner (Microsoft) <brauner@...nel.org>
> > > Signed-off-by: Ondrej Mosnacek <omosnace@...hat.com>
> > > ---
> > >
> > > v1 -> v2: switch to simpler and better solution as suggested by Christian
> > >
> > > v1: https://lore.kernel.org/selinux/CAFqZXNuC7c0Ukx_okYZ7rsKycQY5P1zpMPmmq_T5Qyzbg-x7yQ@mail.gmail.com/T/
> > >
> > >  fs/xattr.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > VFS folks, this should really go through a vfs tree, but if nobody
> > wants to pick it up *and* there are no objections to the change, I can
> > take this via the LSM tree.
>
> I can pick this up as I'm currently massaging the simple xattr
> infrastructure.

Thanks Christian.

> I think the fix is pretty straightforward otherwise.

Agreed.

-- 
paul-moore.com