linux-kernel - Re: [PATCH v2 5/6] LSM: secctx provider check on release

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Message-ID: <CAHC9VhQzva=uKWqFduADzvyTR+NXokCH6R7WNe6RgmDDa-Ge1g@mail.gmail.com>
Date: Tue, 22 Oct 2024 12:28:26 -0400
From: Paul Moore <paul@...l-moore.com>
To: Casey Schaufler <casey@...aufler-ca.com>
Cc: linux-security-module@...r.kernel.org, jmorris@...ei.org, serge@...lyn.com, 
	keescook@...omium.org, john.johansen@...onical.com, 
	penguin-kernel@...ove.sakura.ne.jp, stephen.smalley.work@...il.com, 
	linux-kernel@...r.kernel.org, selinux@...r.kernel.org, mic@...ikod.net
Subject: Re: [PATCH v2 5/6] LSM: secctx provider check on release

On Mon, Oct 21, 2024 at 8:06 PM Casey Schaufler <casey@...aufler-ca.com> wrote:
> On 10/21/2024 4:39 PM, Paul Moore wrote:
> > On Oct 14, 2024 Casey Schaufler <casey@...aufler-ca.com> wrote:
> >> Verify that the LSM releasing the secctx is the LSM that
> >> allocated it. This was not necessary when only one LSM could
> >> create a secctx, but once there can be more than one it is.
> >>
> >> Signed-off-by: Casey Schaufler <casey@...aufler-ca.com>
> >> ---
> >>  security/apparmor/secid.c | 10 ++--------
> >>  security/selinux/hooks.c  | 10 ++--------
> >>  2 files changed, 4 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/security/apparmor/secid.c b/security/apparmor/secid.c
> >> index 5d92fc3ab8b4..974f802cbe5a 100644
> >> --- a/security/apparmor/secid.c
> >> +++ b/security/apparmor/secid.c
> >> @@ -122,14 +122,8 @@ int apparmor_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid)
> >>
> >>  void apparmor_release_secctx(struct lsm_context *cp)
> >>  {
> >> -    /*
> >> -     * stacking scaffolding:
> >> -     * When it is possible for more than one LSM to provide a
> >> -     * release hook, do this check:
> >> -     * if (cp->id == LSM_ID_APPARMOR || cp->id == LSM_ID_UNDEF)
> >> -     */
> >> -
> >> -    kfree(cp->context);
> >> +    if (cp->id == LSM_ID_APPARMOR)
> >> +            kfree(cp->context);
> > Should we set cp->context to NULL too?  One could argue that it's an
> > unecessary assignment, given the cp->id checks, and they wouldn't be
> > wrong, but considering the potential for a BPF LSM to do things with
> > a lsm_context, I wonder if resetting the pointer to NULL is the
> > smart thing to do.
>
> Wouldn't hurt. I'll go ahead and add that. If a BPF LSM does anything
> with a lsm_context we're likely to hear about the many issues quite
> quickly.

Yes, I suspect you're right about that, at least we can protect
against a UAF in this one case :)

-- 
paul-moore.com