Re: [PATCH] cred: clarify that task_struct::cred is only for the current task

[Paul Moore]

On Wed, Aug 12, 2026 at 10:59 AM Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> On Mon, Aug 10, 2026 at 03:41:54PM -0400, Paul Moore wrote:
> > On Aug  6, 2026 Jann Horn <jannh@xxxxxxxxxx> wrote:
> > >
> > > The `cred` field in task_struct is currently marked as __rcu, but that's
> > > not true: It can point to credentials from access_override_creds(), which
> > > do not get freed with RCU delay.
> > >
> > > What actually protects task_struct::cred is that accessing it is only
> > > permitted for the current task (except for setting up a child during fork()
> > > or tearing down a dead process).
> > > (There is currently code in Smack that violates this rule, but that's a bug
> > > and causes UAF, I have sent a separate fix for that.)
> > >
> > > Clarify this, remove the __rcu marker, and remove RCU helpers from all
> > > accesses to this field.
> > >
> > > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> > > ---
> > > For context:
> > > There have been at least two UAFs of struct cred that I'm aware of, both
> > > caused by wrong use of task_struct::cred:
> > >
> > >  - https://git.kernel.org/linus/a3727a8bac0a9e77c70820655fd8715523ba3db7
> > >  - https://lore.kernel.org/all/20260806-smack-uaf-fix-v1-1-26426d389a26@xxxxxxxxxx/
> > > ---
> > >  include/linux/cred.h  | 17 +++++++++++------
> > >  include/linux/sched.h |  8 ++++++--
> > >  kernel/auditsc.c      |  5 +++--
> > >  kernel/cred.c         |  2 +-
> > >  security/lsm_init.c   |  2 +-
> > >  5 files changed, 22 insertions(+), 12 deletions(-)
> >
> > This looks good to me, with one small nit below, but as we are at -rc7
> > I think this is best kept until after the upcoming merge window so it
> > has more than a few days in linux-next.  I'm going to merge this into
> > lsm/dev-staging with the expectation of merging it into lsm/dev after
> > the upcoming merge window closes.  If anyone wishes to object to this
> > patch or add their own review/ACK/test tag before then, please do.
> >
> > Thanks Jann.
> >
>
> Thanks.  FWIW I did also look at this closely.
>
> Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>

Updated, thanks.

-- 
paul-moore.com