linux-kernel - Re: [PATCH] uprobes: convert uprobe.ref to refcount

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Date:   Mon, 11 Feb 2019 12:21:32 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "acme@...nel.org" <acme@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] uprobes: convert uprobe.ref to refcount_t

On Sun, Feb 3, 2019 at 10:51 PM Reshetova, Elena
<elena.reshetova@...el.com> wrote:
>
>
> > * Elena Reshetova <elena.reshetova@...el.com> [2019-01-16 13:20:27]:
> >
> > > atomic_t variables are currently used to implement reference
> > > counters with the following properties:
> > >  - counter is initialized to 1 using atomic_set()
> > >  - a resource is freed upon counter reaching zero
> > >  - once counter reaches zero, its further
> > >    increments aren't allowed
> > >  - counter schema uses basic atomic operations
> > >    (set, inc, inc_not_zero, dec_and_test, etc.)
> > >
> > > Such atomic variables should be converted to a newly provided
> > > refcount_t type and API that prevents accidental counter overflows
> > > and underflows. This is important since overflows and underflows
> > > can lead to use-after-free situation and be exploitable.
> > >
> > > The variable uprobe.ref is used as pure reference counter.
> > > Convert it to refcount_t and fix up the operations.
> > >
> > > **Important note for maintainers:
> > >
> > > Some functions from refcount_t API defined in lib/refcount.c
> > > have different memory ordering guarantees than their atomic
> > > counterparts.
> > > The full comparison can be seen in
> > > https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
> > > in state to be merged to the documentation tree.
> > > Normally the differences should not matter since refcount_t provides
> > > enough guarantees to satisfy the refcounting use cases, but in
> > > some rare cases it might matter.
> > > Please double check that you don't have some undocumented
> > > memory guarantees for this variable usage.
> > >
> > > For the uprobe.ref it might make a difference
> > > in following places:
> > >  - put_uprobe(): decrement in refcount_dec_and_test() only
> > >    provides RELEASE ordering and control dependency on success
> > >    vs. fully ordered atomic counterpart
> > >
> > > Suggested-by: Kees Cook <keescook@...omium.org>
> > > Reviewed-by: David Windsor <dwindsor@...il.com>
> > > Reviewed-by: Hans Liljestrand <ishkamiel@...il.com>
> > > Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>
> > > ---
> > >  kernel/events/uprobes.c | 8 ++++----
> > >  1 file changed, 4 insertions(+), 4 deletions(-)
> > >
> >
> > Looks good to me.
> >
> > Reviewed-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
>
> Thank you very much Srikar!
> Would you be able to take this patch to integration?

Steve can you pick this up with Srikar's Reviewed-by?

-- 
Kees Cook