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

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Message-ID: <2236FBA76BA1254E88B949DDB74E612BA4BA0C70@IRSMSX102.ger.corp.intel.com>
Date:   Mon, 4 Feb 2019 06:50:56 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
CC:     "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>,
        "keescook@...omium.org" <keescook@...omium.org>
Subject: RE: [PATCH] uprobes: convert uprobe.ref to refcount_t


> * 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?

Best Regards,
Elena.