Re: [PATCH] infiniband: fix a possible use-after-free bug

[Gi-Oh Kim]

On Mon, Jun 4, 2018 at 6:46 PM, Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> On Mon, Jun 04, 2018 at 06:23:20PM +0200, Gi-Oh Kim wrote:
>> On Fri, Jun 1, 2018 at 8:31 PM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
>> > ucma_process_join() will free the new allocated "mc" struct,
>> > if there is any error after that, especially the copy_to_user().
>> >
>> > But in parallel, ucma_leave_multicast() could find this "mc"
>> > through idr_find() before ucma_process_join() frees it, since it
>> > is already published.
>> >
>> > So "mc" could be used in ucma_leave_multicast() after it is been
>> > allocated and freed in ucma_process_join(), since we don't refcnt
>> > it.
>> >
>> > Fix this by separating "publish" from ID allocation, so that we
>> > can get an ID first and publish it later after copy_to_user().
>> >
>> > Fixes c8f6a362bf3e ("RDMA/cma: Add multicast communication support")
>> > Reported-by: Noam Rathaus <noamr@xxxxxxxxxxxxxxxxxx>
>> > Cc: Sean Hefty <sean.hefty@xxxxxxxxx>
>> > Cc: Doug Ledford <dledford@xxxxxxxxxx>
>> > Cc: Jason Gunthorpe <jgg@xxxxxxxx>
>> > Cc: linux-rdma@xxxxxxxxxxxxxxx
>> > Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
>> >  drivers/infiniband/core/ucma.c | 6 +++++-
>> >  1 file changed, 5 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/drivers/infiniband/core/ucma.c b/drivers/infiniband/core/ucma.c
>> > index eab43b17e9cf..ec8fb289621f 100644
>> > +++ b/drivers/infiniband/core/ucma.c
>> > @@ -235,7 +235,7 @@ static struct ucma_multicast* ucma_alloc_multicast(struct ucma_context *ctx)
>> >                 return NULL;
>> >
>> >         mutex_lock(&mut);
>> > -       mc->id = idr_alloc(&multicast_idr, mc, 0, 0, GFP_KERNEL);
>> > +       mc->id = idr_alloc(&multicast_idr, NULL, 0, 0, GFP_KERNEL);
>> >         mutex_unlock(&mut);
>> >         if (mc->id < 0)
>> >                 goto error;
>> > @@ -1421,6 +1421,10 @@ static ssize_t ucma_process_join(struct ucma_file *file,
>> >                 goto err3;
>> >         }
>> >
>> > +       mutex_lock(&mut);
>> > +       idr_replace(&multicast_idr, mc, mc->id);
>> > +       mutex_unlock(&mut);
>> > +
>> >         mutex_unlock(&file->mut);
>> >         ucma_put_ctx(ctx);
>> >         return 0;
>> >
>>
>>
>> Hi,
>>
>> Your patch is reasonable to me.
>> Can I ask a question for that?
>> Could it be solved by asymmetric locking as following?
>
> No, there are many other paths that touch multicast_idr that don't
> hold both locks, we should protect all of them from accessing an
> incompletely initialized structure.
>
> Jason

Understood. Thank you.


-- 
GIOH KIM
Linux Kernel Entwickler

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