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

[Gi-Oh Kim]

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
> --- a/drivers/infiniband/core/ucma.c
> +++ 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;
> --
> 2.13.0
>


Hi,

Your patch is reasonable to me.
Can I ask a question for that?
Could it be solved by asymmetric locking as following?


diff --git a/drivers/infiniband/core/ucma.c b/drivers/infiniband/core/ucma.c
index eab43b17e9cf..d8b256baec31 100644
--- a/drivers/infiniband/core/ucma.c
+++ b/drivers/infiniband/core/ucma.c
@@ -1493,6 +1493,7 @@ static ssize_t ucma_leave_multicast(struct
ucma_file *file,
        if (copy_from_user(&cmd, inbuf, sizeof(cmd)))
                return -EFAULT;

+       mutex_lock(&mc->ctx->file->mut);
        mutex_lock(&mut);
        mc = idr_find(&multicast_idr, cmd.id);
        if (!mc)
@@ -1507,11 +1508,11 @@ static ssize_t ucma_leave_multicast(struct
ucma_file *file,

        if (IS_ERR(mc)) {
                ret = PTR_ERR(mc);
+               mutex_unlock(&mc->ctx->file->mut);
                goto out;
        }

        rdma_leave_multicast(mc->ctx->cm_id, (struct sockaddr *) &mc->addr);
-       mutex_lock(&mc->ctx->file->mut);
        ucma_cleanup_mc_events(mc);
        list_del(&mc->list);
        mutex_unlock(&mc->ctx->file->mut);



-- 
GIOH KIM
Linux Kernel Entwickler

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