netdev - Re: [PATCH v2] net:qrtr: fix atomic idr allocation in qrtr_port

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Date:   Sat, 27 Mar 2021 15:31:18 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Du Cheng <ducheng2@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>,
        syzbot+3eec59e770685e3dc879@...kaller.appspotmail.com
Subject: Re: [PATCH v2] net:qrtr: fix atomic idr allocation in
 qrtr_port_assign()

On Sat, Mar 27, 2021 at 10:25:20PM +0800, Du Cheng wrote:
> On Sat, Mar 27, 2021 at 03:12:14PM +0100, Greg Kroah-Hartman wrote:
> > Adding the xarray maintainer...
> > 
> > On Sat, Mar 27, 2021 at 10:07:02PM +0800, Du Cheng wrote:
> > > add idr_preload() and idr_preload_end() around idr_alloc_u32(GFP_ATOMIC)
> > > due to internal use of per_cpu variables, which requires preemption
> > > disabling/enabling.
> > > 
> > > reported as "BUG: "using smp_processor_id() in preemptible" by syzkaller
> > > 
> > > Reported-by: syzbot+3eec59e770685e3dc879@...kaller.appspotmail.com
> > > Signed-off-by: Du Cheng <ducheng2@...il.com>
> > > ---
> > > changelog
> > > v1: change to GFP_KERNEL for idr_alloc_u32() but might sleep
> > > v2: revert to GFP_ATOMIC but add preemption disable/enable protection
> > > 
> > >  net/qrtr/qrtr.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > > 
> > > diff --git a/net/qrtr/qrtr.c b/net/qrtr/qrtr.c
> > > index edb6ac17ceca..6361f169490e 100644
> > > --- a/net/qrtr/qrtr.c
> > > +++ b/net/qrtr/qrtr.c
> > > @@ -722,17 +722,23 @@ static int qrtr_port_assign(struct qrtr_sock *ipc, int *port)
> > >  	mutex_lock(&qrtr_port_lock);
> > >  	if (!*port) {
> > >  		min_port = QRTR_MIN_EPH_SOCKET;
> > > +		idr_preload(GFP_ATOMIC);
> > >  		rc = idr_alloc_u32(&qrtr_ports, ipc, &min_port, QRTR_MAX_EPH_SOCKET, GFP_ATOMIC);
> > > +		idr_preload_end();
> > 
> > This seems "odd" to me.  We are asking idr_alloc_u32() to abide by
> > GFP_ATOMIC, so why do we need to "preload" it with the same type of
> > allocation?
> > 
> > Is there something in the idr/radix/xarray code that can't really handle
> > GFP_ATOMIC during a "normal" idr allocation that is causing this warning
> > to be hit?  Why is this change the "correct" one?
> > 
> > thanks,
> > 
> > greg k-h
> 
> 
> >From the comment above idr_preload() in lib/radix-tree.c:1460
> /**
>  * idr_preload - preload for idr_alloc()
>  * @gfp_mask: allocation mask to use for preloading
>  *
>  * Preallocate memory to use for the next call to idr_alloc().  This function
>  * returns with preemption disabled.  It will be enabled by idr_preload_end().
>  */
> 
> idr_alloc is a very simple wrapper around idr_alloc_u32().
> 
> On top of radix_tree_node_alloc() which is called by idr_alloc_u32(), there is
> this comment at line 244 in the same radix-tree.c
> /*
>  * This assumes that the caller has performed appropriate preallocation, and
>  * that the caller has pinned this thread of control to the current CPU.
>  */
> 
> Therefore the preload/preload_end are necessary, or at least should have
> preemption disabled

Ah, so it's disabling preemption that is the key here.  Still odd, why
is GFP_ATOMIC not sufficient in a normal idr_alloc() call to keep things
from doing stuff like this?  Feels like a lot of "internal knowledge" is
needed here to use this api properly...

Matthew, is the above change really correct?

thanks,

greg k-h