linux-kernel - Re: [PATCH v2 3/4] misc: fastrpc: Remove buffer from list prior to unmap operation

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Message-ID: <wmq356u5rqdjzp27zvvvjwgzsf3ft4j3gpv6fuewxm6ohg5h4s@kuf5j7hxved6>
Date: Tue, 3 Feb 2026 23:15:57 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Jianping <jianping.li@....qualcomm.com>
Cc: srini@...nel.org, amahesh@....qualcomm.com, arnd@...db.de,
        gregkh@...uxfoundation.org, linux-arm-msm@...r.kernel.org,
        Ekansh Gupta <ekansh.gupta@....qualcomm.com>,
        thierry.escande@...aro.org, abelvesa@...nel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        quic_chennak@...cinc.com, stable@...nel.org
Subject: Re: [PATCH v2 3/4] misc: fastrpc: Remove buffer from list prior to
 unmap operation

On Mon, Feb 02, 2026 at 02:51:33PM +0800, Jianping wrote:
> 
> 
> On 1/16/2026 4:47 AM, Dmitry Baryshkov wrote:
> > On Thu, Jan 15, 2026 at 04:28:50PM +0800, Jianping Li wrote:
> > > From: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
> > > 
> > > fastrpc_req_munmap_impl() is called to unmap any buffer. The buffer is
> > > getting removed from the list after it is unmapped from DSP. This can
> > > create potential race conditions if any other thread removes the entry
> > > from list while unmap operation is ongoing. Remove the entry before
> > > calling unmap operation.
> > > 
> > > Fixes: 2419e55e532de ("misc: fastrpc: add mmap/unmap support")
> > > Cc: stable@...nel.org
> > > Co-developed-by: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
> > > Signed-off-by: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
> > > Signed-off-by: Jianping Li <jianping.li@....qualcomm.com>
> > > ---
> > >   drivers/misc/fastrpc.c | 28 ++++++++++++++++++++--------
> > >   1 file changed, 20 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> > > index 4f12fa5a05aa..833c265add5e 100644
> > > --- a/drivers/misc/fastrpc.c
> > > +++ b/drivers/misc/fastrpc.c
> > > @@ -202,6 +202,8 @@ struct fastrpc_buf {
> > >   	/* mmap support */
> > >   	struct list_head node; /* list of user requested mmaps */
> > >   	uintptr_t raddr;
> > > +	/* Lock for buf->node */
> > > +	spinlock_t *list_lock;
> > 
> > Why do you need to lock this? Isn't fl->lock enough?
> 
> According to the discussion in v1 patch:
> https://lore.kernel.org/all/p6cc5lxufmefeulx5bhlh6q6ivwluqf2muj3hu5e5526fsppuu@brcy6arm7epg/
> 
> The lock is stored in fastrpc_buf here.

That was a separate topic. So, why fl->lock isn't enough? What is the
race that isn't prevented by it?

> > 
> > >   };
> > >   struct fastrpc_dma_buf_attachment {
> > > @@ -441,6 +443,7 @@ static int __fastrpc_buf_alloc(struct fastrpc_user *fl, struct device *dev,
> > >   	buf->size = size;
> > >   	buf->dev = dev;
> > >   	buf->raddr = 0;
> > > +	buf->list_lock = &fl->lock;
> > >   	buf->virt = dma_alloc_coherent(dev, buf->size, &buf->dma_addr,
> > >   				       GFP_KERNEL);
> > > @@ -1865,9 +1868,6 @@ static int fastrpc_req_munmap_impl(struct fastrpc_user *fl, struct fastrpc_buf *
> > >   				      &args[0]);
> > >   	if (!err) {
> > >   		dev_dbg(dev, "unmmap\tpt 0x%09lx OK\n", buf->raddr);
> > > -		spin_lock(&fl->lock);
> > > -		list_del(&buf->node);
> > > -		spin_unlock(&fl->lock);
> > >   		fastrpc_buf_free(buf);
> > >   	} else {
> > >   		dev_err(dev, "unmmap\tpt 0x%09lx ERROR\n", buf->raddr);
> > > @@ -1881,6 +1881,7 @@ static int fastrpc_req_munmap(struct fastrpc_user *fl, char __user *argp)
> > >   	struct fastrpc_buf *buf = NULL, *iter, *b;
> > >   	struct fastrpc_req_munmap req;
> > >   	struct device *dev = fl->sctx->dev;
> > > +	int err;
> > >   	if (copy_from_user(&req, argp, sizeof(req)))
> > >   		return -EFAULT;
> > > @@ -1888,6 +1889,7 @@ static int fastrpc_req_munmap(struct fastrpc_user *fl, char __user *argp)
> > >   	spin_lock(&fl->lock);
> > >   	list_for_each_entry_safe(iter, b, &fl->mmaps, node) {
> > >   		if ((iter->raddr == req.vaddrout) && (iter->size == req.size)) {
> > > +			list_del(&iter->node);
> > >   			buf = iter;
> > >   			break;
> > >   		}
> > > @@ -1900,7 +1902,14 @@ static int fastrpc_req_munmap(struct fastrpc_user *fl, char __user *argp)
> > >   		return -EINVAL;
> > >   	}
> > > -	return fastrpc_req_munmap_impl(fl, buf);
> > > +	err = fastrpc_req_munmap_impl(fl, buf);
> > > +	if (err) {
> > > +		spin_lock(buf->list_lock);
> > > +		list_add_tail(&buf->node, &fl->mmaps);
> > > +		spin_unlock(buf->list_lock);
> > > +	}
> > > +
> > > +	return err;
> > >   }
> > >   static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
> > > @@ -1985,20 +1994,23 @@ static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
> > >   		}
> > >   	}
> > > -	spin_lock(&fl->lock);
> > > +	spin_lock(buf->list_lock);
> > >   	list_add_tail(&buf->node, &fl->mmaps);
> > > -	spin_unlock(&fl->lock);
> > > +	spin_unlock(buf->list_lock);
> > >   	if (copy_to_user((void __user *)argp, &req, sizeof(req))) {
> > >   		err = -EFAULT;
> > > -		goto err_assign;
> > > +		goto err_copy;
> > >   	}
> > >   	dev_dbg(dev, "mmap\t\tpt 0x%09lx OK [len 0x%08llx]\n",
> > >   		buf->raddr, buf->size);
> > >   	return 0;
> > > -
> > > +err_copy:
> > > +	spin_lock(buf->list_lock);
> > > +	list_del(&buf->node);
> > > +	spin_unlock(buf->list_lock);
> > >   err_assign:
> > >   	fastrpc_req_munmap_impl(fl, buf);
> > > -- 
> > > 2.43.0
> > > 
> > 
> 

-- 
With best wishes
Dmitry