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

[Jianping <jianping.li@oss.qualcomm.com>]

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@oss.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@kernel.org
>> Co-developed-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
>> Signed-off-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
>> Signed-off-by: Jianping Li <jianping.li@oss.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.
> 
>>   };
>>   
>>   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
>>
>