Re: [PATCH v1 1/2] misc: fastrpc: Fix registered buffer page address

[Ekansh Gupta <quic_ekangupt@quicinc.com>]

On 12/18/2024 4:42 PM, Dmitry Baryshkov wrote:
> On Wed, Dec 18, 2024 at 03:54:28PM +0530, Ekansh Gupta wrote:
>> For registered  buffers, fastrpc driver sends the buffer information
>> to remote subsystem. There is a problem with current implementation
>> where the page address is being sent with an offset leading to
>> improper buffer address on DSP. This is leads to functional failures
>> as DSP expects base address in page information and extracts offset
>> information from remote arguments. Mask the offset and pass the base
>> page address to DSP.
>>
>> Fixes: 80f3afd72bd4 ("misc: fastrpc: consider address offset before sending to DSP")
> This was committed in 2019. Are you saying that the driver has been
> broken since that time? If so, what is the impact? Because I've
> definitely been running fastrpc workload after that moment.
>
> Also, is there any reason for neglecting checkpatch warning?
Hi Dmitry,

This issue is observed is a corner case when some buffer which is registered with fastrpc
framework is passed with some offset by user and then the DSP implementation tried to
read the data. As DSP expects base address and takes care of offsetting with remote
arguments, passing an offsetted address will result in some unexpected data read in DSP.

All generic usecases usually pass the buffer as it is hence is problem is not usually observed. If
someone tries to pass offsetted buffer and then tries to compare data at HLOS and DSP end,
then the ambiguity will be observed.

Apologies for delay in response as I was traveling with very limited internet access.

--ekansh
>
>> Cc: stable <stable@kernel.org>
>> Signed-off-by: Ekansh Gupta <quic_ekangupt@quicinc.com>
>> ---
>>  drivers/misc/fastrpc.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
>> index 48d08eeb2d20..cfa1546c9e3f 100644
>> --- a/drivers/misc/fastrpc.c
>> +++ b/drivers/misc/fastrpc.c
>> @@ -992,7 +992,7 @@ static int fastrpc_get_args(u32 kernel, struct fastrpc_invoke_ctx *ctx)
>>  			mmap_read_lock(current->mm);
>>  			vma = find_vma(current->mm, ctx->args[i].ptr);
>>  			if (vma)
>> -				pages[i].addr += ctx->args[i].ptr -
>> +				pages[i].addr += (ctx->args[i].ptr & PAGE_MASK) -
>>  						 vma->vm_start;
>>  			mmap_read_unlock(current->mm);
>>  
>> -- 
>> 2.34.1
>>