linux-kernel - Re: [PATCH] spi: spi-mem: check if data buffers are on stack

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Date:   Tue, 1 Feb 2022 13:32:07 +0530
From:   Pratyush Yadav <p.yadav@...com>
To:     <Tudor.Ambarus@...rochip.com>
CC:     <broonie@...nel.org>, <michael@...le.cc>,
        <miquel.raynal@...tlin.com>, <tkuw584924@...il.com>,
        <linux-spi@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] spi: spi-mem: check if data buffers are on stack

On 01/02/22 07:44AM, Tudor.Ambarus@...rochip.com wrote:
> On 1/31/22 13:45, Pratyush Yadav wrote:
> > EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> > 
> > The buffers passed in the data phase must be DMA-able. Programmers often
> > don't realise this requirement and pass in buffers that reside on the
> > stack. This can be hard to spot when reviewing code. Reject ops if their
> > data buffer is on the stack to avoid this.
> > 
> > Signed-off-by: Pratyush Yadav <p.yadav@...com>
> > ---
> >  drivers/spi/spi-mem.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
> > index 37f4443ce9a0..b3793a2979ee 100644
> > --- a/drivers/spi/spi-mem.c
> > +++ b/drivers/spi/spi-mem.c
> > @@ -207,6 +207,15 @@ static int spi_mem_check_op(const struct spi_mem_op *op)
> >             !spi_mem_buswidth_is_valid(op->data.buswidth))
> >                 return -EINVAL;
> > 
> > +       /* Buffers must be DMA-able. */
> > +       if (op->data.dir == SPI_MEM_DATA_IN &&
> > +           object_is_on_stack(op->data.buf.in))
> 
> should we also check if the virt addr is valid?
> if (object_is_on_stack(op->data.buf.in) || !virt_addr_valid(op->data.buf.in))

When would virt addr not be valid? When someone passes a bad pointer? I 
generally have not seen kernel APIs checking for pointer validity (other 
than NULL). If you pass a bad pointer then expect bad things to happen. 
Plus a bad pointer might also point to a valid virtual address, and we 
have no way of catching that. Dunno...

-- 
Regards,
Pratyush Yadav
Texas Instruments Inc.