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Date: Wed, 25 Apr 2018 11:31:28 +0100
From: Lee Jones <lee.jones@linaro.org>
To: Kees Cook <keescook@chromium.org>
Cc: Kyle Spiers <ksspiers@google.com>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] rave-sp: Remove VLA
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On Tue, 24 Apr 2018, Kees Cook wrote:

> On Mon, Apr 23, 2018 at 10:43 PM, Lee Jones <lee.jones@linaro.org> wrote:
> > On Mon, 23 Apr 2018, Kyle Spiers wrote:
> >
> >> As part of the effort to remove VLAs from the kernel[1], this creates
> >> constants for the checksum lengths of CCITT and 8B2C and changes
> >> crc_calculated to be the maximum size of a checksum.
> >>
> >> https://lkml.org/lkml/2018/3/7/621
> >>
> >> Signed-off-by: Kyle Spiers <ksspiers@google.com>
> >> ---
> >>  drivers/mfd/rave-sp.c | 11 +++++++++--
> >>  1 file changed, 9 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/mfd/rave-sp.c b/drivers/mfd/rave-sp.c
> >> index 5c858e784a89..99fa482419f9 100644
> >> --- a/drivers/mfd/rave-sp.c
> >> +++ b/drivers/mfd/rave-sp.c
> >> @@ -45,7 +45,9 @@
> >>  #define RAVE_SP_DLE                  0x10
> >>
> >>  #define RAVE_SP_MAX_DATA_SIZE                64
> >> -#define RAVE_SP_CHECKSUM_SIZE                2  /* Worst case scenario on RDU2 */
> >> +#define RAVE_SP_CHECKSUM_8B2C                1
> >> +#define RAVE_SP_CHECKSUM_CCITT               2
> >> +#define RAVE_SP_CHECKSUM_SIZE                RAVE_SP_CHECKSUM_CCITT
> >>  /*
> >>   * We don't store STX, ETX and unescaped bytes, so Rx is only
> >>   * DATA + CSUM
> >> @@ -415,7 +417,12 @@ static void rave_sp_receive_frame(struct rave_sp *sp,
> >>       const size_t payload_length  = length - checksum_length;
> >>       const u8 *crc_reported       = &data[payload_length];
> >>       struct device *dev           = &sp->serdev->dev;
> >> -     u8 crc_calculated[checksum_length];
> >> +     u8 crc_calculated[RAVE_SP_CHECKSUM_SIZE];
> >> +
> >> +     if (unlikely(length > sizeof(crc_calculated))) {
> >
> > Forgive me if I have this wrong (it's still very early here), but this
> > doesn't leave any room for the payload?
> >
> >     <--       length         -->
> >     <ck len><- payload length ->
> >     [CK][CK][D][A][T][A] .. [64]
> >
> > It is my hope that length would always be larger than the size of the
> > checksum, or else there would never be any data?
> >
> > Should this not be:
> >
> >     if (unlikely(length > RAVE_SP_MAX_DATA_SIZE))
> 
> Oh, whoops, no, this should be:
> 
> +     if (unlikely(checksum_lengh > sizeof(crc_calculated))) {
> 
> (To validate the VLA max size.)

That doesn't match the OP's error message though:

  dev_warn(dev, "Dropping oversized frame\n");

Which I assume is designed to complement the existing warning:

  if (unlikely(length <= checksum_length))
          dev_warn(dev, "Dropping short frame\n");

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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