linux-kernel - Re: [PATCH] rave-sp: Remove VLA

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Message-ID: <CAM9OsyKauRJCWfJ360+SAiUAOE1+aBZ1hp_AMDgmzkQ-MYrqxA@mail.gmail.com>
Date:   Wed, 25 Apr 2018 16:51:26 +0000
From:   Kyle Spiers <ksspiers@...gle.com>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rave-sp: Remove VLA

The error message is also wrong. Would "Checksum length too large" be fine?

On Wed, Apr 25, 2018 at 3:31 AM Lee Jones <lee.jones@...aro.org> wrote:

> On Tue, 24 Apr 2018, Kees Cook wrote:

> > On Mon, Apr 23, 2018 at 10:43 PM, Lee Jones <lee.jones@...aro.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@...gle.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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