RE: [PATCH] tpm/tpm_crb: Avoid unaligned reads in crb_recv():

[Winkler, Tomas]

> 
> The current approach to read first 6 bytes from the response and then tail of
> the response, can cause the 2nd memcpy_fromio() to do an unaligned read
> (e.g. read 32-bit word from address aligned to a 16-bits), depending on how
> memcpy_fromio() is implemented. If this happens, the read will fail and the
> memory controller will fill the read with 1's.
> 
> This was triggered by 170d13ca3a2f, which should be probably refined to check
> and react to the address alignment. Before that commit, on x86
> memcpy_fromio() turned out to be memcpy(). By a luck GCC has done the right
> thing (from tpm_crb's perspective) for us so far, but we should not rely on that.
> Thus, it makes sense to fix this also in tpm_crb, not least because the fix can be
> then backported to stable kernels and make them more robust when compiled
> in differing environments.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: James Morris <jmorris@xxxxxxxxx>
> Cc: Tomas Winkler <tomas.winkler@xxxxxxxxx>
> Cc: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>
> Fixes: 30fc8d138e91 ("tpm: TPM 2.0 CRB Interface")
> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> ---
>  drivers/char/tpm/tpm_crb.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm_crb.c b/drivers/char/tpm/tpm_crb.c index
> 36952ef98f90..7f47e43aa9f1 100644
> --- a/drivers/char/tpm/tpm_crb.c
> +++ b/drivers/char/tpm/tpm_crb.c
> @@ -288,18 +288,18 @@ static int crb_recv(struct tpm_chip *chip, u8 *buf,
> size_t count)
>  	unsigned int expected;
> 
>  	/* sanity check */
> -	if (count < 6)
> +	if (count < 8)
>  		return -EIO;
Why don't you already enforce reading at least the whole TPM header 10bytes, we are reading the whole buffer at one call anyway.
Who every is asking for 8 bytes from the protocol level, is doing something wrong.

>  	if (ioread32(&priv->regs_t->ctrl_sts) & CRB_CTRL_STS_ERROR)
>  		return -EIO;
> 
> -	memcpy_fromio(buf, priv->rsp, 6);
> +	memcpy_fromio(buf, priv->rsp, 8);
Maybe a short comment will spare someone looking into git history 
>  	expected = be32_to_cpup((__be32 *) &buf[2]);
> -	if (expected > count || expected < 6)
> +	if (expected > count || expected < 8)
Expected should be at least tpm header, right?
>  		return -EIO;
> 
> -	memcpy_fromio(&buf[6], &priv->rsp[6], expected - 6);
> +	memcpy_fromio(&buf[8], &priv->rsp[8], expected - 8);
> 
>  	return expected;
>  }
Otherwise ready the first 8 bytes looks good. 
Thanks 
Tomas