linux-kernel - Re: [PATCH] tpm/tpm_crb: Avoid unaligned reads in crb

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Date:   Fri, 1 Feb 2019 11:42:14 -0700
From:   Jerry Snitselaar <jsnitsel@...hat.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, stable@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        James Morris <jmorris@...ei.org>,
        Tomas Winkler <tomas.winkler@...el.com>
Subject: Re: [PATCH] tpm/tpm_crb: Avoid unaligned reads in crb_recv():

On Fri Feb 01 19, Jarkko Sakkinen wrote:
>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@...r.kernel.org
>Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>Cc: James Morris <jmorris@...ei.org>
>Cc: Tomas Winkler <tomas.winkler@...el.com>
>Cc: Jerry Snitselaar <jsnitsel@...hat.com>
>Fixes: 30fc8d138e91 ("tpm: TPM 2.0 CRB Interface")
>Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
>---
> 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;
>
> 	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);
> 	expected = be32_to_cpup((__be32 *) &buf[2]);
>-	if (expected > count || expected < 6)
>+	if (expected > count || expected < 8)
> 		return -EIO;
>
>-	memcpy_fromio(&buf[6], &priv->rsp[6], expected - 6);
>+	memcpy_fromio(&buf[8], &priv->rsp[8], expected - 8);
>
> 	return expected;
> }
>-- 
>2.17.1
>

Reviewed-by: Jerry Snitselaar <jsnitsel@...hat.com>