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

From: Jerry Snitselaar
Date: Fri Feb 01 2019 - 13:42:20 EST

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@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;

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;

Reviewed-by: Jerry Snitselaar <jsnitsel@xxxxxxxxxx>