Re: [PATCH] platform/chrome: cros_ec_dev - Fix security issue

[Lee Jones <lee.jones@linaro.org>]

On Wed, 17 Mar 2021, Gwendal Grignou wrote:

> commit 5d749d0bbe811c10d9048cde6dfebc761713abfd upstream.
> 
> Prevent memory scribble by checking that ioctl buffer size parameters
> are sane.
> Without this check, on 32 bits system, if .insize = 0xffffffff - 20 and
> .outsize the amount to scribble, we would overflow, allocate a small
> amounts and be able to write outside of the malloc'ed area.
> Adding a hard limit allows argument checking of the ioctl. With the
> current EC, it is expected .insize and .outsize to be at around 512 bytes
> or less.
> 
> Signed-off-by: Olof Johansson <olof@lixom.net>
> Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
> ---
>  drivers/platform/chrome/cros_ec_dev.c   | 4 ++++
>  drivers/platform/chrome/cros_ec_proto.c | 4 ++--
>  include/linux/mfd/cros_ec.h             | 6 ++++--
>  3 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/platform/chrome/cros_ec_dev.c b/drivers/platform/chrome/cros_ec_dev.c
> index 2b331d5b9e799..e16d82bb36a9d 100644
> --- a/drivers/platform/chrome/cros_ec_dev.c
> +++ b/drivers/platform/chrome/cros_ec_dev.c
> @@ -137,6 +137,10 @@ static long ec_device_ioctl_xcmd(struct cros_ec_dev *ec, void __user *arg)
>  	if (copy_from_user(&u_cmd, arg, sizeof(u_cmd)))
>  		return -EFAULT;
>  
> +	if ((u_cmd.outsize > EC_MAX_MSG_BYTES) ||
> +	    (u_cmd.insize > EC_MAX_MSG_BYTES))
> +		return -EINVAL;
> +
>  	s_cmd = kmalloc(sizeof(*s_cmd) + max(u_cmd.outsize, u_cmd.insize),
>  			GFP_KERNEL);
>  	if (!s_cmd)
> diff --git a/drivers/platform/chrome/cros_ec_proto.c b/drivers/platform/chrome/cros_ec_proto.c
> index 5c285f2b3a650..d20190c8f0c06 100644
> --- a/drivers/platform/chrome/cros_ec_proto.c
> +++ b/drivers/platform/chrome/cros_ec_proto.c
> @@ -311,8 +311,8 @@ int cros_ec_query_all(struct cros_ec_device *ec_dev)
>  			ec_dev->max_response = EC_PROTO2_MAX_PARAM_SIZE;
>  			ec_dev->max_passthru = 0;
>  			ec_dev->pkt_xfer = NULL;
> -			ec_dev->din_size = EC_MSG_BYTES;
> -			ec_dev->dout_size = EC_MSG_BYTES;
> +			ec_dev->din_size = EC_PROTO2_MSG_BYTES;
> +			ec_dev->dout_size = EC_PROTO2_MSG_BYTES;
>  		} else {
>  			/*
>  			 * It's possible for a test to occur too early when
> diff --git a/include/linux/mfd/cros_ec.h b/include/linux/mfd/cros_ec.h
> index 3ab3cede28eac..93c14e9df6309 100644
> --- a/include/linux/mfd/cros_ec.h
> +++ b/include/linux/mfd/cros_ec.h
> @@ -50,9 +50,11 @@ enum {
>  					EC_MSG_TX_TRAILER_BYTES,
>  	EC_MSG_RX_PROTO_BYTES	= 3,
>  
> -	/* Max length of messages */
> -	EC_MSG_BYTES		= EC_PROTO2_MAX_PARAM_SIZE +
> +	/* Max length of messages for proto 2*/
> +	EC_PROTO2_MSG_BYTES		= EC_PROTO2_MAX_PARAM_SIZE +
>  					EC_MSG_TX_PROTO_BYTES,

Nit: Better to not tab the '=' so far and place it all on one line.

Checkpatch now only complains about lines exceeding 100 chars.

Once fixed, feel free to apply my:

  Acked-by: Lee Jones <lee.jones@linaro.org>

> +
> +	EC_MAX_MSG_BYTES		= 64 * 1024,
>  };
>  
>  /*

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