linux-kernel - Re: [PATCH v5 03/11] tpm: Allow PCR 23 to be restricted to kernel-only use

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Y3FXsPbBeZjAKv1/@sol.localdomain>
Date:   Sun, 13 Nov 2022 12:46:40 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Evan Green <evgreen@...omium.org>
Cc:     linux-kernel@...r.kernel.org, corbet@....net,
        linux-integrity@...r.kernel.org, gwendal@...omium.org,
        dianders@...omium.org, apronin@...omium.org,
        Pavel Machek <pavel@....cz>, Ben Boeckel <me@...boeckel.net>,
        rjw@...ysocki.net, jejb@...ux.ibm.com,
        Kees Cook <keescook@...omium.org>, dlunev@...gle.com,
        zohar@...ux.ibm.com, Matthew Garrett <mgarrett@...ora.tech>,
        jarkko@...nel.org, linux-pm@...r.kernel.org,
        Matthew Garrett <mjg59@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Peter Huewe <peterhuewe@....de>
Subject: Re: [PATCH v5 03/11] tpm: Allow PCR 23 to be restricted to
 kernel-only use

On Fri, Nov 11, 2022 at 03:16:28PM -0800, Evan Green wrote:
> Introduce a new Kconfig, TCG_TPM_RESTRICT_PCR, which if enabled

TCG_TPM_RESTRICT_PCR => TCG_TPM2_RESTRICT_PCR

> For systems with TPM1 devices, having this Kconfig enabled completely
> restricts usermode's access to the TPM.

This doesn't appear to actually be the case.

> +config TCG_TPM2_RESTRICT_PCR
> +	bool "Restrict userland access to PCR 23 on TPM2 devices"
> +	depends on TCG_TPM

I assume you also considered making this a once-settable sysctl, or similar?
I guess this kconfig is fine for now, but IMO it does violate the concept of
"kernel provides mechanism, not policy".

> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index 303ce2ea02a4b0..3bc5546fddc792 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -778,3 +778,25 @@ int tpm2_find_cc(struct tpm_chip *chip, u32 cc)
>  
>  	return -1;
>  }
> +
> +#ifdef CONFIG_TCG_TPM2_RESTRICT_PCR
> +int tpm2_cmd_restricted(struct tpm_chip *chip, u8 *buffer, size_t size)
> +{
> +	int cc = tpm2_find_and_validate_cc(chip, NULL, buffer, size);
> +	__be32 *handle;
> +
> +	switch (cc) {
> +	case TPM2_CC_PCR_EXTEND:
> +	case TPM2_CC_PCR_RESET:
> +		if (size < (TPM_HEADER_SIZE + sizeof(u32)))
> +			return -EINVAL;
> +
> +		handle = (__be32 *)&buffer[TPM_HEADER_SIZE];
> +		if (be32_to_cpu(*handle) == TPM_RESTRICTED_PCR)
> +			return -EPERM;

get_unaligned_be32((__be32 *)&buffer[TPM_HEADER_SIZE]),
to avoid an unaligned memory access.

> +		break;
> +	}
> +
> +	return 0;

So, if tpm2_find_and_validate_cc() returns an error code, the command is *not*
restricted, even if it uses one of the forbidden command codes.  Are you sure
there are no loopholes here?

- Eric