linux-kernel - Re: [PATCH v4 1/6] s390/kexec_file: Don't opencode appended signature check.

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Date:   Tue, 25 Jan 2022 11:50:40 -0800
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Michal Suchanek <msuchanek@...e.de>,
        Heiko Carstens <hca@...ux.ibm.com>
Cc:     keyrings@...r.kernel.org, linux-crypto@...r.kernel.org,
        linux-integrity@...r.kernel.org, kexec@...ts.infradead.org,
        Philipp Rudo <prudo@...hat.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Nayna <nayna@...ux.vnet.ibm.com>, Rob Herring <robh@...nel.org>,
        linux-s390@...r.kernel.org, Vasily Gorbik <gor@...ux.ibm.com>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Jessica Yu <jeyu@...nel.org>, linux-kernel@...r.kernel.org,
        David Howells <dhowells@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Paul Mackerras <paulus@...ba.org>,
        Hari Bathini <hbathini@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org,
        Frank van der Linden <fllinden@...zon.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Daniel Axtens <dja@...ens.net>, buendgen@...ibm.com,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Baoquan He <bhe@...hat.com>,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v4 1/6] s390/kexec_file: Don't opencode appended
 signature check.

On Mon, Jan 10, 2022 at 02:49:53PM +0100, Michal Suchanek wrote:
> Module verification already implements appeded signature check.
> 
> Reuse it for kexec_file.
> 
> The kexec_file implementation uses EKEYREJECTED error in some cases when
> there is no key and the common implementation uses ENOPKG or EBADMSG
> instead.
> 
> Signed-off-by: Michal Suchanek <msuchanek@...e.de>
> Acked-by: Heiko Carstens <hca@...ux.ibm.com>
> ---
> v3: Philipp Rudo <prudo@...hat.com>: Update the commit with note about
> change of return value
> ---
>  arch/s390/kernel/machine_kexec_file.c | 22 +++++-----------------
>  1 file changed, 5 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/s390/kernel/machine_kexec_file.c b/arch/s390/kernel/machine_kexec_file.c
> index 8f43575a4dd3..c944d71316c7 100644
> --- a/arch/s390/kernel/machine_kexec_file.c
> +++ b/arch/s390/kernel/machine_kexec_file.c
> @@ -31,6 +31,7 @@ int s390_verify_sig(const char *kernel, unsigned long kernel_len)
>  	const unsigned long marker_len = sizeof(MODULE_SIG_STRING) - 1;
>  	struct module_signature *ms;
>  	unsigned long sig_len;
> +	int ret;
>  
>  	/* Skip signature verification when not secure IPLed. */
>  	if (!ipl_secure_flag)
> @@ -45,25 +46,12 @@ int s390_verify_sig(const char *kernel, unsigned long kernel_len)
>  	kernel_len -= marker_len;
>  
>  	ms = (void *)kernel + kernel_len - sizeof(*ms);
> -	kernel_len -= sizeof(*ms);
> +	ret = mod_check_sig(ms, kernel_len, "kexec");
> +	if (ret)
> +		return ret;
>  
>  	sig_len = be32_to_cpu(ms->sig_len);
> -	if (sig_len >= kernel_len)
> -		return -EKEYREJECTED;

There is a small minor fix here, where by using mod_check_sig() now
decreased the kernel_len by the sizeof(*ms). It is minor though.

> -	kernel_len -= sig_len;
> -
> -	if (ms->id_type != PKEY_ID_PKCS7)
> -		return -EKEYREJECTED;

More importantly is the return value used here changes but given the
Ack by Heiko I suspect this if fine and does not break old userspace,
the only change here is the possible error value returned by the
kexec_file_load() system call.

Reviewed-by: Luis Chamberlain <mcgrof@...nel.org>

   Luis