From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Andrew Donnellan <ajd@linux.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, linux-integrity@vger.kernel.org
Cc:     gregkh@linuxfoundation.org, gcwilson@linux.ibm.com,
        linux-kernel@vger.kernel.org, nayna@linux.ibm.com,
        ruscur@russell.cc, zohar@linux.ibm.com, gjoyce@linux.ibm.com,
        sudhakar@linux.ibm.com, bgray@linux.ibm.com, erichte@linux.ibm.com,
        joel@jms.id.au
Subject: Re: [PATCH v4 02/24] powerpc/pseries: Fix alignment of PLPKS
 structures and buffers
In-Reply-To: <20230120074306.1326298-3-ajd@linux.ibm.com>
References: <20230120074306.1326298-1-ajd@linux.ibm.com>
 <20230120074306.1326298-3-ajd@linux.ibm.com>
Date:   Thu, 26 Jan 2023 00:09:53 +1100
Message-ID: <87pmb2pxpa.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Andrew Donnellan <ajd@linux.ibm.com> writes:
> A number of structures and buffers passed to PKS hcalls have alignment
> requirements, which could on occasion cause problems:
>
> - Authorisation structures must be 16-byte aligned and must not cross a
>   page boundary
>
> - Label structures must not cross page boundaries
>
> - Password output buffers must not cross page boundaries
>
> Round up the allocations of these structures/buffers to the next power of
> 2 to make sure this happens.

It's not the *next* power of 2, it's the *nearest* power of 2, including
the initial value if it's already a power of 2.

That in conjunction with slab's guarantee that power of 2 sized objects
are naturally aligned, and that the relevant structs are smaller than a
page, is what makes this actually work.

So I think the patch is fine, but the change log and comments probably
need to be a bit clearer.

cheers

> Reported-by: Benjamin Gray <bgray@linux.ibm.com>
> Fixes: 2454a7af0f2a ("powerpc/pseries: define driver for Platform KeyStore")
> Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
> Reviewed-by: Russell Currey <ruscur@russell.cc>
> Signed-off-by: Russell Currey <ruscur@russell.cc>
>
> ---
>
> v3: Merge plpks fixes and signed update series with secvar series
>
> v4: Fix typo in commit message
>
>     Move up in series (npiggin)
> ---
>  arch/powerpc/platforms/pseries/plpks.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/platforms/pseries/plpks.c b/arch/powerpc/platforms/pseries/plpks.c
> index 9e85b6d85b0b..a01cf2ff140a 100644
> --- a/arch/powerpc/platforms/pseries/plpks.c
> +++ b/arch/powerpc/platforms/pseries/plpks.c
> @@ -126,7 +126,8 @@ static int plpks_gen_password(void)
>  	u8 *password, consumer = PKS_OS_OWNER;
>  	int rc;
>  
> -	password = kzalloc(maxpwsize, GFP_KERNEL);
> +	// The password must not cross a page boundary, so we align to the next power of 2
> +	password = kzalloc(roundup_pow_of_two(maxpwsize), GFP_KERNEL);
>  	if (!password)
>  		return -ENOMEM;
>  
> @@ -162,7 +163,9 @@ static struct plpks_auth *construct_auth(u8 consumer)
>  	if (consumer > PKS_OS_OWNER)
>  		return ERR_PTR(-EINVAL);
>  
> -	auth = kzalloc(struct_size(auth, password, maxpwsize), GFP_KERNEL);
> +	// The auth structure must not cross a page boundary and must be
> +	// 16 byte aligned. We align to the next largest power of 2
> +	auth = kzalloc(roundup_pow_of_two(struct_size(auth, password, maxpwsize)), GFP_KERNEL);
>  	if (!auth)
>  		return ERR_PTR(-ENOMEM);
>  
> @@ -196,7 +199,8 @@ static struct label *construct_label(char *component, u8 varos, u8 *name,
>  	if (component && slen > sizeof(label->attr.prefix))
>  		return ERR_PTR(-EINVAL);
>  
> -	label = kzalloc(sizeof(*label), GFP_KERNEL);
> +	// The label structure must not cross a page boundary, so we align to the next power of 2
> +	label = kzalloc(roundup_pow_of_two(sizeof(*label)), GFP_KERNEL);
>  	if (!label)
>  		return ERR_PTR(-ENOMEM);
>  
> -- 
> 2.39.0