linux-kernel - Re: [PATCH v2 21/24] x86/sgx: use vmalloc

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Message-ID: <004bc553-4dca-070b-c203-adcb50d4112d@intel.com>
Date:   Tue, 27 Jun 2023 07:54:36 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Julia Lawall <Julia.Lawall@...ia.fr>,
        Jarkko Sakkinen <jarkko@...nel.org>
Cc:     kernel-janitors@...r.kernel.org, keescook@...omium.org,
        christophe.jaillet@...adoo.fr, kuba@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 21/24] x86/sgx: use vmalloc_array and vcalloc

On 6/27/23 07:43, Julia Lawall wrote:
> Use vmalloc_array and vcalloc to protect against
> multiplication overflows.
...
> diff -u -p a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -628,7 +628,7 @@ static bool __init sgx_setup_epc_section
>  	if (!section->virt_addr)
>  		return false;
>  
> -	section->pages = vmalloc(nr_pages * sizeof(struct sgx_epc_page));
> +	section->pages = vmalloc_array(nr_pages, sizeof(struct sgx_epc_page));
>  	if (!section->pages) {

I'm not sure that changelog matches the code.

'nr_pages' here is an 'unsigned long' and The sizeof()==32.  In
practice, the multiplication can be done with a shift, and the ulong is
a *LONG* way from overflowing.

I'll accept that, as a general rule, vmalloc_array() is the preferred
form.  It's totally possible that someone could copy and paste the
nr_foo*sizeof(struct bar) code over to a place where nr_foo is a more
troublesome type.

But, if that's the true motivation, could we please say that in the
changelog?  As it stands, it's a bit silly to be talking about
multiplication overflows, unless I'm missing something totally obvious.