linux-kernel - Re: [PATCH] kernel: dma: Make CMA boot parameters __ro_after

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Date:   Sun, 13 Oct 2019 19:25:43 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Shyam Saini <mayhs11saini@...il.com>
Cc:     kernel-hardening@...ts.openwall.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Christopher Lameter <cl@...ux.com>,
        Kees Cook <keescook@...omium.org>,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] kernel: dma: Make CMA boot parameters __ro_after_init

On Sat, Oct 12, 2019 at 05:59:18PM +0530, Shyam Saini wrote:
> This parameters are not changed after early boot.
> By making them __ro_after_init will reduce any attack surface in the
> kernel.
> 
> Link: https://lwn.net/Articles/676145/
> Cc: Christoph Hellwig <hch@....de>
> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Christopher Lameter <cl@...ux.com>
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Shyam Saini <mayhs11saini@...il.com>
> ---
>  kernel/dma/contiguous.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> index 69cfb4345388..1b689b1303cd 100644
> --- a/kernel/dma/contiguous.c
> +++ b/kernel/dma/contiguous.c
> @@ -42,10 +42,10 @@ struct cma *dma_contiguous_default_area;
>   * Users, who want to set the size of global CMA area for their system
>   * should use cma= kernel parameter.
>   */
> -static const phys_addr_t size_bytes = (phys_addr_t)CMA_SIZE_MBYTES * SZ_1M;
> -static phys_addr_t size_cmdline = -1;
> -static phys_addr_t base_cmdline;
> -static phys_addr_t limit_cmdline;
> +static const phys_addr_t __ro_after_init size_bytes = (phys_addr_t)CMA_SIZE_MBYTES * SZ_1M;

The 0day bot reported an issue with this change with clang:

https://groups.google.com/d/msgid/clang-built-linux/201910140334.nhultlt8%25lkp%40intel.com

kernel/dma/contiguous.c:46:36: error: 'size_cmdline' causes a section type conflict with 'size_bytes'
static phys_addr_t __ro_after_init size_cmdline = -1;
                                   ^
kernel/dma/contiguous.c:45:42: note: declared here
static const phys_addr_t __ro_after_init size_bytes = (phys_addr_t)CMA_SIZE_MBYTES * SZ_1M;
                                         ^
kernel/dma/contiguous.c:47:36: error: 'base_cmdline' causes a section type conflict with 'size_bytes'
static phys_addr_t __ro_after_init base_cmdline;
                                   ^
kernel/dma/contiguous.c:45:42: note: declared here
static const phys_addr_t __ro_after_init size_bytes = (phys_addr_t)CMA_SIZE_MBYTES * SZ_1M;
                                         ^
kernel/dma/contiguous.c:48:36: error: 'limit_cmdline' causes a section type conflict with 'size_bytes'
static phys_addr_t __ro_after_init limit_cmdline;
                                   ^
kernel/dma/contiguous.c:45:42: note: declared here
static const phys_addr_t __ro_after_init size_bytes = (phys_addr_t)CMA_SIZE_MBYTES * SZ_1M;
                                         ^
3 errors generated.

The errors seem kind of cryptic at first but something that is const
should automatically be in the read only section, this part of the
commit seems unnecessary. Removing that part of the change fixes the error.

Cheers,
Nathan