linux-kernel - Re: [PATCH] riscv: mm: Proper page permissions after initmem free

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Date:   Mon, 14 Nov 2022 15:37:10 +0100
From:   Alexandre Ghiti <alex@...ti.fr>
To:     Björn Töpel <bjorn@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org
Cc:     Björn Töpel <bjorn@...osinc.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: mm: Proper page permissions after initmem free

Hi Björn,

On 12/11/2022 12:35, Björn Töpel wrote:
> From: Björn Töpel <bjorn@...osinc.com>
>
> 64-bit RISC-V kernels have the kernel image mapped separately, and in
> addition to the linear map. When the kernel is loaded, the linear map
> of kernel image is set to PAGE_READ permission, and the kernel map is
> set to PAGE_READ and PAGE_EXEC.
>
> When the initmem is freed, the corresponding pages in the linear map
> should be restored to PAGE_READ and PAGE_WRITE. The corresponding
> pages in the kernel map should also be restored to PAGE_READ and
> PAGE_WRITE, by removing the PAGE_EXEC permission, and adding
> PAGE_WRITE.
>
> This is not the case. For 64-bit kernels, only the linear map is
> restored to its proper page permissions at initmem free, and not the
> kernelmap.
>
> In practise this results in that the kernel can potentially jump to
> dead __init code, and start executing invalid 0xcc instructions,
> without getting an exception.
>
> Restore the freed initmem properly, by setting both the alias (kernel
> map) and the linear map to the correct permissions.
>
> Fixes: e5c35fa04019 ("riscv: Map the kernel with correct permissions the first time")
> Signed-off-by: Björn Töpel <bjorn@...osinc.com>
> ---
>   arch/riscv/kernel/setup.c | 10 ++++++----
>   1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index ad76bb59b059..361e635070fe 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -321,10 +321,12 @@ subsys_initcall(topology_init);
>   
>   void free_initmem(void)
>   {
> -	if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
> -		set_kernel_memory(lm_alias(__init_begin), lm_alias(__init_end),
> -				  IS_ENABLED(CONFIG_64BIT) ?
> -					set_memory_rw : set_memory_rw_nx);
> +	if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX)) {
> +		if (IS_ENABLED(CONFIG_64BIT))
> +			set_kernel_memory(lm_alias(__init_begin), lm_alias(__init_end),
> +					  set_memory_rw);
> +		set_kernel_memory(__init_begin, __init_end, set_memory_rw_nx);


That's nits but for 64-bit kernels, do we really want to make the kernel 
mapping writable? I think catching a write access here would be better 
because IMO that should not happen.

Thanks,

Alex


> +	}
>   
>   	free_initmem_default(POISON_FREE_INITMEM);
>   }
>
> base-commit: 442bcbfd2c5401587b983e34bed0b407214735c3