linux-kernel - Re: [PATCH v2] riscv: mm: Proper page permissions after initmem free

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Message-ID: <1057a6ba-3778-f419-fb99-029c26092871@ghiti.fr>
Date:   Tue, 15 Nov 2022 15:53:47 +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,
        Samuel Holland <samuel@...lland.org>
Cc:     Björn Töpel <bjorn@...osinc.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] riscv: mm: Proper page permissions after initmem free

Hi Björn,

On 15/11/2022 10:06, Björn Töpel wrote:
> From: Björn Töpel <bjorn@...osinc.com>
>
> 64-bit RISC-V kernels have the kernel image mapped separately to alias
> the linear map. The linear map and the kernel image map are documented
> as "direct mapping" and "kernel" respectively in [1].
>
> At image load time, the linear map corresponding to the kernel image
> is set to PAGE_READ permission, and the kernel image map is set to
> PAGE_READ|PAGE_EXEC.
>
> When the initmem is freed, the pages in the linear map should be
> restored to PAGE_READ|PAGE_WRITE, whereas the corresponding pages in
> the kernel image map should be restored to PAGE_READ, by removing the
> PAGE_EXEC permission.
>
> 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
> kernel image map.
>
> In practise this results in that the kernel can potentially jump to
> dead __init code, and start executing invalid instructions, without
> getting an exception.
>
> Restore the freed initmem properly, by setting both the kernel image
> map to the correct permissions.
>
> [1] Documentation/riscv/vm-layout.rst
>
> Fixes: e5c35fa04019 ("riscv: Map the kernel with correct permissions the first time")
> Signed-off-by: Björn Töpel <bjorn@...osinc.com>
> ---
> v2: * Do not set the kernel image map to PAGE_WRITE. (Alex)
>      * Massaged the commit message a bit.
>      
> Samuel, I removed your Reviewed-by:/Tested-by: for the v2.
> ---
>   arch/riscv/kernel/setup.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index 67ec1fadcfe2..86acd690d529 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -322,10 +322,11 @@ 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)) {
> +		set_kernel_memory(lm_alias(__init_begin), lm_alias(__init_end), set_memory_rw_nx);
> +		if (IS_ENABLED(CONFIG_64BIT))
> +			set_kernel_memory(__init_begin, __init_end, set_memory_nx);
> +	}
>   
>   	free_initmem_default(POISON_FREE_INITMEM);
>   }


This looks good to me, I tested it on both defconfig and rv32_defconfig 
on qemu, so you can add:

Reviewed-by: Alexandre Ghiti <alex@...ti.fr>
Tested-by: Alexandre Ghiti <alex@...ti.fr>

Thanks,

Alex


> base-commit: 22dce2b89d6043d5c3f68384285fff5506109317