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

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Date:   Sun, 13 Nov 2022 15:21:50 -0600
From:   Samuel Holland <samuel@...lland.org>
To:     Björn Töpel <bjorn@...nel.org>
Cc:     Björn Töpel <bjorn@...osinc.com>,
        linux-kernel@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, Alexandre Ghiti <alex@...ti.fr>
Subject: Re: [PATCH] riscv: mm: Proper page permissions after initmem free

On 11/12/22 05:35, 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(-)

Reviewed-by: Samuel Holland <samuel@...lland.org>
Tested-by: Samuel Holland <samuel@...lland.org> # on D1