linux-kernel - Re: [PATCH v2] powerpc: fix alignment bug whithin the init sections

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Message-ID: <dacc0535-217d-1a35-fcfe-7ff730e32735@csgroup.eu>
Date:   Mon, 11 Jan 2021 08:06:16 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Ariel Marcovitch <arielmarcovitch@...il.com>, mpe@...erman.id.au
Cc:     benh@...nel.crashing.org, paulus@...ba.org, keescook@...omium.org,
        dja@...ens.net, npiggin@...il.com, naveen.n.rao@...ux.vnet.ibm.com,
        maskray@...gle.com, ariel.marcovitch@...il.com, oss@...error.net,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] powerpc: fix alignment bug whithin the init sections



Le 02/01/2021 à 21:11, Ariel Marcovitch a écrit :
> This is a bug that causes early crashes in builds with a
> .exit.text section smaller than a page and a .init.text section that
> ends in the beginning of a physical page (this is kinda random, which
> might explain why this wasn't really encountered before).
> 
> The init sections are ordered like this:
> 	.init.text
> 	.exit.text
> 	.init.data
> 
> Currently, these sections aren't page aligned.
> 
> Because the init code might become read-only at runtime and because the
> .init.text section can potentially reside on the same physical page as
> .init.data, the beginning of .init.data might be mapped read-only along
> with .init.text.
> 
> Then when the kernel tries to modify a variable in .init.data (like
> kthreadd_done, used in kernel_init()) the kernel panics.
> 
> To avoid this, make _einittext page aligned and also align .exit.text
> to make sure .init.data is always seperated from the text segments.
> 
> Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
> Signed-off-by: Ariel Marcovitch <ariel.marcovitch@...il.com>

Reviewed-by: Christophe Leroy <christophe.leroy@...roup.eu>

> ---
>   arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
> index 6db90cdf11da..b6c765d8e7ee 100644
> --- a/arch/powerpc/kernel/vmlinux.lds.S
> +++ b/arch/powerpc/kernel/vmlinux.lds.S
> @@ -187,6 +187,11 @@ SECTIONS
>   	.init.text : AT(ADDR(.init.text) - LOAD_OFFSET) {
>   		_sinittext = .;
>   		INIT_TEXT
> +
> +		/* .init.text might be RO so we must
> +		* ensure this section ends in a page boundary.
> +		*/
> +		. = ALIGN(PAGE_SIZE);
>   		_einittext = .;
>   #ifdef CONFIG_PPC64
>   		*(.tramp.ftrace.init);
> @@ -200,6 +205,8 @@ SECTIONS
>   		EXIT_TEXT
>   	}
>   
> +	. = ALIGN(PAGE_SIZE);
> +
>   	.init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
>   		INIT_DATA
>   	}
> 
> base-commit: 2c85ebc57b3e1817b6ce1a6b703928e113a90442
>