linux-hardening - Re: [PATCH] stackleak: don't modify ctl

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Date: Fri, 3 May 2024 12:31:04 -0700
From: Kees Cook <keescook@...omium.org>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: Alexander Popov <alex.popov@...ux.com>, linux-hardening@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Luis Chamberlain <mcgrof@...nel.org>,
	Joel Granados <j.granados@...sung.com>
Subject: Re: [PATCH] stackleak: don't modify ctl_table argument

On Fri, May 03, 2024 at 03:44:09PM +0200, Thomas Weißschuh wrote:
> Sysctl handlers are not supposed to modify the ctl_table passed to them.
> Adapt the logic to work with a temporary
> variable, similar to how it is done in other parts of the kernel.
> 
> This is also a prerequisite to enforce the immutability of the argument
> through the callbacks prototy.
                        ^^^^^^^

Was this supposed to be "prototype"? I couldn't quite figure out what
was meant there; the sentence reads fine to me without the word there at
all. :)

> 
> Fixes: 964c9dff0091 ("stackleak: Allow runtime disabling of kernel stack erasing")
> Cc: stable@...r.kernel.org
> Acked-by: Kees Cook <keescook@...omium.org>
> Reviewed-by: Luis Chamberlain <mcgrof@...nel.org>
> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> ---
> This was split out of my sysctl-const-handler series [0].
> 
> As that series will take some more time, submit the patch on its own,
> as it is a generic bugfix that is valuable on its own.
> And I can get it out of my books.
> 
> Changelog in contrast to the patch in the series:
> * Reword commit message to remove strong relation to the constification
> * Cc stable
> 
> [0] https://lore.kernel.org/lkml/20240423-sysctl-const-handler-v3-0-e0beccb836e2@weissschuh.net/
> 
> Cc: Joel Granados <j.granados@...sung.com>
> ---
>  kernel/stackleak.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
> index 34c9d81eea94..b292e5ca0b7d 100644
> --- a/kernel/stackleak.c
> +++ b/kernel/stackleak.c
> @@ -27,10 +27,11 @@ static int stack_erasing_sysctl(struct ctl_table *table, int write,
>  	int ret = 0;
>  	int state = !static_branch_unlikely(&stack_erasing_bypass);
>  	int prev_state = state;
> +	struct ctl_table tmp = *table;
>  
> -	table->data = &state;
> -	table->maxlen = sizeof(int);
> -	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> +	tmp.data = &state;
> +	tmp.maxlen = sizeof(int);
> +	ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);

In looking at this yet again, I can't figure out why maxlen is being
set. It starts its life as sizeof(int):

static struct ctl_table stackleak_sysctls[] = {
        {
                .procname       = "stack_erasing",
                .data           = NULL,
                .maxlen         = sizeof(int),

-Kees

>  	state = !!state;
>  	if (ret || !write || state == prev_state)
>  		return ret;
> 
> ---
> base-commit: f03359bca01bf4372cf2c118cd9a987a5951b1c8
> change-id: 20240503-sysctl-const-stackleak-af3e67bc65b0
> 
> Best regards,
> -- 
> Thomas Weißschuh <linux@...ssschuh.net>
> 

-- 
Kees Cook