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@xxxxxxxxxxxxxxx
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Reviewed-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> Signed-off-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxxx/
>
> Cc: Joel Granados <j.granados@xxxxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxxx>
>
--
Kees Cook
