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

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Message-Id: <20240503-sysctl-const-stackleak-v1-1-603fecb19170@weissschuh.net>
Date: Fri, 03 May 2024 15:44:09 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Kees Cook <keescook@...omium.org>, 
 Alexander Popov <alex.popov@...ux.com>
Cc: linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org, 
 stable@...r.kernel.org, Luis Chamberlain <mcgrof@...nel.org>, 
 Thomas Weißschuh <linux@...ssschuh.net>, 
 Joel Granados <j.granados@...sung.com>
Subject: [PATCH] stackleak: don't modify ctl_table argument

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.

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);
 	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>