[PATCH] sysctl: Check name array length in deprecated_sysctl_warning()

[PATCH] sysctl: Check name array length in deprecated_sysctl_warning()

[Mateusz Jurczyk]

Prevent use of uninitialized memory (originating from the stack frame of
do_sysctl()) by verifying that the name array is filled with sufficient
input data before comparing its specific entries with integer constants.

Through timing measurement or analyzing the kernel debug logs, a user-mode
program could potentially infer the results of comparisons against the
uninitialized memory, and acquire some (very limited) information about the
state of the kernel stack. The change also eliminates possible future
warnings by tools such as KMSAN and other code checkers /
instrumentations.

Signed-off-by: Mateusz Jurczyk <mjurczyk@google.com>
---
 kernel/sysctl_binary.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sysctl_binary.c b/kernel/sysctl_binary.c
index ece4b177052b..38d6ba22a209 100644
--- a/kernel/sysctl_binary.c
+++ b/kernel/sysctl_binary.c
@@ -1346,7 +1346,7 @@ static void deprecated_sysctl_warning(const int *name, int nlen)
 	 * CTL_KERN/KERN_VERSION is used by older glibc and cannot
 	 * ever go away.
 	 */
-	if (name[0] == CTL_KERN && name[1] == KERN_VERSION)
+	if (nlen >= 2 && name[0] == CTL_KERN && name[1] == KERN_VERSION)
 		return;
 
 	if (printk_ratelimit()) {
-- 
2.13.0.219.gdb65acc882-goog

Re: [PATCH] sysctl: Check name array length in deprecated_sysctl_warning()

[Kees Cook]

On Wed, May 24, 2017 at 5:21 AM, Mateusz Jurczyk <mjurczyk@google.com> wrote:
> Prevent use of uninitialized memory (originating from the stack frame of
> do_sysctl()) by verifying that the name array is filled with sufficient
> input data before comparing its specific entries with integer constants.
>
> Through timing measurement or analyzing the kernel debug logs, a user-mode
> program could potentially infer the results of comparisons against the
> uninitialized memory, and acquire some (very limited) information about the
> state of the kernel stack. The change also eliminates possible future
> warnings by tools such as KMSAN and other code checkers /
> instrumentations.
>
> Signed-off-by: Mateusz Jurczyk <mjurczyk@google.com>

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  kernel/sysctl_binary.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sysctl_binary.c b/kernel/sysctl_binary.c
> index ece4b177052b..38d6ba22a209 100644
> --- a/kernel/sysctl_binary.c
> +++ b/kernel/sysctl_binary.c
> @@ -1346,7 +1346,7 @@ static void deprecated_sysctl_warning(const int *name, int nlen)
>          * CTL_KERN/KERN_VERSION is used by older glibc and cannot
>          * ever go away.
>          */
> -       if (name[0] == CTL_KERN && name[1] == KERN_VERSION)
> +       if (nlen >= 2 && name[0] == CTL_KERN && name[1] == KERN_VERSION)
>                 return;
>
>         if (printk_ratelimit()) {
> --
> 2.13.0.219.gdb65acc882-goog
>



-- 
Kees Cook
Pixel Security