Re: [PATCH] sysctl: redefine zero as a unsigned long

[Kees Cook]

On Thu, Apr 4, 2019 at 11:48 PM Hou Tao <houtao1@xxxxxxxxxx> wrote:
>
> We have got KASAN splat when tried to set /proc/sys/fs/file-max:
>
>   BUG: KASAN: global-out-of-bounds in __do_proc_doulongvec_minmax+0x3e4/0x8f0
>   Read of size 8 at addr ffff20000f9b2980 by task file-max.sh/36819
>
>   Call trace:
>    dump_backtrace+0x0/0x3f8
>    show_stack+0x3c/0x60
>    dump_stack+0x150/0x1a8
>    print_address_description+0x2b8/0x5a0
>    kasan_report+0x278/0x648
>    __asan_load8+0x124/0x170
>    __do_proc_doulongvec_minmax+0x3e4/0x8f0
>    proc_doulongvec_minmax+0x80/0xa0
>    proc_sys_call_handler+0x188/0x2a0
>    proc_sys_write+0x5c/0x80
>    __vfs_write+0x118/0x578
>    vfs_write+0x184/0x418
>    ksys_write+0xfc/0x1e8
>    __arm64_sys_write+0x88/0xa8
>    el0_svc_common+0x1a4/0x500
>    el0_svc_handler+0xb8/0x180
>    el0_svc+0x8/0xc
>
>   The buggy address belongs to the variable:
>    zero+0x0/0x40
>
> The cause is that proc_doulongvec_minmax() is trying to cast an int
> pointer (namely &zero) to a unsigned long pointer, and dereferencing it.
>
> Although the warning seems does no harm, because zero will be placed
> in a .bss section, but it's better to kill the KASAN warning by
> redefining zero as a unsigned long, so it's OK whenever it is accessed
> as an int or a a unsigned long.
>
> An alternative fix seems to be set the minimal value of file-max to be 1,
> so one_ul can be used instead, but I'm not sure whether or not a file-max
> with a value of zero has special purpose (e.g., prohibit the file-related
> activities of all no-privileged users).
>
> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  kernel/sysctl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index e5da394d1ca3..03846e015013 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -124,7 +124,7 @@ static int sixty = 60;
>
>  static int __maybe_unused neg_one = -1;
>
> -static int zero;
> +static unsigned long zero;
>  static int __maybe_unused one = 1;
>  static int __maybe_unused two = 2;
>  static int __maybe_unused four = 4;
> --
> 2.16.2.dirty
>


-- 
Kees Cook