linux-kernel - Re: [PATCH v2 1/2] mm/page_alloc: Fix panic caused by passing debug_guardpage

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Message-ID: <20180924142408.GC18685@dhcp22.suse.cz>
Date:   Mon, 24 Sep 2018 16:24:08 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     zhe.he@...driver.com
Cc:     akpm@...ux-foundation.org, vbabka@...e.cz,
        pasha.tatashin@...cle.com, mgorman@...hsingularity.net,
        aaron.lu@...el.com, osalvador@...e.de, iamjoonsoo.kim@....com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/page_alloc: Fix panic caused by passing
 debug_guardpage_minorder or kernelcore to command line

On Sat 22-09-18 22:53:32, zhe.he@...driver.com wrote:
> From: He Zhe <zhe.he@...driver.com>
> 
> debug_guardpage_minorder_setup and cmdline_parse_kernelcore do not check
> input argument before using it. The argument would be a NULL pointer if
> "debug_guardpage_minorder" or "kernelcore", without its value, is set in
> command line and thus causes the following panic.
> 
> PANIC: early exception 0xe3 IP 10:ffffffffa08146f1 error 0 cr2 0x0
> [    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.19.0-rc4-yocto-standard+ #11
> [    0.000000] RIP: 0010:parse_option_str+0x11/0x90
> ...
> [    0.000000] Call Trace:
> [    0.000000]  cmdline_parse_kernelcore+0x19/0x41
> [    0.000000]  do_early_param+0x57/0x8e
> [    0.000000]  parse_args+0x208/0x320
> [    0.000000]  ? rdinit_setup+0x30/0x30
> [    0.000000]  parse_early_options+0x29/0x2d
> [    0.000000]  ? rdinit_setup+0x30/0x30
> [    0.000000]  parse_early_param+0x36/0x4d
> [    0.000000]  setup_arch+0x336/0x99e
> [    0.000000]  start_kernel+0x6f/0x4ee
> [    0.000000]  x86_64_start_reservations+0x24/0x26
> [    0.000000]  x86_64_start_kernel+0x6f/0x72
> [    0.000000]  secondary_startup_64+0xa4/0xb0
> 
> This patch adds a check to prevent the panic

Is this something we deeply care about? The kernel command line
interface is to be used by admins who know what they are doing.  Using
random or wrong values for these parameters can have detrimental effects
on the system. This particular case would blow up early, good. At least
it is visible immediately. This and many other parameters could have a
seemingly valid input (e.g. not a missing value) and subtle runtime
effect. You won't blow up immediately but the system is hardly usable
and the early checking cannot possible catch all those cases. Take a
mem=$N copied from one machine to another with a different memory
layout. While 2G can be perfectly fine on one a different machine might
result on a completely unusable system because the available RAM is
place higher.

So I am really wondering. Do we really want a lot of code to catch
kernel command line incorrect inputs? Does it really lead to better
quality overall? IMHO, we do have a proper documentation and we should
trust those starting the kernel.

> and adds KBUILD_MODNAME to prints.

This doesn't seem to be done in this patch. Probably a left over
from the previous version.

> Signed-off-by: He Zhe <zhe.he@...driver.com>
> Cc: stable@...r.kernel.org
> Cc: akpm@...ux-foundation.org
> Cc: mhocko@...e.com
> Cc: vbabka@...e.cz
> Cc: pasha.tatashin@...cle.com
> Cc: mgorman@...hsingularity.net
> Cc: aaron.lu@...el.com
> Cc: osalvador@...e.de
> Cc: iamjoonsoo.kim@....com
> ---
> v2:
> Use more clear error info
> Split the addition of KBUILD_MODNAME out
> 
>  mm/page_alloc.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 89d2a2a..f34cae1 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -630,6 +630,12 @@ static int __init debug_guardpage_minorder_setup(char *buf)
>  {
>  	unsigned long res;
>  
> +	if (!buf) {
> +		pr_err("kernel option debug_guardpage_minorder requires an \
> +			argument\n");
> +		return -EINVAL;
> +	}
> +
>  	if (kstrtoul(buf, 10, &res) < 0 ||  res > MAX_ORDER / 2) {
>  		pr_err("Bad debug_guardpage_minorder value\n");
>  		return 0;
> @@ -6952,6 +6958,11 @@ static int __init cmdline_parse_core(char *p, unsigned long *core,
>   */
>  static int __init cmdline_parse_kernelcore(char *p)
>  {
> +	if (!p) {
> +		pr_err("kernel option kernelcore requires an argument\n");
> +		return -EINVAL;
> +	}
> +
>  	/* parse kernelcore=mirror */
>  	if (parse_option_str(p, "mirror")) {
>  		mirrored_kernelcore = true;
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs