Re: [PATCH v2 1/2] mm/page_alloc: Fix panic caused by passing debug_guardpage_minorder or kernelcore to command line

[Andrew Morton]

On Mon, 24 Sep 2018 16:24:08 +0200 Michal Hocko <mhocko@xxxxxxxxxx> wrote:

> On Sat 22-09-18 22:53:32, zhe.he@xxxxxxxxxxxxx wrote:
> > From: He Zhe <zhe.he@xxxxxxxxxxxxx>
> > 
> > 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.

No, it's not very important.  It might help some people understand why
their kernel went splat in rare circumstances.  And it's __init code so
the runtime impact is nil.

It bothers me that there are many other kernel parameters which have
the same undesirable behaviour.  I'd much prefer a general fixup which
gave all of them this treatment, but it's unclear how to do this.