Re: [PATCH V2] fs: perform the check when page without mapping but page->mapping contains junk or random bitscribble

[Matthew Wilcox]

On Tue, Apr 11, 2023 at 05:15:36PM -0700, Andrew Morton wrote:
> On Tue, 11 Apr 2023 13:16:18 +0100 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> 
> > On Mon, Apr 10, 2023 at 09:45:02AM +0800, xiaosong.ma wrote:
> > > perform the check in dump_mapping() to print warning info and avoid crash with invalid non-NULL page->mapping.
> > > For example, a panic with following backtraces show dump_page will show wrong info and panic when the bad page
> > > is non-NULL mapping and page->mapping is 0x80000000000.
> > > 
> > >     crash_arm64> bt
> > >     PID: 232    TASK: ffffff80e8c2c340  CPU: 0   COMMAND: "Binder:232_2"
> > >      #0 [ffffffc013e5b080] sysdump_panic_event$b2bce43a479f4f7762201bfee02d7889 at ffffffc0108d7c2c
> > >      #1 [ffffffc013e5b0c0] atomic_notifier_call_chain at ffffffc010300228
> > >      #2 [ffffffc013e5b2c0] panic at ffffffc0102c926c
> > >      #3 [ffffffc013e5b370] die at ffffffc010267670
> > >      #4 [ffffffc013e5b3a0] die_kernel_fault at ffffffc0102808a4
> > >      #5 [ffffffc013e5b3d0] __do_kernel_fault at ffffffc010280820
> > >      #6 [ffffffc013e5b410] do_bad_area at ffffffc01028059c
> > >      #7 [ffffffc013e5b440] do_translation_fault$4df5decbea5d08a63349aa36f07426b2 at ffffffc0111149c8
> > >      #8 [ffffffc013e5b470] do_mem_abort at ffffffc0100a4488
> > >      #9 [ffffffc013e5b5e0] el1_ia at ffffffc0100a6c00
> > >      #10 [ffffffc013e5b5f0] __dump_page at ffffffc0104beecc
> > 
> > This doesn't show a crash in dump_mapping(), it shows a crash in
> > __dump_page().
> 
> um, yes.
> 
> But if page->mapping is corrupted, where does __dump_page() dereference it?

I don't see anywhere that it does, so I'm suspicious that we have the
correct diagnosis here.

> The initial patch
> (https://lkml.kernel.org/r/1680587425-4683-1-git-send-email-Xiaosong.Ma@xxxxxxxxxx)
> prevented __dump_page() from calling dump_mapping() if page->mapping is
> bad, and that presumably fixed things.

Right, but doesn't the _existing_ get_kernel_nofault(host, &mapping->host)
already prevent us from blindly dereferencing a bad mapping pointer?

> > > -	if (get_kernel_nofault(host, &mapping->host) ||
> > > +	if (get_kernel_nofault(mapping, &mapping) ||
> > > +	    get_kernel_nofault(host, &mapping->host) ||
> > 
> > This patch makes no sense.  Essentially, you're saying
> > 	mapping = &mapping
> > which is obviously wrong.
> 
> We're checking for mapping==junk, so this could be 
> 
> 	get_kernel_nofault(tmp, mapping)

Why will that be better than get_kernel_nofault(host, &mapping->host)?
I see no tangible difference between get_kernel_nofault(0x8000'0000) and
get_kernel_nofault(0x8000'0084) (or whatever the offset is).

> or go direct to copy_from_kernel_nofault().  We used to have a
> probe_kernel_address() for this...
> 
> So confusion reigns.  I think making dump_mapping() tolerant of a wild
> mapping pointer makes sense, but I don't think we actually know why the
> reporter's kernel crashed.

In my mind dump_mapping() is already tolerant of a wild page->mapping
pointer.  I think the problem is something entirely different.