Re: [PATCH v2] kasan: remove kasan_find_vm_area() to prevent possible deadlock

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Jul 11, 2025 at 11:11:00AM +0900, Byungchul Park wrote:
> On Fri, Jul 11, 2025 at 11:08:58AM +0900, Byungchul Park wrote:
> > On Thu, Jul 10, 2025 at 02:43:15PM +0200, Andrey Konovalov wrote:
> > > On Thu, Jul 3, 2025 at 8:10 PM Yeoreum Yun <yeoreum.yun@arm.com> wrote:
> > > >
> > > > find_vm_area() couldn't be called in atomic_context.
> > > > If find_vm_area() is called to reports vm area information,
> > > > kasan can trigger deadlock like:
> > > >
> > > > CPU0                                CPU1
> > > > vmalloc();
> > > >  alloc_vmap_area();
> > > >   spin_lock(&vn->busy.lock)
> > > >                                     spin_lock_bh(&some_lock);
> > > >    <interrupt occurs>
> > > >    <in softirq>
> > > >    spin_lock(&some_lock);
> > > >                                     <access invalid address>
> > > >                                     kasan_report();
> > > >                                      print_report();
> > > >                                       print_address_description();
> > > >                                        kasan_find_vm_area();
> > > >                                         find_vm_area();
> > > >                                          spin_lock(&vn->busy.lock) // deadlock!
> > > >
> > > > To prevent possible deadlock while kasan reports, remove kasan_find_vm_area().
> > > >
> > > > Fixes: c056a364e954 ("kasan: print virtual mapping info in reports")
> > > > Reported-by: Yunseong Kim <ysk@kzalloc.com>
> > > > Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> > > 
> > > As a fix:
> > > 
> > > Acked-by: Andrey Konovalov <andreyknvl@gmail.com>
> > > 
> > > But it would be great to figure out a way to eventually restore this
> > > functionality; I'll file a bug for this once this patch lands. The
> > > virtual mapping info helps with real issues: e.g. just recently it
> > > helped me to quickly see the issue that caused a false-positive report
> > 
> > I checked the critical section by &vn->busy.lock in find_vm_area().  The
> > time complextity looks O(log N).  I don't think an irq disabled section
> > of O(log N) is harmful.  I still think using
> > spin_lock_irqsave(&vn->busy.lock) can resolve this issue with no worry
> > of significant irq delay.  Am I missing something?
> 
> I prefer this one tho.
> 
> 	Byungchul
> > 
> > If it's unacceptable for some reasons, why don't we introduce kind of
> > try_find_vm_area() using trylock so as to go ahead only if there's no
> > lock contention?
> > 
>
I wish we get rid of using the find_vm_area() from already existing
users including KASAN outside of vmalloc. In some sense it is not
safe to access a VA because of "use after free" issues.

--
Uladzislau Rezki