linux-kernel - Re: [PATCH v2] kasan: remove kasan_find_vm

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Message-ID: <20250711020858.GA78977@system.software.com>
Date: Fri, 11 Jul 2025 11:08:58 +0900
From: Byungchul Park <byungchul@...com>
To: Andrey Konovalov <andreyknvl@...il.com>
Cc: Yeoreum Yun <yeoreum.yun@....com>, akpm@...ux-foundation.org,
	glider@...gle.com, dvyukov@...gle.com, vincenzo.frascino@....com,
	bigeasy@...utronix.de, clrkwllms@...nel.org, rostedt@...dmis.org,
	max.byungchul.park@...il.com, ysk@...lloc.com,
	kasan-dev@...glegroups.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	kernel_team@...ynix.com
Subject: Re: [PATCH v2] kasan: remove kasan_find_vm_area() to prevent
 possible deadlock

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@....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@...lloc.com>
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@....com>
> 
> As a fix:
> 
> Acked-by: Andrey Konovalov <andreyknvl@...il.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?

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?

	Byungchul

> [1].
> 
> [1] https://lore.kernel.org/all/CA+fCnZfzHOFjVo43UZK8H6h3j=OHjfF13oFJvT0P-SM84Oc4qQ@mail.gmail.com/