linux-kernel - Re: [PATCH] s390/mm: Fix in_atomic() handling in do_secure_storage

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Message-ID: <20250605100743.7808A03-hca@linux.ibm.com>
Date: Thu, 5 Jun 2025 12:07:43 +0200
From: Heiko Carstens <hca@...ux.ibm.com>
To: Christian Borntraeger <borntraeger@...ux.ibm.com>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Sven Schnelle <svens@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] s390/mm: Fix in_atomic() handling in
 do_secure_storage_access()

On Thu, Jun 05, 2025 at 11:06:29AM +0200, Christian Borntraeger wrote:
> Am 05.06.25 um 11:04 schrieb Alexander Gordeev:
> > On Wed, Jun 04, 2025 at 07:40:43PM +0200, Claudio Imbrenda wrote:
> > > > > > This could trigger WARN_ON_ONCE() in handle_fault_error_nolock():
> > > > > > 
> > > > > > 		if (WARN_ON_ONCE(!si_code))
> > > > > > 			si_code = SEGV_MAPERR;
> > > > > > 
> > > > > > Would this warning be justified in this case (aka user_mode(regs) ==
> > > > > > true)?
> > > > > 
> > > > > I think so, because if we are in usermode, we should never trigger
> > > > > faulthandler_disabled()
> > > > 
> > > > I think I do not get you. We are in a system call and also in_atomic(),
> > > > so faulthandler_disabled() is true and handle_fault_error_nolock(regs, 0)
> > > > is called (above).
> > > 
> > > what is the psw in regs?
> > > is it not the one that was being used when the exception was triggered?
> > 
> > Hmm, right. I assume is_kernel_fault() returns false not because
> > user_mode(regs) is true, but because we access the secondary AS.
> > 
> > Still, to me it feels wrong to trigger that warning due to a user
> > process activity. But anyway:
> > 
> > Acked-by: Alexander Gordeev <agordeev@...ux.ibm.com>
> 
> Can we trigger a WARN from userspace?

No. If the warning triggers, then this indicates a bug in the kernel (exit to
user with faulthandler_disabled() == true). I managed to screw up the kernel
exactly with such a bug. See commit 588a9836a4ef ("s390/stacktrace: Use break
instead of return statement"), which lead to random unexplainable user space
crashes.

Note that we have the identical check/code in do_exception().