linux-kernel - Re: [PATCH 02/13] x86/fault: Check user_mode(regs) when validating a stack extension

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Message-ID: <20181120081339.GA35948@gmail.com>
Date:   Tue, 20 Nov 2018 09:13:39 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 02/13] x86/fault: Check user_mode(regs) when validating a
 stack extension


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Andy Lutomirski <luto@...nel.org> wrote:
> 
> > The fault handling code tries to validate that a page fault from
> > user mode that would extend the stack is within a certain range of
> > the user SP.  regs->sp is only equal to the user SP if
> > user_mode(regs).  In the extremely unlikely event that that
> > sw_error_code had the USER bit set but the faulting instruction was
> > in the kernel (i.e. the faulting instruction was WRUSS), then the
> > *kernel* stack pointer would have been checked, which would be an
> > info leak.
> > 
> > Note to -stable maintainers: don't backport this unless you backport
> > CET.  The bug it fixes is unobservable in current kernels.
> > 
> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
> > ---
> >  arch/x86/mm/fault.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index 91d4d2722f2e..eae7ee3ce89b 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -1377,7 +1377,7 @@ void do_user_addr_fault(struct pt_regs *regs,
> >  		bad_area(regs, sw_error_code, address);
> >  		return;
> >  	}
> > -	if (sw_error_code & X86_PF_USER) {
> > +	if (user_mode(regs)) {
> >  		/*
> >  		 * Accessing the stack below %sp is always a bug.
> >  		 * The large cushion allows instructions like enter
> 
> Note that this check is gone now due to:
> 
>   1d8ca3be86eb: x86/mm/fault: Allow stack access below %rsp
> 
> Thanks,

Ok, I like your series - I have applied the first ~7 patches of it to 
tip:x86/mm, the rest is interacting with 1d8ca3be86eb - will apply the 
rest as well once you send a v2.

Thanks,

	Ingo