Re: [PATCH v2] arm64: print a fault message when attempting to write RO memory

[Will Deacon]

Hi Stephen,

On Fri, Feb 24, 2017 at 05:39:08PM -0800, Stephen Boyd wrote:
> Quoting James Morse (2017-02-20 03:10:10)
> > On 17/02/17 15:53, Stephen Boyd wrote:
> > > Quoting James Morse (2017-02-17 03:00:39)
> > >> On 17/02/17 01:19, Stephen Boyd wrote:
> > >>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> > >>> index 156169c6981b..8bd4e7f11c70 100644
> > >>> --- a/arch/arm64/mm/fault.c
> > >>> +++ b/arch/arm64/mm/fault.c
> > >>> @@ -177,9 +193,19 @@ static void __do_kernel_fault(struct mm_struct *mm, unsigned long addr,
> > >>>        * No handler, we'll have to terminate things with extreme prejudice.
> > >>>        */
> > >>>       bust_spinlocks(1);
> > >>> -     pr_alert("Unable to handle kernel %s at virtual address %08lx\n",
> > >>> -              (addr < PAGE_SIZE) ? "NULL pointer dereference" :
> > >>> -              "paging request", addr);
> > >>> +
> > >>> +     if (is_permission_fault(esr, regs)) {
> > >>
> > >> is_permission_fault() was previously guarded with a 'addr<USER_DS' check, this
> > >> is because it assumes software-PAN is relevant.
> > >>
> > >> The corner case is when the kernel accesses TTBR1-mapped memory while
> > >> software-PAN happens to have swivelled TTBR0. Translation faults will be matched
> > >> by is_permission_fault(), but permission faults won't.
> > > 
> > > If I understand correctly, and I most definitely don't because there are
> > > quite a few combinations, you're saying that __do_kernel_fault() could
> > > be called if the kernel attempts to access some userspace address with
> > > software PAN? That won't be caught in do_page_fault() with the previous
> > > is_permission_fault() check?
> > 
> > You're right the user-address side of things will get caught in do_page_fault().
> > I was trying to badly-explain 'is_permission_fault(esr)' isn't as general
> > purpose as its name and prototype suggest, it only gives the results that the
> > PAN checks expect when called with a user address.
> 
> Ok. I'd rather not change the function in this patch because I'm only
> moving the code around to use it higher up in the file. But if you
> prefer I can combine the code movement with the addition of a new 'addr'
> argument to this function and rework things based on that.

Are you planning to send a v3 of this?

Will