Re: [PATCH v3 4/5] powerpc/fault: Avoid heavy search_exception_tables() verification

[Christophe Leroy]

Le 08/12/2020 à 16:07, Christophe Leroy a écrit :
> Le 08/12/2020 à 15:52, Aneesh Kumar K.V a écrit :
> > Christophe Leroy <christophe.leroy@xxxxxxxxxx> writes:
> >
> > > search_exception_tables() is an heavy operation, we have to avoid it.
> > > When KUAP is selected, we'll know the fault has been blocked by KUAP.
> > > Otherwise, it behaves just as if the address was already in the TLBs
> > > and no fault was generated.
> > >
> > > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > > Reviewed-by: Nicholas Piggin <npiggin@xxxxxxxxx>
> > > ---
> > > v3: rebased
> > > v2: Squashed with the preceeding patch which was re-ordering tests that get removed in this patch.
> > > ---
> > >   arch/powerpc/mm/fault.c | 23 +++++++----------------
> > >   1 file changed, 7 insertions(+), 16 deletions(-)
> > >
> > > diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> > > index 3fcd34c28e10..1770b41e4730 100644
> > > --- a/arch/powerpc/mm/fault.c
> > > +++ b/arch/powerpc/mm/fault.c
> > > @@ -210,28 +210,19 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code,
> > >           return true;
> > >       }
> > > -    if (!is_exec && address < TASK_SIZE && (error_code & (DSISR_PROTFAULT | DSISR_KEYFAULT)) &&
> > > -        !search_exception_tables(regs->nip)) {
> > > -        pr_crit_ratelimited("Kernel attempted to access user page (%lx) - exploit attempt? (uid: 
> > > %d)\n",
> > > -                    address,
> > > -                    from_kuid(&init_user_ns, current_uid()));
> > > -    }
> > > -
> > >       // Kernel fault on kernel address is bad
> > >       if (address >= TASK_SIZE)
> > >           return true;
> > > -    // Fault on user outside of certain regions (eg. copy_tofrom_user()) is bad
> > > -    if (!search_exception_tables(regs->nip))
> > > -        return true;
> > > -
> > > -    // Read/write fault in a valid region (the exception table search passed
> > > -    // above), but blocked by KUAP is bad, it can never succeed.
> > > -    if (bad_kuap_fault(regs, address, is_write))
> > > +    // Read/write fault blocked by KUAP is bad, it can never succeed.
> > > +    if (bad_kuap_fault(regs, address, is_write)) {
> > > +        pr_crit_ratelimited("Kernel attempted to %s user page (%lx) - exploit attempt? (uid: 
> > > %d)\n",
> > > +                    is_write ? "write" : "read", address,
> > > +                    from_kuid(&init_user_ns, current_uid()));
> > >           return true;
> > > +    }
> >
> >
> > With this I am wondering whether the WARN() in bad_kuap_fault() is
> > needed. A direct access of userspace address will trigger this, whereas
> > previously we used bad_kuap_fault() only to identify incorrect restore
> > of AMR register (ie, to identify kernel bugs). Hence a WARN() there was
> > useful. We loose that differentiation now?
>
> Yes, I wanted to remove the WARN(), see 
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/cc9129bdda1dbc2f0a09cf45fece7d0b0e690784.1605541983.git.christophe.leroy@xxxxxxxxxx/ 
>
> but I understood from Michael that maybe it was not a good idea, so I left it aside for now when 
> rebasing to v3.
>
> Yes previously we were able to differentiate between a direct access of userspace and a valid access 
> triggering a KUAP fault, but at the cost of the heavy search_exception_tables().
> The issue was reported by Nick through https://github.com/linuxppc/issues/issues/317
>
> Should be perform the search_exception_tables() once we have hit the KUAP fault and WARN() only in 
> that case ?

I sent out v4 which does that: only emit the warning once we know it is a KUAP fault within an 
uaccess routine. With that, we should be back more or less as before: warning only if we hit KUAP 
fault AND it is a place where a userspace access should be granted.
We are not anymore in the fast hot path, so calling search_exception_tables() there should be a 
performance issue.

Christophe


> I was wondering also if we should keep the WARN() only when CONFIG_PPC_KUAP_DEBUG is set ?