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

[Christophe Leroy]

Le 08/12/2020 à 14:00, Aneesh Kumar K.V a écrit :
> On 12/8/20 2:07 PM, Christophe Leroy wrote:
> > 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;
> > +    }
>
>
> Should we update bad_kuap_fault to check for !is_kernel_addr() and error_code & (DSISIR_PROT_FAULT | 
> DSISIR_KEYFAULT). I am wondering whether we can take another fault w.r.t kernel address/user address 
> and end up reporting that as KUAP fault?

Just before this we do:

	if (address >= TASK_SIZE)
		return true;

About the error code, I don't know. Can we take a fault that is not a DSISR_PROT_FAULT |
 DSISR_KEYFAULT and that is not a KUAP fault ?

Previously (before this patch), the error code was taken into account for the call to 
search_exception_tables(), but has never been for the bad_kuap_fault().

> > -    // What's left? Kernel fault on user in well defined regions (extable
> > -    // matched), and allowed by KUAP in the faulting context.
> > +    // What's left? Kernel fault on user and allowed by KUAP in the faulting context.
> >       return false;
> >   }
>
>
> -aneesh

Christophe