Re: [PATCH v1 4/5] powerpc/fault: Avoid heavy search_exception_tables() verification

[Christophe Leroy]

Le 09/09/2020 à 08:04, 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>
> > ---
> >   arch/powerpc/mm/fault.c | 20 +++++---------------
> >   1 file changed, 5 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> > index 525e0c2b5406..edde169ba3a6 100644
> > --- a/arch/powerpc/mm/fault.c
> > +++ b/arch/powerpc/mm/fault.c
> > @@ -214,24 +214,14 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code,
> >   	if (address >= TASK_SIZE)
> >   		return true;
> >   
> > -	if (!is_exec && (error_code & DSISR_PROTFAULT) &&
> > -	    !search_exception_tables(regs->nip)) {
> > +	// 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 access user page (%lx) - exploit attempt? (uid: %d)\n",
> > -				    address,
> > -				    from_kuid(&init_user_ns, current_uid()));
> > -	}
> > -
> > -	// Fault on user outside of certain regions (eg. copy_tofrom_user()) is bad
> > -	if (!search_exception_tables(regs->nip))
> > -		return true;
>
> We still need to keep this ? Without that we detect the lack of
> exception tables pretty late.

Is that a problem at all to detect the lack of exception tables late ?
That case is very unlikely and will lead to failure anyway. So, is it 
worth impacting performance of the likely case which will always have an 
exception table and where we expect the exception to run as fast as 
possible ?

The other architectures I have looked at (arm64 and x86) only have the 
exception table search together with the down_read_trylock(&mm->mmap_sem).

Christophe