linux-kernel - Re: [PATCH 1/1] pgo: Fix allocate_node() handling of non-vmlinux nodes.

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Date:   Wed, 02 Jun 2021 21:52:49 +0300
From:   jarmo.tiitto@...il.com
To:     Jarmo Tiitto <jarmo.tiitto@...il.com>,
        Kees Cook <keescook@...omium.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Bill Wendling <wcw@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org,
        morbo@...gle.com
Subject: Re: [PATCH 1/1] pgo: Fix allocate_node() handling of non-vmlinux nodes.

Kees Cook wrote keskiviikkona 2. kesäkuuta 2021 20.41.28 EEST:
> On Wed, Jun 02, 2021 at 03:57:04AM +0300, Jarmo Tiitto wrote:
> > Currently allocate_node() will reserve nodes even if *p
> > doesn't point into __llvm_prf_data_start - __llvm_prf_data_end
> > range.
> > 
> > Fix it by checking if p points into vmlinux range
> > and otherwise return NULL.
> > 
> > Signed-off-by: Jarmo Tiitto <jarmo.tiitto@...il.com>
> > ---
> >  kernel/pgo/instrument.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/kernel/pgo/instrument.c b/kernel/pgo/instrument.c
> > index 0e07ee1b17d9..9bca535dfa91 100644
> > --- a/kernel/pgo/instrument.c
> > +++ b/kernel/pgo/instrument.c
> > @@ -55,6 +55,10 @@ void prf_unlock(unsigned long flags)
> >  static struct llvm_prf_value_node *allocate_node(struct llvm_prf_data *p,
> >  						 u32 index, u64 value)
> >  {
> > +	/* check if p points into vmlinux. If not, don't allocate. */
> > +	if (p < __llvm_prf_data_start || p >= __llvm_prf_data_end)
> > +		return NULL;
> 
> This should be a tighter check (struct llvm_prf_data has size, so just
> checking for p < __llvm_prf_data_end isn't sufficient. I recommend using
> the memory_contains() helper.
> 
> And I think this should be louder as it's entirely unexpected right
> now. Perhaps:
> 
> 	if (WARN_ON_ONCE(!memory_contains(__llvm_prf_data_start,
> 					  __llvm_prf_data_end,
> 					  p, sizeof(*p))))
> 		return NULL;
> 
> > +
> >  	if (&__llvm_prf_vnds_start[current_node + 1] >= __llvm_prf_vnds_end)
> >  		return NULL; /* Out of nodes */
> >  
> > -- 
> > 2.31.1
> > 
> 
> -- 
> Kees Cook
> 


Well, if you do that the WARN_ON_ONCE() will always trigger, unless CONFIG_MODULES=n 😇

This is because 'struct llvm_prf_data *p' argument is expected
(at least I think so) to point into __llvm_prf_data section in vmlinux
and also into each module's own __llvm_prf_data section.

So in the end the compiler supplied pointer is likely correct,
but the current v9 PGO patch attempts to reserve all vnodes
from vmlinux __llvm_prf_vnds section, instead of respective module section.

I think it would be normal to ignore pointers here,
until my module PGO machinery is ready.

But I agree on using memory_contains() for checking if p is within bounds.
I will follow on with v2 of this patch. :-)

-Jarmo