Re: [PATCH v2 2/2] net: openvswitch: Annotate struct mask_array with __counted_by

[Christophe JAILLET]

Le 15/10/2023 à 06:53, Julia Lawall a écrit :
> On Sat, 14 Oct 2023, Kees Cook wrote:
>
> > On Sat, Oct 14, 2023 at 08:34:53AM +0200, Christophe JAILLET wrote:
> > > Prepare for the coming implementation by GCC and Clang of the __counted_by
> > > attribute. Flexible array members annotated with __counted_by can have
> > > their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
> > > (for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
> > > functions).
> > >
> > > Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
> > > ---
> > > v2: Fix the subject  [Ilya Maximets]
> > >      fix the field name used with __counted_by  [Ilya Maximets]
> > >
> > > v1: https://lore.kernel.org/all/f66ddcf1ef9328f10292ea75a17b584359b6cde3.1696156198.git.christophe.jaillet@xxxxxxxxxx/
> > >
> > >
> > > This patch is part of a work done in parallel of what is currently worked
> > > on by Kees Cook.
> > >
> > > My patches are only related to corner cases that do NOT match the
> > > semantic of his Coccinelle script[1].
>
> What was the problem with the semantic patch in this case?


The allocation in tbl_mask_array_alloc() looks like:
	new = kzalloc(sizeof(struct mask_array) +
		      sizeof(struct sw_flow_mask *) * size +
		      sizeof(u64) * size, GFP_KERNEL);


We allocated the struct, the ending flex aray *and* some more memory at 
the same time.

IIUC the cocci script, this extra space is not taken into account with 
the current script and it won't match.

CJ


> thanks,
> julia
>
>
> > > In this case, in tbl_mask_array_alloc(), several things are allocated with
> > > a single allocation. Then, some pointer arithmetic computes the address of
> > > the memory after the flex-array.
> > >
> > > [1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci
> > > ---
> > >   net/openvswitch/flow_table.h | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/net/openvswitch/flow_table.h b/net/openvswitch/flow_table.h
> > > index 9e659db78c05..f524dc3e4862 100644
> > > --- a/net/openvswitch/flow_table.h
> > > +++ b/net/openvswitch/flow_table.h
> > > @@ -48,7 +48,7 @@ struct mask_array {
> > >   	int count, max;
> > >   	struct mask_array_stats __percpu *masks_usage_stats;
> > >   	u64 *masks_usage_zero_cntr;
> > > -	struct sw_flow_mask __rcu *masks[];
> > > +	struct sw_flow_mask __rcu *masks[] __counted_by(max);
> > >   };
> >
> > Yup, this looks correct to me. Thanks!
> >
> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> >
> > --
> > Kees Cook