Re: [PATCH] kexec: Annotate struct crash_mem with __counted_by

[Baoquan He <bhe@redhat.com>]

On 09/22/23 at 08:25pm, Kees Cook wrote:
> On Sat, Sep 23, 2023 at 08:46:47AM +0800, Baoquan He wrote:
> > On 09/22/23 at 10:52am, Kees Cook 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).
> > > 
> > > As found with Coccinelle[1], add __counted_by for struct crash_mem.
> > > 
> > > [1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci
> > > 
> > > Cc: Eric Biederman <ebiederm@xmission.com>
> > > Cc: kexec@lists.infradead.org
> > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > > ---
> > >  include/linux/crash_core.h | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h
> > > index 3426f6eef60b..5126a4fecb44 100644
> > > --- a/include/linux/crash_core.h
> > > +++ b/include/linux/crash_core.h
> > > @@ -131,7 +131,7 @@ static inline void __init reserve_crashkernel_generic(char *cmdline,
> > >  struct crash_mem {
> > >  	unsigned int max_nr_ranges;
> > >  	unsigned int nr_ranges;
> > > -	struct range ranges[];
> > > +	struct range ranges[] __counted_by(max_nr_ranges);
> > 
> > This __counted_by() only makes sense when there's a obvious upper
> > boundary, max_nr_ranges in this case.
> 
> Yes; it's designed to be the array element count used for the
> allocation. For example with the above case:
> 
>         nr_ranges += 2;
>         cmem = vzalloc(struct_size(cmem, ranges, nr_ranges));
>         if (!cmem)
>                 return NULL;
> 
>         cmem->max_nr_ranges = nr_ranges;
>         cmem->nr_ranges = 0;
> 
> nr_ranges is the max count of the elements.
> 
> _However_, if a structure (like this one) has _two_ counters, one for
> "in use" and another for "max available", __counted_by could specify the
> "in use" case, as long as array indexing only happens when that "in use"
> has been updated. So, if it were:
> 
> struct crash_mem {
>     unsigned int max_nr_ranges;
>     unsigned int nr_ranges;
>     struct range ranges[] __counted_by(nr_ranges);
> };
> 
> then this would trigger the bounds checking:
> 
> 	cmem->ranges[0] = some_range;	/* "nr_ranges" is still 0 so index 0 isn't allowed */
> 	cmem->nr_ranges ++;
> 
> but this would not:
> 
> 	cmem->nr_ranges ++;		/* index 0 is now available for use. */
> 	cmem->ranges[0] = some_range;
> 
> > This heavily depends and isn't much in kernel?
> 
> Which "this" do you mean? The tracking of max allocation is common.
> Tracking max and "in use" happens in some places (like here), but is
> less common.

I thought usually it may not have a max counter of the variable length
array embeded in struct, seems I was wrong. Here 'this' means the
__counted_by() adding for the variable length array.

> 
> > E.g struct swap_info_struct->avail_lists[].
> 
> This is even less common: tracking the count externally from the struct,
> as done there with nr_node_ids. Shakeel asked a very similar question
> and also pointed out nr_node_ids:
> https://lore.kernel.org/all/202309221128.6AC35E3@keescook/
> 
> > Just curious, not related to this patch though.
> 
> I'm happy to answer questions! Yeah, as I said in the above thread,
> I expect to expand what __counted_by can use, and I suspect (hope)
> a global would be easier to add than an arbitrary expression. :)

Thanks a lot for these explanation, Kees.

LGTM,
Acked-by: Baoquan He <bhe@redhat.com>