Re: [PATCH] arm64: cacheinfo: Avoid out-of-bounds write to cacheinfo array

[Will Deacon <will@kernel.org>]

On Tue, Feb 04, 2025 at 10:55:53AM -0500, Radu Rendec wrote:
> On Tue, 2025-02-04 at 12:39 +0000, Will Deacon wrote:
> > On Thu, Jan 23, 2025 at 01:11:59PM -0500, Radu Rendec wrote:
> > > diff --git a/arch/arm64/kernel/cacheinfo.c b/arch/arm64/kernel/cacheinfo.c
> > > index d9c9218fa1fdd..77ffda7284754 100644
> > > --- a/arch/arm64/kernel/cacheinfo.c
> > > +++ b/arch/arm64/kernel/cacheinfo.c
> > > @@ -101,16 +101,18 @@ int populate_cache_leaves(unsigned int cpu)
> > >  	unsigned int level, idx;
> > >  	enum cache_type type;
> > >  	struct cpu_cacheinfo *this_cpu_ci = get_cpu_cacheinfo(cpu);
> > > -	struct cacheinfo *this_leaf = this_cpu_ci->info_list;
> > > +	struct cacheinfo *infos = this_cpu_ci->info_list;
> > >  
> > >  	for (idx = 0, level = 1; level <= this_cpu_ci->num_levels &&
> > > -	     idx < this_cpu_ci->num_leaves; idx++, level++) {
> > > +	     idx < this_cpu_ci->num_leaves; level++) {
> > >  		type = get_cache_type(level);
> > >  		if (type == CACHE_TYPE_SEPARATE) {
> > > -			ci_leaf_init(this_leaf++, CACHE_TYPE_DATA, level);
> > > -			ci_leaf_init(this_leaf++, CACHE_TYPE_INST, level);
> > > +			if (idx + 2 > this_cpu_ci->num_leaves)
> > > +				break;
> > 
> > Why are you checking 'idx + 2' rather than 'idx + 1'?
> 
> I don't like "magic constants", and I thought "idx + 2" would be more
> suggestive since 2 elements were added.
> 
> The check is correct though. For example, if this_cpu_ci->num_leaves = 3
> (the array size is 3) and idx = 2, then 2 + 2 > 3 is true, so you can't
> add two more elements. On the other hand, if idx = 1, then 1 + 2 > 3 is
> false, so you can add the two elements (at indices 1 and 2).
> 
> If there's a strong preference for "idx + 1", I can change it. But then
> of course ">" will need to change to ">=" as well.

Might just be me, but I'd personally find that clearer given that we're
assigning to infos[idx] and infos[idx + 1].

Thanks,

Will