Re: [PATCH] percpu: add basic double free check

[Dennis Zhou <dennis@kernel.org>]

Hi Chris,

On Wed, Jan 14, 2026 at 04:22:00AM -0800, Chris Mason wrote:
> On Fri, 19 Dec 2025 16:27:37 -0800 Dennis Zhou <dennis@kernel.org> wrote:
> 
> > This adds a basic double free check by validating the first bit of the
> > allocation in alloc_map and bound_map are set. If the alloc_map bit is
> > not set, then this means the area is currently unallocated. If the
> > bound_map bit is not set, then we are not freeing from the beginning of
> > the allocation.
> 
> Hi Dennis,
> 
> I ran mm-new through the AI review prompts, and this one was flagged:
> 

Wow that's cool, thanks!

> > diff --git a/mm/percpu.c b/mm/percpu.c
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> 
> [ ... ]
> 
> > @@ -1285,6 +1286,11 @@ static int pcpu_free_area(struct pcpu_chunk *chunk, int off)
> >
> > 	bit_off = off / PCPU_MIN_ALLOC_SIZE;
> >
> > +	/* check double free */
> > +	if (!test_bit(bit_off, chunk->alloc_map) ||
> > +	    !test_bit(bit_off, chunk->bound_map))
> > +		return 0;
> 
> Does this early return corrupt pcpu_stats.nr_cur_alloc and
> chunk->nr_alloc?
> 
> Looking at pcpu_free_area(), earlier in the function (before this new
> check) there is a call to pcpu_stats_area_dealloc(chunk) which decrements
> these counters:
> 
>     static int pcpu_free_area(struct pcpu_chunk *chunk, int off)
>     {
>         ...
>         lockdep_assert_held(&pcpu_lock);
>         pcpu_stats_area_dealloc(chunk);   <-- decrements counters here
>         ...
>         bit_off = off / PCPU_MIN_ALLOC_SIZE;
> 
>         /* check double free */
>         if (!test_bit(bit_off, chunk->alloc_map) ||
>             !test_bit(bit_off, chunk->bound_map))
>             return 0;                     <-- returns without actual free
>         ...
>     }
> 
> When a double-free is detected:
> 
>   1. pcpu_stats_area_dealloc() runs, decrementing pcpu_stats.nr_cur_alloc
>      and chunk->nr_alloc
>   2. The check detects the double-free and returns 0
>   3. No actual memory is freed, but the counters are now wrong
> 
> Should pcpu_stats_area_dealloc() be moved after the double-free check,
> or should there be a compensating increment on the early return path?

I moved pcpu_stats_area_dealloc() to the end of the function in v2.

> 
> This could also affect percpu_stats_show() in mm/percpu-stats.c, which
> uses chunk->nr_alloc via find_max_nr_alloc() to size a buffer:
> 
>     buffer = vmalloc_array(2 * max_nr_alloc + 1, sizeof(int));
> 
> If nr_alloc is underreported due to this bug, the buffer may be undersized
> for the actual number of allocations tracked in alloc_map, which
> chunk_map_stats() iterates based on the actual bitmap contents.
>