linux-kernel - Re: [PATCH v2 09/14] mm: Pcpu chunk address tag reset

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Message-ID: <CA+fCnZdZpiu+guJjE20f8kwzwoPkx4X=JveQpeU38USEvFyZ7g@mail.gmail.com>
Date: Thu, 20 Mar 2025 18:47:59 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, kees@...nel.org, 
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Subject: Re: [PATCH v2 09/14] mm: Pcpu chunk address tag reset

On Thu, Mar 20, 2025 at 6:40 PM Andrey Ryabinin <ryabinin.a.a@...il.com> wrote:
>
> On Tue, Feb 18, 2025 at 9:19 AM Maciej Wieczor-Retman
> <maciej.wieczor-retman@...el.com> wrote:
> >
> > The problem presented here is related to NUMA systems and tag-based
> > KASAN mode. Getting to it can be explained in the following points:
> >
> >         1. A new chunk is created with pcpu_create_chunk() and
> >            vm_structs are allocated. On systems with one NUMA node only
> >            one is allocated, but with more NUMA nodes at least a second
> >            one will be allocated too.
> >
> >         2. chunk->base_addr is assigned the modified value of
> >            vms[0]->addr and thus inherits the tag of this allocated
> >            structure.
> >
> >         3. In pcpu_alloc() for each possible cpu pcpu_chunk_addr() is
> >            executed which calculates per cpu pointers that correspond to
> >            the vms structure addresses. The calculations are based on
> >            adding an offset from a table to chunk->base_addr.
> >
> > Here the problem presents itself since for addresses based on vms[1] and
> > up, the tag will be different than the ones based on vms[0] (base_addr).
> > The tag mismatch happens and an error is reported.
> >
> > Reset the base_addr tag, since it will disable tag checks for pointers
> > derived arithmetically from base_addr that would inherit its tag.
> >
> > Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
> > ---
> >  mm/percpu-vm.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/percpu-vm.c b/mm/percpu-vm.c
> > index cd69caf6aa8d..e13750d804f7 100644
> > --- a/mm/percpu-vm.c
> > +++ b/mm/percpu-vm.c
> > @@ -347,7 +347,7 @@ static struct pcpu_chunk *pcpu_create_chunk(gfp_t gfp)
> >         }
> >
> >         chunk->data = vms;
> > -       chunk->base_addr = vms[0]->addr - pcpu_group_offsets[0];
> > +       chunk->base_addr = kasan_reset_tag(vms[0]->addr) - pcpu_group_offsets[0];
>
> This looks like a generic tags mode bug. I mean that arm64 is also
> affected by this.
> I assume it just wasn't noticed before because arm64 with multiple
> NUMAs are much less common.
>
> With this change tag-mode KASAN won't be able to catch bugus accesses
> to pcpu areas.
> I'm thinking it would be better to fix this on the pcpu_get_vm_areas()
> area side by replacing
> this
>     for (area = 0; area < nr_vms; area++)
>         vms[area]->addr = kasan_unpoison_vmalloc(vms[area]->addr,
>                                              vms[area]->size,
> KASAN_VMALLOC_PROT_NORMAL);
>
> with something like
>     kasan_unpoison_vmap_areas(vms, nr_vms);
> which will unpoison all areas using the same tag.
>
> Thoughts?

Just a side note: KASAN doesn't have proper handling of the percpu
areas anyway, I even had to remove a related test; see [1] and [2]. So
I think for now we can go with the simplest/consistent solution that
prevents false-positives, whichever that solution is.

[1] https://bugzilla.kernel.org/show_bug.cgi?id=215019
[2] https://bugzilla.kernel.org/show_bug.cgi?id=215758