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

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Message-ID: <CAPAsAGxDRv_uFeMYu9TwhBVWHCCtkSxoWY4xmFB_vowMbi8raw@mail.gmail.com>
Date: Thu, 20 Mar 2025 18:39:35 +0100
From: Andrey Ryabinin <ryabinin.a.a@...il.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
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Subject: Re: [PATCH v2 09/14] mm: Pcpu chunk address tag reset

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?