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    SubjectRe: [mm/slub] 3616799128: BUG_kmalloc-#(Not_tainted):kmalloc_Redzone_overwritten
    On Tue, Aug 02, 2022 at 06:30:44PM +0800, Dmitry Vyukov wrote:
    > .On Tue, 2 Aug 2022 at 11:43, Vlastimil Babka <vbabka@suse.cz> wrote:
    > >
    > > On 8/2/22 09:06, Dmitry Vyukov wrote:
    > > > On Tue, 2 Aug 2022 at 08:55, Feng Tang <feng.tang@intel.com> wrote:
    > > >>
    > > >> On Mon, Aug 01, 2022 at 10:23:23PM +0800, Vlastimil Babka wrote:
    > > >> > On 8/1/22 08:21, Feng Tang wrote:
    > > >> [snip]
    > > >> > > Cc kansan mail list.
    > > >> > >
    > > >> > > This is really related with KASAN debug, that in free path, some
    > > >> > > kmalloc redzone ([orig_size+1, object_size]) area is written by
    > > >> > > kasan to save free meta info.
    > > >> > >
    > > >> > > The callstack is:
    > > >> > >
    > > >> > > kfree
    > > >> > > slab_free
    > > >> > > slab_free_freelist_hook
    > > >> > > slab_free_hook
    > > >> > > __kasan_slab_free
    > > >> > > ____kasan_slab_free
    > > >> > > kasan_set_free_info
    > > >> > > kasan_set_track
    > > >> > >
    > > >> > > And this issue only happens with "kmalloc-16" slab. Kasan has 2
    > > >> > > tracks: alloc_track and free_track, for x86_64 test platform, most
    > > >> > > of the slabs will reserve space for alloc_track, and reuse the
    > > >> > > 'object' area for free_track. The kasan free_track is 16 bytes
    > > >> > > large, that it will occupy the whole 'kmalloc-16's object area,
    > > >> > > so when kmalloc-redzone is enabled by this patch, the 'overwritten'
    > > >> > > error is triggered.
    > > >> > >
    > > >> > > But it won't hurt other kmalloc slabs, as kasan's free meta won't
    > > >> > > conflict with kmalloc-redzone which stay in the latter part of
    > > >> > > kmalloc area.
    > > >> > >
    > > >> > > So the solution I can think of is:
    > > >> > > * skip the kmalloc-redzone for kmalloc-16 only, or
    > > >> > > * skip kmalloc-redzone if kasan is enabled, or
    > > >> > > * let kasan reserve the free meta (16 bytes) outside of object
    > > >> > > just like for alloc meta
    > > >> >
    > > >> > Maybe we could add some hack that if both kasan and SLAB_STORE_USER is
    > > >> > enabled, we bump the stored orig_size from <16 to 16? Similar to what
    > > >> > __ksize() does.
    > > >>
    > > >> How about the following patch:
    > > >>
    > > >> ---
    > > >> diff --git a/mm/slub.c b/mm/slub.c
    > > >> index added2653bb0..33bbac2afaef 100644
    > > >> --- a/mm/slub.c
    > > >> +++ b/mm/slub.c
    > > >> @@ -830,6 +830,16 @@ static inline void set_orig_size(struct kmem_cache *s,
    > > >> if (!slub_debug_orig_size(s))
    > > >> return;
    > > >>
    > > >> +#ifdef CONFIG_KASAN
    > > >> + /*
    > > >> + * When kasan is enabled, it could save its free meta data in the
    > > >> + * start part of object area, so skip the kmalloc redzone check
    > > >> + * for small kmalloc slabs to avoid the data conflict.
    > > >> + */
    > > >> + if (s->object_size <= 32)
    > > >> + orig_size = s->object_size;
    > > >> +#endif
    > > >> +
    > > >> p += get_info_end(s);
    > > >> p += sizeof(struct track) * 2;
    > > >>
    > > >> I extend the size to 32 for potential's kasan meta data size increase.
    > > >> This is tested locally, if people are OK with it, I can ask for 0Day's
    > > >> help to verify this.
    > >
    > > Is there maybe some KASAN macro we can use instead of hardcoding 32?
    >
    > kasan_free_meta is placed in the object data after freeing, so it can
    > be sizeof(kasan_free_meta)

    'kasan_free_meta' is defined in mm/kasan/kasan.h, to use it we need to
    include "../kasan/kasan.h" in slub.c, or move its definition to
    "include/linux/kasan.h"

    Another idea is to save the info in kasan_info, like:

    ---
    diff --git a/include/linux/kasan.h b/include/linux/kasan.h
    index b092277bf48d..97e899948d0b 100644
    --- a/include/linux/kasan.h
    +++ b/include/linux/kasan.h
    @@ -100,6 +100,7 @@ static inline bool kasan_has_integrated_init(void)
    struct kasan_cache {
    int alloc_meta_offset;
    int free_meta_offset;
    + int free_meta_size;
    bool is_kmalloc;
    };

    diff --git a/mm/kasan/common.c b/mm/kasan/common.c
    index c40c0e7b3b5f..7bd82c5ec264 100644
    --- a/mm/kasan/common.c
    +++ b/mm/kasan/common.c
    @@ -178,6 +178,8 @@ void __kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
    return;
    }

    + cache->kasan_info.free_meta_size = sizeof(struct free_meta_offset);
    +
    /*
    * Add free meta into redzone when it's not possible to store
    * it in the object. This is the case when:
    Thanks,
    Feng

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