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    SubjectRe: [PATCH] slub: Fixes freepointer encoding for single free
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    On 2024/4/25 23:02, Nicolas Bouchinet wrote:
    > Hy,
    >
    > First of all, thanks a lot for your time.
    >
    > On 4/25/24 10:36, Chengming Zhou wrote:
    >> On 2024/4/24 20:47, Nicolas Bouchinet wrote:
    >>> From: Nicolas Bouchinet<nicolas.bouchinet@ssi.gouv.fr>
    >>>
    >>> Commit 284f17ac13fe ("mm/slub: handle bulk and single object freeing
    >>> separately") splits single and bulk object freeing in two functions
    >>> slab_free() and slab_free_bulk() which leads slab_free() to call
    >>> slab_free_hook() directly instead of slab_free_freelist_hook().
    >> Right.
    >>
    >>> If `init_on_free` is set, slab_free_hook() zeroes the object.
    >>> Afterward, if `slub_debug=F` and `CONFIG_SLAB_FREELIST_HARDENED` are
    >>> set, the do_slab_free() slowpath executes freelist consistency
    >>> checks and try to decode a zeroed freepointer which leads to a
    >>> "Freepointer corrupt" detection in check_object().
    >> IIUC, the "freepointer" can be checked on the free path only when
    >> it's outside the object memory. Here slab_free_hook() zeroed the
    >> freepointer and caused the problem.
    >>
    >> But why we should zero the memory outside the object_size? It seems
    >> more reasonable to only zero the object_size when init_on_free is set?
    >
    > The original purpose was to avoid leaking information through the object and its metadata / tracking information as described in init_on_free initial Commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options").
    >
    > I have to admit I didn't read the entire lore about the original patchset yet, though it could be interesting to know a bit more the threat models, specifically regarding the object metadata init.

    Thank you for the reference! I also don't get why it needs to zero
    the metadata and tracking information.

    >
    > The patch could also be optimized a bit by restricting set_freepointer() call to the `CONFIG_SLAB_FREELIST_HARDENED` option value.
    >

    Yeah. Maybe memcg_alloc_abort_single() needs this too.

    Thanks.

    > Thanks again, Nicolas
    >
    >>
    >> Thanks.
    >>
    >>> Object's freepointer thus needs to be properly set using
    >>> set_freepointer() after init_on_free.
    >>>
    >>> To reproduce, set `slub_debug=FU init_on_free=1 log_level=7` on the
    >>> command line of a kernel build with `CONFIG_SLAB_FREELIST_HARDENED=y`.
    >>>
    >>> dmesg sample log:
    >>> [   10.708715] =============================================================================
    >>> [   10.710323] BUG kmalloc-rnd-05-32 (Tainted: G    B           T ): Freepointer corrupt
    >>> [   10.712695] -----------------------------------------------------------------------------
    >>> [   10.712695]
    >>> [   10.712695] Slab 0xffffd8bdc400d580 objects=32 used=4 fp=0xffff9d9a80356f80 flags=0x200000000000a00(workingset|slab|node=0|zone=2)
    >>> [   10.716698] Object 0xffff9d9a80356600 @offset=1536 fp=0x7ee4f480ce0ecd7c
    >>> [   10.716698]
    >>> [   10.716698] Bytes b4 ffff9d9a803565f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    >>> [   10.720703] Object   ffff9d9a80356600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    >>> [   10.720703] Object   ffff9d9a80356610: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    >>> [   10.724696] Padding  ffff9d9a8035666c: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    >>> [   10.724696] Padding  ffff9d9a8035667c: 00 00 00 00                                      ....
    >>> [   10.724696] FIX kmalloc-rnd-05-32: Object at 0xffff9d9a80356600 not freed
    >>>
    >>> Signed-off-by: Nicolas Bouchinet<nicolas.bouchinet@ssi.gouv.fr>
    >>> ---
    >>>   mm/slub.c | 8 +++++++-
    >>>   1 file changed, 7 insertions(+), 1 deletion(-)
    >>>
    >>> diff --git a/mm/slub.c b/mm/slub.c
    >>> index 3aa12b9b323d9..71dbff9ad8f17 100644
    >>> --- a/mm/slub.c
    >>> +++ b/mm/slub.c
    >>> @@ -4342,10 +4342,16 @@ static __fastpath_inline
    >>>   void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
    >>>              unsigned long addr)
    >>>   {
    >>> +    bool init = false;
    >>> +
    >>>       memcg_slab_free_hook(s, slab, &object, 1);
    >>> +    init = slab_want_init_on_free(s);
    >>>   -    if (likely(slab_free_hook(s, object, slab_want_init_on_free(s))))
    >>> +    if (likely(slab_free_hook(s, object, init))) {
    >>> +        if (init)
    >>> +            set_freepointer(s, object, NULL);
    >>>           do_slab_free(s, slab, object, object, 1, addr);
    >>> +    }
    >>>   }
    >>>     static __fastpath_inline

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