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    Subject[PATCH AUTOSEL 5.4 257/330] mm/slub: fix incorrect interpretation of s->offset
    Date
    From: Waiman Long <longman@redhat.com>

    [ Upstream commit cbfc35a48609ceac978791e3ab9dde0c01f8cb20 ]

    In a couple of places in the slub memory allocator, the code uses
    "s->offset" as a check to see if the free pointer is put right after the
    object. That check is no longer true with commit 3202fa62fb43 ("slub:
    relocate freelist pointer to middle of object").

    As a result, echoing "1" into the validate sysfs file, e.g. of dentry,
    may cause a bunch of "Freepointer corrupt" error reports like the
    following to appear with the system in panic afterwards.

    =============================================================================
    BUG dentry(666:pmcd.service) (Tainted: G B): Freepointer corrupt
    -----------------------------------------------------------------------------

    To fix it, use the check "s->offset == s->inuse" in the new helper
    function freeptr_outside_object() instead. Also add another helper
    function get_info_end() to return the end of info block (inuse + free
    pointer if not overlapping with object).

    Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
    Signed-off-by: Waiman Long <longman@redhat.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
    Reviewed-by: Kees Cook <keescook@chromium.org>
    Acked-by: Rafael Aquini <aquini@redhat.com>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: Vitaly Nikolenko <vnik@duasynt.com>
    Cc: Silvio Cesare <silvio.cesare@gmail.com>
    Cc: Pekka Enberg <penberg@kernel.org>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: Markus Elfring <Markus.Elfring@web.de>
    Cc: Changbin Du <changbin.du@gmail.com>
    Link: http://lkml.kernel.org/r/20200429135328.26976-1-longman@redhat.com
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    mm/slub.c | 45 ++++++++++++++++++++++++++++++---------------
    1 file changed, 30 insertions(+), 15 deletions(-)

    diff --git a/mm/slub.c b/mm/slub.c
    index 822ba07245291..d69934eac9e94 100644
    --- a/mm/slub.c
    +++ b/mm/slub.c
    @@ -533,15 +533,32 @@ static void print_section(char *level, char *text, u8 *addr,
    metadata_access_disable();
    }

    +/*
    + * See comment in calculate_sizes().
    + */
    +static inline bool freeptr_outside_object(struct kmem_cache *s)
    +{
    + return s->offset >= s->inuse;
    +}
    +
    +/*
    + * Return offset of the end of info block which is inuse + free pointer if
    + * not overlapping with object.
    + */
    +static inline unsigned int get_info_end(struct kmem_cache *s)
    +{
    + if (freeptr_outside_object(s))
    + return s->inuse + sizeof(void *);
    + else
    + return s->inuse;
    +}
    +
    static struct track *get_track(struct kmem_cache *s, void *object,
    enum track_item alloc)
    {
    struct track *p;

    - if (s->offset)
    - p = object + s->offset + sizeof(void *);
    - else
    - p = object + s->inuse;
    + p = object + get_info_end(s);

    return p + alloc;
    }
    @@ -682,10 +699,7 @@ static void print_trailer(struct kmem_cache *s, struct page *page, u8 *p)
    print_section(KERN_ERR, "Redzone ", p + s->object_size,
    s->inuse - s->object_size);

    - if (s->offset)
    - off = s->offset + sizeof(void *);
    - else
    - off = s->inuse;
    + off = get_info_end(s);

    if (s->flags & SLAB_STORE_USER)
    off += 2 * sizeof(struct track);
    @@ -776,7 +790,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,
    * object address
    * Bytes of the object to be managed.
    * If the freepointer may overlay the object then the free
    - * pointer is the first word of the object.
    + * pointer is at the middle of the object.
    *
    * Poisoning uses 0x6b (POISON_FREE) and the last byte is
    * 0xa5 (POISON_END)
    @@ -810,11 +824,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct page *page,

    static int check_pad_bytes(struct kmem_cache *s, struct page *page, u8 *p)
    {
    - unsigned long off = s->inuse; /* The end of info */
    -
    - if (s->offset)
    - /* Freepointer is placed after the object. */
    - off += sizeof(void *);
    + unsigned long off = get_info_end(s); /* The end of info */

    if (s->flags & SLAB_STORE_USER)
    /* We also have user information there */
    @@ -900,7 +910,7 @@ static int check_object(struct kmem_cache *s, struct page *page,
    check_pad_bytes(s, page, p);
    }

    - if (!s->offset && val == SLUB_RED_ACTIVE)
    + if (!freeptr_outside_object(s) && val == SLUB_RED_ACTIVE)
    /*
    * Object and freepointer overlap. Cannot check
    * freepointer while object is allocated.
    @@ -3585,6 +3595,11 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
    *
    * This is the case if we do RCU, have a constructor or
    * destructor or are poisoning the objects.
    + *
    + * The assumption that s->offset >= s->inuse means free
    + * pointer is outside of the object is used in the
    + * freeptr_outside_object() function. If that is no
    + * longer true, the function needs to be modified.
    */
    s->offset = size;
    size += sizeof(void *);
    --
    2.25.1
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