On Wed, Apr 29, 2020 at 09:53:28AM -0400, Waiman Long wrote:
> 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.
>
> [ 38.579769] =============================================================================
> [ 38.580845] BUG dentry(666:pmcd.service) (Tainted: G B): Freepointer corrupt
> [ 38.581948] -----------------------------------------------------------------------------
>
> 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@xxxxxxxxxx>
> ---
> mm/slub.c | 45 ++++++++++++++++++++++++++++++---------------
> 1 file changed, 30 insertions(+), 15 deletions(-)
>
> [v3: Change function name to freeptr_outside_object(), update check & add comment]
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 9bf44955c4f1..b762450fc9f0 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -551,15 +551,32 @@ 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;
> }
> @@ -686,10 +703,7 @@ 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);
> @@ -782,7 +796,7 @@ 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)
> @@ -816,11 +830,7 @@ 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 */
> @@ -907,7 +917,7 @@ 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.
> @@ -3587,6 +3597,11 @@ 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 *);
> --
> 2.18.1
>
>
Acked-by: Rafael Aquini <aquini@xxxxxxxxxx>
