On Wed, Oct 4, 2023 at 9:19 AM Gustavo A. R. Silva
<gustavo...@kernel.org> wrote:
>
> commit d61491a51f7e ("net/sched: cls_u32: Replace one-element array
> with flexible-array member") incorrecly replaced an instance of
> `sizeof(*tp_c)` with `struct_size(tp_c, hlist->ht, 1)`. This results
> in a an over-allocation of 8 bytes.
>
> This change is wrong because `hlist` in `struct tc_u_common` is a
> pointer:
>
> net/sched/cls_u32.c:
> struct tc_u_common {
> struct tc_u_hnode __rcu *hlist;
> void *ptr;
> int refcnt;
> struct idr handle_idr;
> struct hlist_node hnode;
> long knodes;
> };
>
> So, the use of `struct_size()` makes no sense: we don't need to allocate
> any extra space for a flexible-array member. `sizeof(*tp_c)` is just fine.
>
> So, `struct_size(tp_c, hlist->ht, 1)` translates to:
>
> sizeof(*tp_c) + sizeof(tp_c->hlist->ht) ==
> sizeof(struct tc_u_common) + sizeof(struct tc_u_knode *) ==
> 144 + 8 == 0x98 (byes)
> ^^^
> |
> unnecessary extra
> allocation size
>
> $ pahole -C tc_u_common net/sched/cls_u32.o
> struct tc_u_common {
> struct tc_u_hnode * hlist; /* 0 8 */
> void * ptr; /* 8 8 */
> int refcnt; /* 16 4 */
>
> /* XXX 4 bytes hole, try to pack */
>
> struct idr handle_idr; /* 24 96 */
> /* --- cacheline 1 boundary (64 bytes) was 56 bytes ago --- */
> struct hlist_node hnode; /* 120 16 */
> /* --- cacheline 2 boundary (128 bytes) was 8 bytes ago --- */
> long int knodes; /* 136 8 */
>
> /* size: 144, cachelines: 3, members: 6 */
> /* sum members: 140, holes: 1, sum holes: 4 */
> /* last cacheline: 16 bytes */
> };
>
> And with `sizeof(*tp_c)`, we have:
>
> sizeof(*tp_c) == sizeof(struct tc_u_common) == 144 == 0x90 (bytes)
>
> which is the correct and original allocation size.
>
> Fix this issue by replacing `struct_size(tp_c, hlist->ht, 1)` with
> `sizeof(*tp_c)`, and avoid allocating 8 too many bytes.
>
> The following difference in binary output is expected and reflects the
> desired change:
>
> | net/sched/cls_u32.o
> | @@ -6148,7 +6148,7 @@
> | include/linux/slab.h:599
> | 2cf5: mov 0x0(%rip),%rdi # 2cfc <u32_init+0xfc>
> | 2cf8: R_X86_64_PC32 kmalloc_caches+0xc
> |- 2cfc: mov $0x98,%edx
> |+ 2cfc: mov $0x90,%edx
>
> Reported-by: Alejandro Colomar <a...@kernel.org>
> Closes:
> https://lore.kernel.org/lkml/09b4a2ce-da74-3a19-6961-67883f634...@kernel.org/
> Signed-off-by: Gustavo A. R. Silva <gustavo...@kernel.org>
Acked-by: Jamal Hadi Salim <j...@mojatatu.com>
cheers,
jamal
> Changes in v2:
> - Update subject line.
> - Update changelog text.
>
> v1:
> - Link: https://lore.kernel.org/linux-hardening/ZN5DvRyq6JNz20l1@work/
>
> net/sched/cls_u32.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
> index da4c179a4d41..6663e971a13e 100644
> --- a/net/sched/cls_u32.c
> +++ b/net/sched/cls_u32.c
> @@ -366,7 +366,7 @@ static int u32_init(struct tcf_proto *tp)
> idr_init(&root_ht->handle_idr);
>
> if (tp_c == NULL) {
> - tp_c = kzalloc(struct_size(tp_c, hlist->ht, 1), GFP_KERNEL);
> + tp_c = kzalloc(sizeof(*tp_c), GFP_KERNEL);
> if (tp_c == NULL) {
> kfree(root_ht);
> return -ENOBUFS;
> --
> 2.34.1
>
