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Message-ID: <CAM0EoMkMfvpmxkcSyqC0dOLKDH8_JiJ74u06x7sqUHSehgjOtQ@mail.gmail.com> Date: Wed, 4 Oct 2023 11:16:50 -0400 From: Jamal Hadi Salim <jhs@...atatu.com> To: "Gustavo A. R. Silva" <gustavoars@...nel.org> Cc: Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org, Kees Cook <keescook@...omium.org> Subject: Re: [PATCH v2] net: sched: cls_u32: Fix allocation size in u32_init() On Wed, Oct 4, 2023 at 9:19 AM Gustavo A. R. Silva <gustavoars@...nel.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 <alx@...nel.org> > Closes: https://lore.kernel.org/lkml/09b4a2ce-da74-3a19-6961-67883f634d98@kernel.org/ > Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org> Acked-by: Jamal Hadi Salim <jhs@...atatu.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 >
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