[PATCH AUTOSEL 6.0 72/77] net: sched: cls_u32: Avoid memcpy() false-positive warning

Sasha Levin
2022-10-09T22:20:55Z

From: Kees Cook <keescook@chromium.org>

[ Upstream commit 7cba18332e3635aaae60e4e7d4e52849de50d91b ]

To work around a misbehavior of the compiler's ability to see into
composite flexible array structs (as detailed in the coming memcpy()
hardening series[1]), use unsafe_memcpy(), as the sizing,
bounds-checking, and allocation are all very tightly coupled here.
This silences the false-positive reported by syzbot:

memcpy: detected field-spanning write (size 80) of single field "&n->sel" at net/sched/cls_u32.c:1043 (size 16)

[1] https://lore.kernel.org/linux-hardening/20220901065914.1417829-2-keescook@chromium.org

Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Reported-by: syzbot+a2c4601efc75848ba321@syzkaller.appspotmail.com
Link: https://lore.kernel.org/lkml/000000000000a96c0b05e97f0444@google.com/
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://lore.kernel.org/r/20220927153700.3071688-1-keescook@chromium.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/sched/cls_u32.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index 4d27300c287c..5f33472aad36 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -1040,7 +1040,11 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
}
#endif

- memcpy(&n->sel, s, sel_size);
+ unsafe_memcpy(&n->sel, s, sel_size,
+ /* A composite flex-array structure destination,
+ * which was correctly sized with struct_size(),
+ * bounds-checked against nla_len(), and allocated
+ * above. */);
RCU_INIT_POINTER(n->ht_up, ht);
n->handle = handle;
n->fshift = s->hmask ? ffs(ntohl(s->hmask)) - 1 : 0;
--
2.35.1