Re: [PATCH bpf-next v4 5/6] bpf, sockmap: fix integer overflow in bpf_msg_pop_data() bounds check

[John Fastabend <john.fastabend@gmail.com>]

On Mon, Jun 15, 2026 at 10:19:58AM +0800, Jiayuan Chen wrote:
>From: Sechang Lim <rhkrqnwk98@gmail.com>
>
>start and len are u32, so
>
>	u64 last = start + len;
>
>evaluates start + len in 32-bit and wraps before storing it in last.
>The bounds check
>
>	if (start >= offset + l || last > msg->sg.size)
>		return -EINVAL;
>
>can then be passed with an out-of-range start/len, after which the pop
>loop runs off the end of the scatterlist and sk_msg_shift_left() calls
>put_page() on the empty msg->sg.end slot:
>
>  Oops: general protection fault, probably for non-canonical address
>  0xdffffc0000000001: 0000 [#1] SMP KASAN PTI
>  KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
>  RIP: 0010:sk_msg_shift_left net/core/filter.c:2957 [inline]
>  RIP: 0010:____bpf_msg_pop_data net/core/filter.c:3103 [inline]
>  RIP: 0010:bpf_msg_pop_data+0x753/0x1a10 net/core/filter.c:2984
>  Call Trace:
>   <TASK>
>   bpf_prog_4cc92c278f4d5d56+0x1b1/0x1e8
>   bpf_prog_run_pin_on_cpu+0x107/0x320 include/linux/filter.h:746
>   sk_psock_msg_verdict+0x357/0x7f0 net/core/skmsg.c:934
>   tcp_bpf_send_verdict net/ipv4/tcp_bpf.c:420 [inline]
>   tcp_bpf_sendmsg+0x766/0x1ae0 net/ipv4/tcp_bpf.c:583
>   __sock_sendmsg+0x153/0x1c0 net/socket.c:802
>   __sys_sendto+0x326/0x430 net/socket.c:2265
>   __x64_sys_sendto+0xe3/0x100 net/socket.c:2268
>   do_syscall_64+0x14c/0x480
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
>   </TASK>
>
>Widen the addition with a (u64) cast so the bound is evaluated in
>64-bit and a len near U32_MAX no longer wraps below msg->sg.size.
>
>While here, change pop from int to u32. It counts bytes against the
>unsigned scatterlist lengths and can never be negative, so the signed
>type only invites sign-confusion in the pop loop.
>
>Fixes: 7246d8ed4dcc ("bpf: helper to pop data from messages")
>Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
>Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
>Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
>Signed-off-by: Sechang Lim <rhkrqnwk98@gmail.com>
>Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>

Reviewed-by: John Fastabend <john.fastabend@gmail.com>