Re: [PATCH bpf v2 1/2] bpf, sockmap: fix integer overflow in bpf_msg_pop_data() bounds check

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Wed Jun 10, 2026 at 1:11 AM PDT, Sechang Lim wrote:
> 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")
> Signed-off-by: Sechang Lim <rhkrqnwk98@gmail.com>
> ---
>  net/core/filter.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 80439767e0ee..9cdfec2ca11e 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2974,8 +2974,8 @@ BPF_CALL_4(bpf_msg_pop_data, struct sk_msg *, msg, u32, start,
>  	   u32, len, u64, flags)
>  {
>  	u32 i = 0, l = 0, space, offset = 0;
> -	u64 last = start + len;
> -	int pop;
> +	u64 last = (u64)start + len;

sashiko is correct that there are 4 other issue in very similar code path
all in skmsg. Please fix them all in one go.

pw-bot: cr