CVE-2026-74256: bpf, sockmap: fix integer overflow in bpf_msg_pop_data() bounds check

In the Linux kernel, the following vulnerability has been resolved:

bpf, sockmap: fix integer overflow in bpfmsgpopdata() bounds check

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 skmsgshiftleft() calls putpage() 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:skmsgshiftleft net/core/filter.c:2957 [inline] RIP: 0010:bpfmsgpopdata net/core/filter.c:3103 [inline] RIP: 0010:bpfmsgpopdata+0x753/0x1a10 net/core/filter.c:2984 Call Trace: <TASK> bpfprog4cc92c278f4d5d56+0x1b1/0x1e8 bpfprogrunpinoncpu+0x107/0x320 include/linux/filter.h:746 skpsockmsgverdict+0x357/0x7f0 net/core/skmsg.c:934 tcpbpfsendverdict net/ipv4/tcpbpf.c:420 [inline] tcpbpfsendmsg+0x766/0x1ae0 net/ipv4/tcpbpf.c:583 socksendmsg+0x153/0x1c0 net/socket.c:802 syssendto+0x326/0x430 net/socket.c:2265 x64syssendto+0xe3/0x100 net/socket.c:2268 dosyscall64+0x14c/0x480 entrySYSCALL64afterhwframe+0x77/0x7f </TASK>

Widen the addition with a (u64) cast so the bound is evaluated in 64-bit and a len near U32MAX 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.