CVE-2025-40079: riscv, bpf: Sign extend struct ops return values properly
In the Linux kernel, the following vulnerability has been resolved:

riscv, bpf: Sign extend struct ops return values properly

The ns_bpf_qdisc selftest triggers a kernel panic:

    Unable to handle kernel paging request at virtual address ffffffffa38dbf58
    Current test_progs pgtable: 4K pagesize, 57-bit VAs, pgdp=0x00000001109cc000
    [ffffffffa38dbf58] pgd=000000011fffd801, p4d=000000011fffd401, pud=000000011fffd001, pmd=0000000000000000
    Oops [#1]
    Modules linked in: bpf_testmod(OE) xt_conntrack nls_iso8859_1 [...] [last unloaded: bpf_testmod(OE)]
    CPU: 1 UID: 0 PID: 23584 Comm: test_progs Tainted: G        W  OE       6.17.0-rc1-g2465bb83e0b4 #1 NONE
    Tainted: [W]=WARN, [O]=OOT_MODULE, [E]=UNSIGNED_MODULE
    Hardware name: Unknown Unknown Product/Unknown Product, BIOS 2024.01+dfsg-1ubuntu5.1 01/01/2024
    epc : __qdisc_run+0x82/0x6f0
     ra : __qdisc_run+0x6e/0x6f0
    epc : ffffffff80bd5c7a ra : ffffffff80bd5c66 sp : ff2000000eecb550
     gp : ffffffff82472098 tp : ff60000096895940 t0 : ffffffff8001f180
     t1 : ffffffff801e1664 t2 : 0000000000000000 s0 : ff2000000eecb5d0
     s1 : ff60000093a6a600 a0 : ffffffffa38dbee8 a1 : 0000000000000001
     a2 : ff2000000eecb510 a3 : 0000000000000001 a4 : 0000000000000000
     a5 : 0000000000000010 a6 : 0000000000000000 a7 : 0000000000735049
     s2 : ffffffffa38dbee8 s3 : 0000000000000040 s4 : ff6000008bcda000
     s5 : 0000000000000008 s6 : ff60000093a6a680 s7 : ff60000093a6a6f0
     s8 : ff60000093a6a6ac s9 : ff60000093140000 s10: 0000000000000000
     s11: ff2000000eecb9d0 t3 : 0000000000000000 t4 : 0000000000ff0000
     t5 : 0000000000000000 t6 : ff60000093a6a8b6
    status: 0000000200000120 badaddr: ffffffffa38dbf58 cause: 000000000000000d
    [<ffffffff80bd5c7a>] __qdisc_run+0x82/0x6f0
    [<ffffffff80b6fe58>] __dev_queue_xmit+0x4c0/0x1128
    [<ffffffff80b80ae0>] neigh_resolve_output+0xd0/0x170
    [<ffffffff80d2daf6>] ip6_finish_output2+0x226/0x6c8
    [<ffffffff80d31254>] ip6_finish_output+0x10c/0x2a0
    [<ffffffff80d31446>] ip6_output+0x5e/0x178
    [<ffffffff80d2e232>] ip6_xmit+0x29a/0x608
    [<ffffffff80d6f4c6>] inet6_csk_xmit+0xe6/0x140
    [<ffffffff80c985e4>] __tcp_transmit_skb+0x45c/0xaa8
    [<ffffffff80c995fe>] tcp_connect+0x9ce/0xd10
    [<ffffffff80d66524>] tcp_v6_connect+0x4ac/0x5e8
    [<ffffffff80cc19b8>] __inet_stream_connect+0xd8/0x318
    [<ffffffff80cc1c36>] inet_stream_connect+0x3e/0x68
    [<ffffffff80b42b20>] __sys_connect_file+0x50/0x88
    [<ffffffff80b42bee>] __sys_connect+0x96/0xc8
    [<ffffffff80b42c40>] __riscv_sys_connect+0x20/0x30
    [<ffffffff80e5bcae>] do_trap_ecall_u+0x256/0x378
    [<ffffffff80e69af2>] handle_exception+0x14a/0x156
    Code: 892a 0363 1205 489c 8bc1 c7e5 2d03 084a 2703 080a (2783) 0709
    ---[ end trace 0000000000000000 ]---

The bpf_fifo_dequeue prog returns a skb which is a pointer. The pointer
is treated as a 32bit value and sign extend to 64bit in epilogue. This
behavior is right for most bpf prog types but wrong for struct ops which
requires RISC-V ABI.

So let's sign extend struct ops return values according to the function
model and RISC-V ABI([0]).

  [0]: https://riscv.org/wp-content/uploads/2024/12/riscv-calling.pdf