decode_compound_hdr() in fs/nfs/nfs4xdr.c adds the taglen supplied by
the server to a pointer and then dereferences it, but does not first
check taglen for sanity:

        p = xdr_inline_decode(xdr, 8);
        ...;
        hdr->taglen = be32_to_cpup(p);
        ...;
        p = xdr_inline_decode(xdr, hdr->taglen + 4);
        ...;
        p += XDR_QUADLEN(hdr->taglen);
        hdr->nops = be32_to_cpup(p);

The second xdr_inline_decode() limits the opportunities for an
attacker-controlled pointer dereference, but a taglen of 0xfffffffc
will cause a kernel page fault.

I've attached a program that tickles the bug on my kernel 5.15
machine:

# uname -a
Linux (none) 5.15.0-rc7-dirty #15 SMP Thu Nov 4 19:20:36 UTC 2021 riscv64 riscv64 riscv64 GNU/Linux
# cc nfs_1.c
# ./a.out
mount:mount.nfs: timeout set for Thu Jan  1 00:02:12 1970
mount.nfs: trying text-based options 'vers=4.2,addr=127.0.0.1,clientaddr=127.0.0.1'
accept returned 4
proc 0
proc 1
exception pc=0xffffffff8022dcd8 cause=d symbolic tval=0xffffffe102c5aad8
[   16.101267] Unable to handle kernel paging request at virtual address ffffffe102c5aad8
[   16.112762] Oops [#1]
[   16.116973] Modules linked in:
[   16.122429] CPU: 0 PID: 60 Comm: mount.nfs Not tainted 5.15.0-rc7-dirty #13
[   16.131634] Hardware name: ucbbar,riscvemu-bare (DT)
[   16.138694] epc : decode_compound_hdr+0x96/0x12e
[   16.146706]  ra : decode_compound_hdr+0x82/0x12e
[   16.154151] epc : ffffffff8022dcd8 ra : ffffffff8022dcc4 sp : ffffffd00057b6e0
...
[   16.272291] status: 0000000200000121 badaddr: ffffffe102c5aad8 cause: 000000000000000d
[   16.282369] [<ffffffff8022dcd8>] decode_compound_hdr+0x96/0x12e
[   16.290699] [<ffffffff80239c2a>] nfs4_xdr_dec_exchange_id+0x32/0x57e
[   16.299265] [<ffffffff8071af5c>] rpcauth_unwrap_resp_decode+0x12/0x1a
[   16.307926] [<ffffffff8071bc18>] rpcauth_unwrap_resp+0x12/0x1a
[   16.316196] [<ffffffff80711fb4>] call_decode+0x112/0x176
[   16.323488] [<ffffffff8071a498>] __rpc_execute+0x76/0x216
[   16.330751] [<ffffffff8071aab6>] rpc_execute+0x58/0x7e
[   16.337966] [<ffffffff80713340>] rpc_run_task+0x12c/0x16c
[   16.345113] [<ffffffff80224eba>] nfs4_run_exchange_id+0x1d8/0x262
[   16.353364] [<ffffffff80224f68>] _nfs4_proc_exchange_id+0x24/0x2ba
[   16.361556] [<ffffffff8022cfc4>] nfs4_proc_exchange_id+0x30/0x50
[   16.369829] [<ffffffff8023ea28>] nfs41_discover_server_trunking+0x1c/0xa8
[   16.378421] [<ffffffff80240d4e>] nfs4_discover_server_trunking+0x7c/0x1e8
[   16.386958] [<ffffffff802490fe>] nfs4_init_client+0x92/0xf6
[   16.394014] [<ffffffff80205412>] nfs_get_client+0x36a/0x394
[   16.401147] [<ffffffff8024882e>] nfs4_set_client+0xd6/0x13e
[   16.410346] [<ffffffff8024981a>] nfs4_create_server+0xb8/0x208
[   16.421493] [<ffffffff80241606>] nfs4_try_get_tree+0x16/0x4c
[   16.432759] [<ffffffff80218bac>] nfs_get_tree+0x34a/0x3ac
[   16.442283] [<ffffffff8012bce4>] vfs_get_tree+0x18/0x88
[   16.451889] [<ffffffff8014a28e>] path_mount+0x4f4/0x77a
[   16.461619] [<ffffffff8014a560>] do_mount+0x4c/0x7e
[   16.470833] [<ffffffff8014a912>] sys_mount+0xca/0x14e
[   16.480401] [<ffffffff80003046>] ret_from_syscall+0x0/0x2