[Bug 260176] nfsrvd_verify() passes sfp=NULL to nfsv4_loadattr(), which can crash

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=260176

            Bug ID: 260176
           Summary: nfsrvd_verify() passes sfp=NULL to nfsv4_loadattr(),
                    which can crash
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: rtm@lcs.mit.edu
 Attachment #229846 text/plain
         mime type:

Created attachment 229846
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=229846&action=edit
Crash an NFS v4 server with a VERIFY that mentions FILESAVAIL

nfsrvd_verify() passes NULL for sfp to nfsv4_loadattr(), but if the
client includes an attribute like FILESAVAIL that uses sfp,
nfsv4_loadattr() will crash.

I've attached a demo:

# uname -a
FreeBSD  14.0-CURRENT FreeBSD 14.0-CURRENT #122
main-n250906-d95bc6b0bf4c-dirty: Thu Dec  2 05:26:06 EST 2021    
rtm@xxx:/usr/obj/usr/rtm/symbsd/src/riscv.riscv64/sys/RTM  riscv
# cc fnfsd_9.c
# ./a.out
...
panic: Fatal page fault at 0xffffffc00020a678: 0x00000000000028
--- exception 13, tval = 0x28
nfsv4_loadattr() at nfsv4_loadattr+0x1a94
nfsrvd_verify() at nfsrvd_verify+0xb6
nfsrvd_dorpc() at nfsrvd_dorpc+0x147a
nfssvc_program() at nfssvc_program+0x5a8
svc_run_internal() at svc_run_internal+0x810
svc_run() at svc_run+0x1a2
nfsrvd_nfsd() at nfsrvd_nfsd+0x30c
nfssvc_nfsd() at nfssvc_nfsd+0x3ac
sys_nfssvc() at sys_nfssvc+0xd0
do_trap_user() at do_trap_user+0x220
cpu_exception_handler_user() at cpu_exception_handler_user+0x72

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