[Bug 260046] An NFS v4 client can crash the server with a LISTXATTRS RPC

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=260046

            Bug ID: 260046
           Summary: An NFS v4 client can crash the server with a
                    LISTXATTRS RPC
           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 #229724 text/plain
         mime type:

Created attachment 229724
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=229724&action=edit
Demo of an NFS client crashing a server with LISTXATTRS.

nfsrvd_listxattr() calls nfsvno_listxattr(). The latter can return
zero at the same time as leaving buf == NULL. But if the return value
is zero, nfsrvd_listxattr() dereferences the NULL buf, causing a
crash.

I've attached a demo:

# uname -a
FreeBSD  14.0-CURRENT FreeBSD 14.0-CURRENT #99 main-n250901-77e3db078984-dirty:
Wed Nov 24 05:42:01 EST 2021    
rtm@xxx:/usr/obj/usr/rtm/symbsd/src/riscv.riscv64/sys/RTM  riscv
# cc fnfsd_3.c
# ./a.out
...
panic: Fatal page fault at 0xffffffc00026f256: 0000000000000000
cpuid = 0
time = 1637841164
KDB: stack backtrace:
db_trace_self() at db_trace_self
db_trace_self_wrapper() at db_trace_self_wrapper+0x38
kdb_backtrace() at kdb_backtrace+0x2c
vpanic() at vpanic+0x154
panic() at panic+0x2a
page_fault_handler() at page_fault_handler+0x1ee
do_trap_supervisor() at do_trap_supervisor+0x76
cpu_exception_handler_supervisor() at cpu_exception_handler_supervisor+0x70
--- exception 13, tval = 0
nfsrvd_listxattr() at nfsrvd_listxattr+0x306
nfsrvd_dorpc() at nfsrvd_dorpc+0x154c
nfssvc_program() at nfssvc_program+0x5a4
svc_run_internal() at svc_run_internal+0x808
svc_run() at svc_run+0x1a8
nfsrvd_nfsd() at nfsrvd_nfsd+0x30e
nfssvc_nfsd() at nfssvc_nfsd+0x386
sys_nfssvc() at sys_nfssvc+0xd0
do_trap_user() at do_trap_user+0x206
cpu_exception_handler_user() at cpu_exception_handler_user+0x72
--- exception 8, tval = 0

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