[Bug 260012] NFS v4 client can crash server with a bad LAYOUTRETURN RPC

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=260012

            Bug ID: 260012
           Summary: NFS v4 client can crash server with a bad LAYOUTRETURN
                    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 #229690 text/plain
         mime type:

Created attachment 229690
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=229690&action=edit
An NFS client that crashes the server with a bad LAYOUTRETURN.

If an NFS v4 client sends a LAYOUTRETURN RPC with lr_returntype of
NFSV4LAYOURRET_FILE and an lrf_body<> size <= 0, nfsrvd_layoutreturn()
will pass a NULL layp to nfsrv_layoutreturn(). If the RPC also has
layouttype==NFSV4LAYOUT_FLEXFILE (4), nfsrv_layoutreturn() will call
nfsrv_flexlayouterr(). The latter dereferences layp (as tl) without
checking for NULL.

I've attached a demo:

# uname -a
FreeBSD  14.0-CURRENT FreeBSD 14.0-CURRENT #96 main-n250901-77e3db078984-dirty:
Tue Nov 23 16:58:28 EST 2021    
rtm@xxx:/usr/obj/usr/rtm/symbsd/src/riscv.riscv64/sys/RTM  riscv
# cc fnfsd_1.c
# ./a.out
...
nfsrv_layoutreturn: updatemdsattr failed=2
panic: Fatal page fault at 0xffffffc00025ce8e: 0000000000000000
cpuid = 0
time = 1637591542
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
nfsrv_layoutreturn() at nfsrv_layoutreturn+0xa6
nfsrvd_layoutreturn() at nfsrvd_layoutreturn+0x35c
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

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