[Bug 260071] bad slotid in SEQUENCE reply can crash NFS v4 client

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=260071

            Bug ID: 260071
           Summary: bad slotid in SEQUENCE reply can crash NFS v4 client
           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 #229750 text/plain
         mime type:

Created attachment 229750
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=229750&action=edit
A demo of an nfs v4 server crashing the client with a negative slotid.

This code in newnfs_request() uses a slot number sent by the server
to index into an array without bounds-checking:

                                        slot = fxdr_unsigned(int, *tl++);
                                        freeslot = slot;
                                        if (retseq !=
sep->nfsess_slotseq[slot])
                                                printf("retseq diff 0x%x\n",
                                                    retseq);

I've attached a demo:

# uname -a
FreeBSD  14.0-CURRENT FreeBSD 14.0-CURRENT #103
main-n250903-2a29c1558e82-dirty: Fri Nov 26 13:54:28 EST 2021    
rtm@xxx:/usr/obj/usr/rtm/symbsd/src/riscv.riscv64/sys/RTM  riscv
# cc fnfs_3.c
# ./a.out
...
panic: Fatal page fault at 0xffffffc000207f4c: 0xffffffce02bf4cc0
--- exception 13, tval = 0xffffffce02bf4cc0
newnfs_request() at newnfs_request+0x1002
nfsrpc_reclaimcomplete() at nfsrpc_reclaimcomplete+0x92
nfsrpc_setclient() at nfsrpc_setclient+0x630
nfscl_getcl() at nfscl_getcl+0x53e
mountnfs() at mountnfs+0x722
nfs_mount() at nfs_mount+0x161e
vfs_mount_sigdefer() at vfs_mount_sigdefer+0x20
vfs_domount_first() at vfs_domount_first+0x1d4
vfs_domount() at vfs_domount+0x21c
vfs_donmount() at vfs_donmount+0x79c
sys_nmount() at sys_nmount+0x66
do_trap_user() at do_trap_user+0x206
cpu_exception_handler_user() at cpu_exception_handler_user+0x72

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