NFS: fix general protection fault in nfs_mount

While fuzzing the kernel with syzkaller, the following issue was triggered:

Call Trace:
 strlen usr/src/kernel/include/linux/string.h:306 [inline]
 nfs_mount+0x316/0x740 usr/src/kernel/fs/nfs/mount_clnt.c:174
 ? __kernel_text_address+0x12/0x40 usr/src/kernel/kernel/extable.c:95
 ? mnt_xdr_dec_mountres+0x3a0/0x3a0 usr/src/kernel/fs/nfs/mount_clnt.c:371
 ? arch_stack_walk+0xa2/0xf0 usr/src/kernel/arch/x86/kernel/stacktrace.c:26
 ? mark_usage usr/src/kernel/kernel/locking/lockdep.c:3426 [inline]
 ? __lock_acquire+0x557/0x5ed0 usr/src/kernel/kernel/locking/lockdep.c:3752
 nfs_request_mount.constprop.0+0x42e/0x680 usr/src/kernel/fs/nfs/super.c:1817
 ? nfs_fs_mount+0x2870/0x2870 usr/src/kernel/fs/nfs/super.c:2080
 ? lockdep_hardirqs_on+0x580/0x580 usr/src/kernel/kernel/locking/lockdep.c:3264
 ? lockdep_hardirqs_on+0x580/0x580 usr/src/kernel/kernel/locking/lockdep.c:3264
 nfs_try_mount_request usr/src/kernel/fs/nfs/super.c:1839 [inline]
 nfs_try_mount+0x25b/0x93a usr/src/kernel/fs/nfs/super.c:1907
 ? lock_downgrade+0x770/0x770 usr/src/kernel/kernel/locking/lockdep.c:4043
 ? nfs_request_mount.constprop.0+0x680/0x680 usr/src/kernel/fs/nfs/super.c:1800
 ? lock_downgrade+0x770/0x770 usr/src/kernel/kernel/locking/lockdep.c:4043
 ? rwlock_bug.part.0+0x90/0x90 usr/src/kernel/include/linux/sched.h:1250
 ? kasan_check_read+0x11/0x20 usr/src/kernel/mm/kasan/common.c:94
 ? atomic_read usr/src/kernel/include/asm-generic/atomic-instrumented.h:26 [inline]
 ? queued_spin_is_locked usr/src/kernel/include/asm-generic/qspinlock.h:26 [inline]
 ? debug_spin_unlock usr/src/kernel/kernel/locking/spinlock_debug.c:98 [inline]
 ? do_raw_spin_unlock+0x59/0x260 usr/src/kernel/kernel/locking/spinlock_debug.c:138
 ? __raw_spin_unlock usr/src/kernel/include/linux/spinlock_api_smp.h:152 [inline]
 ? _raw_spin_unlock+0x32/0x50 usr/src/kernel/kernel/locking/spinlock.c:183
 ? find_nfs_version+0xe6/0x110 usr/src/kernel/include/linux/err.h:26
 nfs_fs_mount+0xe83/0x2870 usr/src/kernel/fs/nfs/super.c:2750
 ? nfs_remount+0x19e0/0x19e0 usr/src/kernel/include/net/ipv6.h:517
 ? trace_kfree usr/src/kernel/include/trace/events/kmem.h:137 [inline]
 ? kfree+0x194/0x2c0 usr/src/kernel/mm/slub.c:3983
 ? nfs_clone_super+0x420/0x420 usr/src/kernel/fs/nfs/super.c:2421
 ? nfs_parse_mount_options+0x2400/0x2400 usr/src/kernel/fs/nfs/super.c:1396
 ? vfs_parse_fs_string+0x11b/0x170 usr/src/kernel/fs/fs_context.c:190
 ? vfs_parse_fs_param+0x540/0x540 usr/src/kernel/fs/fs_context.c:163
 ? nfs_remount+0x19e0/0x19e0 usr/src/kernel/include/net/ipv6.h:517
 legacy_get_tree+0x10f/0x220 usr/src/kernel/fs/fs_context.c:659
 ? legacy_parse_monolithic+0x124/0x180 usr/src/kernel/fs/fs_context.c:643
 vfs_get_tree+0x98/0x3a0 usr/src/kernel/fs/super.c:1478
 do_new_mount usr/src/kernel/fs/namespace.c:2801 [inline]
 do_mount+0x1381/0x1c20 usr/src/kernel/fs/namespace.c:3121
 ? copy_mount_string+0x40/0x40 usr/src/kernel/fs/namespace.c:3024
 ? kasan_kmalloc+0x9/0x10 usr/src/kernel/mm/kasan/common.c:509
 ? kmem_cache_alloc_trace+0x147/0x310 usr/src/kernel/mm/slub.c:2810
 ? __sanitizer_cov_trace_const_cmp4+0x16/0x20 usr/src/kernel/kernel/kcov.c:189
 ? memset usr/src/kernel/include/linux/string.h:369 [inline]
 ? copy_mount_options+0x2d9/0x3d0 usr/src/kernel/fs/namespace.c:3017
 ksys_mount+0xd8/0x130 usr/src/kernel/fs/namespace.c:3330
 __do_sys_mount usr/src/kernel/fs/namespace.c:3344 [inline]
 __se_sys_mount usr/src/kernel/fs/namespace.c:3341 [inline]
 __x64_sys_mount+0xc3/0x150 usr/src/kernel/fs/namespace.c:3341
 do_syscall_64+0xc7/0x600 usr/src/kernel/arch/x86/entry/common.c:295
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

It can be reproduced using the following simplified program:
 #include <stdlib.h>
 #include <sys/mount.h>

 int main()
 {
       mount(NULL, "./file0", "nfs", 0x400, "\x07\000\000\000");

       return 0;
 }

It was introduced by the follwing yocto-specific commit:
commit ad818f6b
Author: Jason Wessel <jason.wessel@windriver.com>
Date:   Wed Jan 7 00:59:33 2009 -0500

    NFS: allow nfs root mount to use alternate rpc ports

    Allow an nfs root mount to use alternate RPC ports for mountd and nfsd.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
    [forward port to 2.6.33+]
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>

Since hardknott, "NFS: allow nfs root mount to use alternate rpc ports" patch is
no longer applied by yocto-kernel-cache (since a4283cc6be0c65 ("kver: update
-dev to v5.6-rc5")) due to a merge conflict. Also, in later kernels (since v5.6
commit f2aedb71 ("NFS: Add fs_context support")), the nfs parser has been
refactored to use generic fs_context.

The null pointer dereference takes place only when data->version is 0x07,
because that is the only switch case that does not perform struct
nfs_parsed_mount_data initialization (case 6 in the switch). All other
switch cases fall through case 6 and perform the initializations properly.
To fix this, also run legacy version 6 binary mount logic when NFS version
is 0x07.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>