[syzbot] general protection fault in legacy_parse_param

[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit:    77dd11439b86 Merge tag 'drm-fixes-2021-08-27' of git://ano..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10636bde300000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2fd902af77ff1e56
dashboard link: https://syzkaller.appspot.com/bug?extid=d1e3b1d92d25abf97943
compiler:       Debian clang version 11.0.1-2, GNU ld (GNU Binutils for Debian) 2.35.1
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=126d084d300000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16216eb1300000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d1e3b1d92d25abf97943 at syzkaller.appspotmail.com

general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 0 PID: 8435 Comm: syz-executor272 Not tainted 5.14.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:memchr+0x2f/0x70 lib/string.c:1054
Code: 41 54 53 48 89 d3 41 89 f7 45 31 f6 49 bc 00 00 00 00 00 fc ff df 0f 1f 44 00 00 48 85 db 74 3b 48 89 fd 48 89 f8 48 c1 e8 03 <42> 0f b6 04 20 84 c0 75 0f 48 ff cb 48 8d 7d 01 44 38 7d 00 75 db
RSP: 0018:ffffc9000d9f7d08 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000001 RCX: ffff88801c1f3880
RDX: 0000000000000001 RSI: 000000000000002c RDI: 0000000000000000
RBP: 0000000000000000 R08: ffffffff81e3db46 R09: ffffffff81e3d8e2
R10: 0000000000000002 R11: ffff88801c1f3880 R12: dffffc0000000000
R13: 1ffff92001b3efcc R14: 0000000000000000 R15: 000000000000002c
FS:  0000000000deb300(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020000044 CR3: 0000000037173000 CR4: 00000000001506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 legacy_parse_param+0x49b/0x810 fs/fs_context.c:555
 vfs_parse_fs_param+0x1df/0x460 fs/fs_context.c:146
 vfs_fsconfig_locked fs/fsopen.c:265 [inline]
 __do_sys_fsconfig fs/fsopen.c:439 [inline]
 __se_sys_fsconfig+0xba9/0xff0 fs/fsopen.c:314
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x43ee69
Code: 28 c3 e8 2a 14 00 00 66 2e 0f 1f 84 00 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc5e9e0b98 EFLAGS: 00000246 ORIG_RAX: 00000000000001af
RAX: ffffffffffffffda RBX: 0000000000400488 RCX: 000000000043ee69
RDX: 0000000020000080 RSI: 0000000000000001 RDI: 0000000000000003
RBP: 0000000000402e50 R08: 0000000000000000 R09: 0000000000400488
R10: 00000000200000c0 R11: 0000000000000246 R12: 0000000000402ee0
R13: 0000000000000000 R14: 00000000004ac018 R15: 0000000000400488
Modules linked in:
---[ end trace 74baf661f3b47b0a ]---
RIP: 0010:memchr+0x2f/0x70 lib/string.c:1054
Code: 41 54 53 48 89 d3 41 89 f7 45 31 f6 49 bc 00 00 00 00 00 fc ff df 0f 1f 44 00 00 48 85 db 74 3b 48 89 fd 48 89 f8 48 c1 e8 03 <42> 0f b6 04 20 84 c0 75 0f 48 ff cb 48 8d 7d 01 44 38 7d 00 75 db
RSP: 0018:ffffc9000d9f7d08 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000001 RCX: ffff88801c1f3880
RDX: 0000000000000001 RSI: 000000000000002c RDI: 0000000000000000
RBP: 0000000000000000 R08: ffffffff81e3db46 R09: ffffffff81e3d8e2
R10: 0000000000000002 R11: ffff88801c1f3880 R12: dffffc0000000000
R13: 1ffff92001b3efcc R14: 0000000000000000 R15: 000000000000002c
FS:  0000000000deb300(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fed5f8146c0 CR3: 0000000037173000 CR4: 00000000001506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
----------------
Code disassembly (best guess):
   0:	41 54                	push   %r12
   2:	53                   	push   %rbx
   3:	48 89 d3             	mov    %rdx,%rbx
   6:	41 89 f7             	mov    %esi,%r15d
   9:	45 31 f6             	xor    %r14d,%r14d
   c:	49 bc 00 00 00 00 00 	movabs $0xdffffc0000000000,%r12
  13:	fc ff df
  16:	0f 1f 44 00 00       	nopl   0x0(%rax,%rax,1)
  1b:	48 85 db             	test   %rbx,%rbx
  1e:	74 3b                	je     0x5b
  20:	48 89 fd             	mov    %rdi,%rbp
  23:	48 89 f8             	mov    %rdi,%rax
  26:	48 c1 e8 03          	shr    $0x3,%rax
* 2a:	42 0f b6 04 20       	movzbl (%rax,%r12,1),%eax <-- trapping instruction
  2f:	84 c0                	test   %al,%al
  31:	75 0f                	jne    0x42
  33:	48 ff cb             	dec    %rbx
  36:	48 8d 7d 01          	lea    0x1(%rbp),%rdi
  3a:	44 38 7d 00          	cmp    %r15b,0x0(%rbp)
  3e:	75 db                	jne    0x1b