fs: proc: gpf in find_entry

[Sasha Levin <sasha.levin@oracle.com>]

Hi all,

While fuzzing with trinity inside a KVM tools guest running the latest -next
kernel, I've stumbled on the following spew:

[ 2015.960381] general protection fault: 0000 [#1] PREEMPT SMP KASAN
[ 2015.961912] Dumping ftrace buffer:
[ 2015.962803]    (ftrace buffer empty)
[ 2015.963370] Modules linked in:
[ 2015.963895] CPU: 1 PID: 16983 Comm: trinity-c126 Not tainted 3.18.0-next-20141219-sasha-00047-gaab33f6-dirty #1627
[ 2015.965991] task: ffff88080e478000 ti: ffff88080e474000 task.ti: ffff88080e474000
[ 2015.968196] RIP: find_entry (fs/proc/proc_sysctl.c:95)
[ 2015.970534] RSP: 0018:ffff88080e4779d8  EFLAGS: 00010246
[ 2015.970534] RAX: 0000000000000000 RBX: ffff88000003a960 RCX: 0000000000000073
[ 2015.970534] RDX: 1ffff10101c8f3c4 RSI: 0000000000000000 RDI: ffff88080e479e20
[ 2015.970534] RBP: ffff88080e477a28 R08: 0000000000000066 R09: 0000000000000073
[ 2015.970534] R10: ffffda0017d55630 R11: dfffe90000000000 R12: ffff88005fc644b8
[ 2015.970534] R13: dfffe90000000000 R14: ffffffff92464884 R15: 0000000000000000
[ 2015.970534] FS:  00007f9cd6c6f700(0000) GS:ffff8800c2200000(0000) knlGS:0000000000000000
[ 2015.970534] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2015.970534] CR2: 00007f9cd0b9c000 CR3: 00000008193fb000 CR4: 00000000000006a0
[ 2015.970534] DR0: ffffffffff000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 2015.970534] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
[ 2015.970534] Stack:
[ 2015.970534]  ffff88080e477a28 ffff88080e477a50 00000003beaab0c0 ffff8800beaab0f8
[ 2015.970534]  0000000000000001 ffff8800beaab0f8 ffff8800beaab0c0 ffff8806079af638
[ 2015.970534]  0000000000000003 ffff88045f3cc000 ffff88080e477a88 ffffffff81c4e0ef
[ 2015.970534] Call Trace:
[ 2015.970534] proc_sys_lookup (fs/proc/proc_sysctl.c:303 fs/proc/proc_sysctl.c:452)
[ 2015.970534] ? d_alloc (fs/dcache.c:1499)
[ 2015.970534] lookup_real (fs/namei.c:1371)
[ 2015.970534] __lookup_hash (fs/namei.c:1390)
[ 2015.970534] link_path_walk (fs/namei.c:1496 fs/namei.c:1576 fs/namei.c:1830)
[ 2015.970534] ? preempt_count_sub (kernel/sched/core.c:2620)
[ 2015.970534] ? get_parent_ip (kernel/sched/core.c:2564)
[ 2015.970534] ? get_parent_ip (kernel/sched/core.c:2564)
[ 2015.970534] ? __cmpxchg_double_slab.isra.4 (./arch/x86/include/asm/preempt.h:95 include/linux/bit_spinlock.h:81 mm/slub.c:346 mm/slub.c:386)
[ 2015.970534] path_init (fs/namei.c:1947)
[ 2015.970534] ? deactivate_slab (include/linux/spinlock.h:349 mm/slub.c:1945)
[ 2015.970534] path_lookupat (fs/namei.c:1989)
[ 2015.970534] ? alloc_debug_processing (mm/slub.c:1044)
[ 2015.970534] filename_lookup (fs/namei.c:2025)
[ 2015.970534] kern_path_create (fs/namei.c:3309)
[ 2015.970534] ? getname_flags (fs/namei.c:159)
[ 2015.970534] ? vtime_account_user (kernel/sched/cputime.c:701)
[ 2015.970534] user_path_create (fs/namei.c:3381)
[ 2015.970534] SyS_mknod (fs/namei.c:3443 fs/namei.c:3431 fs/namei.c:3475 fs/namei.c:3473)
[ 2015.970534] tracesys_phase2 (arch/x86/kernel/entry_64.S:529)
[ 2015.970534] Code: e8 03 42 80 3c 28 00 0f 85 ff 01 00 00 4c 8b 7b 18 4d 85 ff 0f 84 de 01 00 00 41 f6 c7 07 0f 85 d4 01 00 00 4c 89 f8 48 c1 e8 03 <42> 80 3c 28 00 0f 85 b5 01 00 00 4d 8b 37 4d 85 ff 0f 84 55 02
All code
========
   0:	e8 03 42 80 3c       	callq  0x3c804208
   5:	28 00                	sub    %al,(%rax)
   7:	0f 85 ff 01 00 00    	jne    0x20c
   d:	4c 8b 7b 18          	mov    0x18(%rbx),%r15
  11:	4d 85 ff             	test   %r15,%r15
  14:	0f 84 de 01 00 00    	je     0x1f8
  1a:	41 f6 c7 07          	test   $0x7,%r15b
  1e:	0f 85 d4 01 00 00    	jne    0x1f8
  24:	4c 89 f8             	mov    %r15,%rax
  27:	48 c1 e8 03          	shr    $0x3,%rax
  2b:*	42 80 3c 28 00       	cmpb   $0x0,(%rax,%r13,1)		<-- trapping instruction
  30:	0f 85 b5 01 00 00    	jne    0x1eb
  36:	4d 8b 37             	mov    (%r15),%r14
  39:	4d 85 ff             	test   %r15,%r15
  3c:	0f                   	.byte 0xf
  3d:	84 55 02             	test   %dl,0x2(%rbp)
	...

Code starting with the faulting instruction
===========================================
   0:	42 80 3c 28 00       	cmpb   $0x0,(%rax,%r13,1)
   5:	0f 85 b5 01 00 00    	jne    0x1c0
   b:	4d 8b 37             	mov    (%r15),%r14
   e:	4d 85 ff             	test   %r15,%r15
  11:	0f                   	.byte 0xf
  12:	84 55 02             	test   %dl,0x2(%rbp)
	...
[ 2015.970534] RIP find_entry (fs/proc/proc_sysctl.c:95)
[ 2015.970534]  RSP <ffff88080e4779d8>
[ 2016.028073] ---[ end trace 142d37d0fb80aa87 ]---
[ 2016.028925] Kernel panic - not syncing: Fatal exception
[ 2016.030263] Dumping ftrace buffer:
[ 2016.030847]    (ftrace buffer empty)
[ 2016.031399] Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[ 2016.032890] Rebooting in 1 seconds..


Thanks,
Sasha