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SubjectOops (bad memory deref) in slab_alloc() due to filp_cachep holding incorrect values
From
Happens under 2.6.39-rc4.

Here's probably more readable version (without lines being wrapped):
http://alt.swiecki.net/linux_kernel/sys_open-kmem_cache_alloc-2.6.39-rc4.txt

The most interesting part is probably the KGDB one. I'm still
debugging, but in case others might figure it out faster, attaching my
analysis here. The initial observation is that values in 'filp_cachep'
are incorrect.

============================================================================
KDB:
============================================================================

Stack traceback for pid 11235
0xffff88011ae91720 11235 11878 1 1 R 0xffff88011ae91ba0 *killall
<c> ffff88011ae1dd48<c> 0000000000000018<c> ffffffff8111ed78<c>
ffff88012bc4e280<c>
<c> ffff88012bffbe00<c> 0000000000000001<c> ffff88011ae1de48<c>
ffff880107947900<c>
<c> 0000000000000001<c> ffff88011ae1df28<c> ffff88010374e000<c>
ffff88011ae1dda8<c>
Call Trace:
[<ffffffff8111ed78>] ? release_pages+0x16f/0x181
[<ffffffff81162e5f>] ? get_empty_filp+0x6b/0x11f
[<ffffffff8116cca7>] ? path_openat+0x33/0x345
[<ffffffff8116cff1>] ? do_filp_open+0x38/0x86
[<ffffffff8108a703>] ? get_parent_ip+0x11/0x41
[<ffffffff8108a703>] ? get_parent_ip+0x11/0x41
[<ffffffff8108a87c>] ? sub_preempt_count+0x97/0xaa
[<ffffffff81ed24cd>] ? _raw_spin_unlock+0x2d/0x38
[<ffffffff811781a1>] ? alloc_fd+0x10b/0x11d
[<ffffffff8116127b>] ? do_sys_open+0x6e/0x100
[<ffffffff8116132d>] ? sys_open+0x20/0x22
[<ffffffff81ed3582>] ? system_call_fastpath+0x16/0x1b

---> *killall

sysname Linux
release 2.6.39-rc4
version #2 SMP PREEMPT Tue May 10 20:39:51 CEST 2011
machine x86_64
nodename ise-test
domainname (none)
ccversion CCVERSION
date 2011-05-22 07:01:01 tz_minuteswest -120
uptime 11 days 12:13
load avg 19.87 22.44 23.24

MemTotal: 993069 kB
MemFree: 130318 kB
Buffers: 44686 kB


<1>[994415.765292] BUG: unable to handle kernel paging request at
00000000ffffffff
<1>[994415.766002] IP: [<ffffffff81150e83>] kmem_cache_alloc+0x57/0xe7
<4>[994415.766002] PGD 10a3ba067 PUD 0
<0>[994415.766002] Oops: 0000 [#1] PREEMPT SMP
<0>[994415.766002] last sysfs file:
/sys/devices/system/cpu/cpu1/cache/index2/shared_cpu_map


Entering kdb (current=0xffff88011ae91720, pid 11235) on processor 1
due to cpu switch
[1]kdb> rd
ax: 0000000000000000 bx: ffff88011ae1de48 cx: ffff88011ae1df28
dx: 0000006c68931281 si: 0000006c68931281 di: ffffffff81162e5f
bp: ffff88011ae1dd88 sp: ffff88011ae1dd48 r8: 0000000000000041
r9: ffff88012074fa50 r10: 0000000000000000 r11: 0000000000000246
r12: 00000000ffffffff r13: ffff88012b002300 r14: 00000000000080d0
r15: ffff88010374e000 ip: ffffffff81150e83 flags: 00010206 cs: 00000010
ss: 00000018

Code: 00 48 8b 50 08 4c 8b 20 4d 85 e4 75 16 48 89 f9 83 ca ff 44 89
f6 4c 89 ef e8 b0 fa ff ff 49 89 c4 eb 20 49 63 45 20 49 8b 75 00 <49>
8b 1c 04 48 8d 4a 40 4c 89 e0 65 48 0f c7 0e 0f 94 c0 90 84
All code
========
0: 00 48 8b add %cl,-0x75(%rax)
3: 50 push %rax
4: 08 4c 8b 20 or %cl,0x20(%rbx,%rcx,4)
8: 4d 85 e4 test %r12,%r12
b: 75 16 jne 0x23
d: 48 89 f9 mov %rdi,%rcx
10: 83 ca ff or $0xffffffffffffffff,%edx
13: 44 89 f6 mov %r14d,%esi
16: 4c 89 ef mov %r13,%rdi
19: e8 b0 fa ff ff callq 0xffffffffffffface
1e: 49 89 c4 mov %rax,%r12
21: eb 20 jmp 0x43
23: 49 63 45 20 movslq 0x20(%r13),%rax
27: 49 8b 75 00 mov 0x0(%r13),%rsi
2b:* 49 8b 1c 04 mov (%r12,%rax,1),%rbx <--
trapping instruction
2f: 48 8d 4a 40 lea 0x40(%rdx),%rcx
33: 4c 89 e0 mov %r12,%rax
36: 65 48 0f c7 0e cmpxchg16b %gs:(%rsi)
3b: 0f 94 c0 sete %al
3e: 90 nop
3f: 84 .byte 0x84

Code starting with the faulting instruction
===========================================
0: 49 8b 1c 04 mov (%r12,%rax,1),%rbx
4: 48 8d 4a 40 lea 0x40(%rdx),%rcx
8: 4c 89 e0 mov %r12,%rax
b: 65 48 0f c7 0e cmpxchg16b %gs:(%rsi)
10: 0f 94 c0 sete %al
13: 90 nop
14: 84 .byte 0x84






============================================================================
GDB:
============================================================================
(gdb) disassemble 0xffffffff81150e83
Dump of assembler code for function kmem_cache_alloc:
0xffffffff81150e2c <+0>: push rbp
0xffffffff81150e2d <+1>: mov rbp,rsp
0xffffffff81150e30 <+4>: push r15
0xffffffff81150e32 <+6>: push r14
0xffffffff81150e34 <+8>: push r13
0xffffffff81150e36 <+10>: push r12
0xffffffff81150e38 <+12>: push rbx
0xffffffff81150e39 <+13>: sub rsp,0x18
0xffffffff81150e3d <+17>: call 0xffffffff81ed32c0
0xffffffff81150e42 <+22>: mov r13,rdi
0xffffffff81150e45 <+25>: mov r14d,esi
0xffffffff81150e48 <+28>: mov rdi,QWORD PTR [rbp+0x8]
0xffffffff81150e4c <+32>: mov rax,QWORD PTR [r13+0x0]
0xffffffff81150e50 <+36>: add rax,QWORD PTR gs:0xdd40
0xffffffff81150e59 <+45>: mov rdx,QWORD PTR [rax+0x8]
0xffffffff81150e5d <+49>: mov r12,QWORD PTR [rax]
0xffffffff81150e60 <+52>: test r12,r12
0xffffffff81150e63 <+55>: jne 0xffffffff81150e7b <kmem_cache_alloc+79>
0xffffffff81150e65 <+57>: mov rcx,rdi
0xffffffff81150e68 <+60>: or edx,0xffffffffffffffff
0xffffffff81150e6b <+63>: mov esi,r14d
0xffffffff81150e6e <+66>: mov rdi,r13
0xffffffff81150e71 <+69>: call 0xffffffff81150926 <__slab_alloc>
0xffffffff81150e76 <+74>: mov r12,rax
0xffffffff81150e79 <+77>: jmp 0xffffffff81150e9b <kmem_cache_alloc+111>
0xffffffff81150e7b <+79>: movsxd rax,DWORD PTR [r13+0x20]
0xffffffff81150e7f <+83>: mov rsi,QWORD PTR [r13+0x0]
0xffffffff81150e83 <+87>: mov rbx,QWORD PTR [r12+rax*1]
0xffffffff81150e87 <+91>: lea rcx,[rdx+0x40]
0xffffffff81150e8b <+95>: mov rax,r12
0xffffffff81150e8e <+98>: call 0xffffffff813d2c70
0xffffffff81150e93 <+103>: nop DWORD PTR [rax+0x0]
0xffffffff81150e97 <+107>: test al,al
---Type <return> to continue, or q <return> to quit---
0xffffffff81150e99 <+109>: je 0xffffffff81150e4c <kmem_cache_alloc+32>
0xffffffff81150e9b <+111>: test r12,r12
0xffffffff81150e9e <+114>: je 0xffffffff81150eb4 <kmem_cache_alloc+136>
0xffffffff81150ea0 <+116>: test r14d,0x8000
0xffffffff81150ea7 <+123>: je 0xffffffff81150eb4 <kmem_cache_alloc+136>
0xffffffff81150ea9 <+125>: movsxd rcx,DWORD PTR [r13+0x1c]
0xffffffff81150ead <+129>: xor eax,eax
0xffffffff81150eaf <+131>: mov rdi,r12
0xffffffff81150eb2 <+134>: rep stos BYTE PTR es:[rdi],al
0xffffffff81150eb4 <+136>: movsxd rax,DWORD PTR [r13+0x18]
0xffffffff81150eb8 <+140>: movsxd r15,DWORD PTR [r13+0x1c]
0xffffffff81150ebc <+144>: mov QWORD PTR [rbp-0x38],rax
0xffffffff81150ec0 <+148>: mov r13,QWORD PTR [rbp+0x8]
0xffffffff81150ec4 <+152>: jmp 0xffffffff81150ec9 <kmem_cache_alloc+157>
0xffffffff81150ec9 <+157>: jmp 0xffffffff81150f01 <kmem_cache_alloc+213>
0xffffffff81150ecb <+159>: call 0xffffffff8114dacf
<rcu_read_lock_sched_notrace>
0xffffffff81150ed0 <+164>: mov rbx,QWORD PTR [rip+0x1abdfc9]
# 0xffffffff82c0eea0
0xffffffff81150ed7 <+171>: test rbx,rbx
0xffffffff81150eda <+174>: je 0xffffffff81150efc <kmem_cache_alloc+208>
0xffffffff81150edc <+176>: mov rdi,QWORD PTR [rbx+0x8]
0xffffffff81150ee0 <+180>: mov r9d,r14d
0xffffffff81150ee3 <+183>: mov r8,QWORD PTR [rbp-0x38]
0xffffffff81150ee7 <+187>: mov rcx,r15
0xffffffff81150eea <+190>: mov rdx,r12
0xffffffff81150eed <+193>: mov rsi,r13
0xffffffff81150ef0 <+196>: call QWORD PTR [rbx]
0xffffffff81150ef2 <+198>: add rbx,0x10
0xffffffff81150ef6 <+202>: cmp QWORD PTR [rbx],0x0
0xffffffff81150efa <+206>: jmp 0xffffffff81150eda <kmem_cache_alloc+174>
0xffffffff81150efc <+208>: call 0xffffffff81150cb1
<rcu_read_unlock_sched_notrace>
0xffffffff81150f01 <+213>: add rsp,0x18
0xffffffff81150f05 <+217>: mov rax,r12
0xffffffff81150f08 <+220>: pop rbx
0xffffffff81150f09 <+221>: pop r12
---Type <return> to continue, or q <return> to quit---
0xffffffff81150f0b <+223>: pop r13
0xffffffff81150f0d <+225>: pop r14
0xffffffff81150f0f <+227>: pop r15
0xffffffff81150f11 <+229>: leave
0xffffffff81150f12 <+230>: ret







============================================================================
KGDB:
============================================================================

(gdb) set remotebaud 115200
(gdb) target remote /dev/ttyS0
Remote debugging using /dev/ttyS0
0xffffffff81150e83 in slab_alloc (s=0xffff88012b002300,
gfpflags=32976) at mm/slub.c:1943
1943 if (unlikely(!this_cpu_cmpxchg_double(
(gdb) bt
#0 0xffffffff81150e83 in slab_alloc (s=0xffff88012b002300,
gfpflags=32976) at mm/slub.c:1943
#1 kmem_cache_alloc (s=0xffff88012b002300, gfpflags=32976) at mm/slub.c:1971
#2 0xffffffff81162e5f in kmem_cache_zalloc () at include/linux/slab.h:310
#3 get_empty_filp () at fs/file_table.c:123
#4 0xffffffff8116cca7 in path_openat (dfd=<value optimized out>,
pathname=0xffff88010374e000 "/proc/1133/stat",
nd=0xffff88011ae1de48, op=0xffff88011ae1df28, flags=<value
optimized out>) at fs/namei.c:2324
#5 0xffffffff8116cff1 in do_filp_open (dfd=-100,
pathname=0xffff88010374e000 "/proc/1133/stat",
op=0xffff88011ae1df28, flags=1) at fs/namei.c:2380
#6 0xffffffff8116127b in do_sys_open (dfd=-100, filename=<value
optimized out>, flags=32768,
mode=<value optimized out>) at fs/open.c:1001
#7 0xffffffff8116132d in sys_open (filename=<value optimized out>,
flags=<value optimized out>,
mode=<value optimized out>) at fs/open.c:1022
#8 <signal handler called>
#9 0x00007f17ee73bff0 in __brk_reservation_fn_dmi_alloc__ ()
#10 0xffff880101141720 in __brk_reservation_fn_dmi_alloc__ ()
#11 0xffffffff82a1bb50 in ?? ()
#12 0x0000000200020000 in __brk_reservation_fn_dmi_alloc__ ()
#13 0x0000000300000001 in __brk_reservation_fn_dmi_alloc__ ()
#14 0x00007ffffffff000 in __brk_reservation_fn_dmi_alloc__ ()
#15 0xffffffff810a4b51 in sys_restart_syscall () at kernel/signal.c:2085
#16 0x0000000000000000 in ?? ()

(gdb) frame 0
#0 0xffffffff81150e83 in slab_alloc (s=0xffff88012b002300,
gfpflags=32976) at mm/slub.c:1943
1943 if (unlikely(!this_cpu_cmpxchg_double(
(gdb) list
1938 * 3. If they were not changed replace tid and freelist
1939 *
1940 * Since this is without lock semantics the protection is only against
1941 * code executing on this cpu *not* from access by other cpus.
1942 */
1943 if (unlikely(!this_cpu_cmpxchg_double(
1944 s->cpu_slab->freelist, s->cpu_slab->tid,
1945 object, tid,
1946 get_freepointer(s, object), next_tid(tid)))) {
1947

(gdb) x/i $pc
=> 0xffffffff81150e83 <kmem_cache_alloc+87>: mov rbx,QWORD PTR [r12+rax*1]

(gdb) info registers
rax 0x0 0
rbx 0xffff88011ae1de48 -131936649355704
rcx 0xffff88011ae1df28 -131936649355480
rdx 0x6c68931281 465610936961
rsi 0x6c68931281 465610936961
rdi 0xffffffff81162e5f -2129252769
rbp 0xffff88011ae1dd88 0xffff88011ae1dd88
rsp 0xffff88011ae1dd48 0xffff88011ae1dd48
r8 0x41 65
r9 0xffff88012074fa50 -131936555828656
r10 0x0 0
r11 0x246 582
r12 0xffffffff 4294967295
r13 0xffff88012b002300 -131936378936576
r14 0x80d0 32976
r15 0xffff88010374e000 -131937042374656
rip 0xffffffff81150e83 0xffffffff81150e83 <kmem_cache_alloc+87>
eflags 0x10206 [ PF IF RF ]
cs 0x10 16
ss 0x18 24
ds Could not fetch register "ds"; remote failure reply 'E22'

(gdb) p object
$36 = (void **) 0xffffffff

(gdb) p s
$1 = (struct kmem_cache *) 0xffff88012b002300
(gdb) p s->cpu_slab
$2 = (struct kmem_cache_cpu *) 0x15300

(gdb) p *s
$17 = {cpu_slab = 0x15300, flags = 0, min_partial = 7, size = 192,
objsize = 192, offset = 0, oo = {x = 21}, max = {
x = 21}, min = {x = 21}, allocflags = 0, refcount = 13, ctor = 0,
inuse = 192, align = 8, reserved = 0,
name = 0xffff88012b000010 "kmalloc-192", list = {next =
0xffff88012b002268, prev = 0xffff88012b002468}, kobj = {
name = 0xffff880123c7c830 ":t-0000192", entry = {next =
0xffff88012b002280, prev = 0xffff88012b002480},
parent = 0xffff8801249a74f8, kset = 0xffff8801249a74e0, ktype =
0xffffffff82a37790, sd = 0xffff880123ca6fa0,
kref = {refcount = {counter = 1}}, state_initialized = 1,
state_in_sysfs = 1, state_add_uevent_sent = 1,
state_remove_uevent_sent = 0, uevent_suppress = 0},
remote_node_defrag_ratio = 1000, node = {0xffff88012b0010c0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x15320, 0x0, 0x5, 0x800000008,
0x0, 0x200, 0x200, 0x200, 0x100000000, 0x0,
0x800000008, 0x0, 0xffff88012b000020, 0xffff88012b002368,
0xffff88012b002568, 0xffff880123c7c800,
0xffff88012b002380, 0xffff88012b002580, 0xffff8801249a74f8,
0xffff8801249a74e0, 0xffffffff82a37790,
0xffff880123ca65f0, 0x700000001, 0x3e8, 0xffff88012b001100, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x15340, 0x0, 0x5,
0x1000000010, 0x0, 0x100, 0x100, 0x100, 0x500000000, 0x0,
0x800000010, 0x0, 0xffff88012b000030,
0xffff88012b002468, 0xffff88012b002668, 0xffff880123c7c7d0,
0xffff88012b002480, 0xffff88012b002680,
0xffff8801249a74f8, 0xffff8801249a74e0, 0xffffffff82a37790,
0xffff880123ca5c30, 0x700000001, 0x3e8}}

(gdb) p/x tid
$20 = 0x6c68931281

(gdb) up
#1 kmem_cache_alloc (s=0xffff88012b002300, gfpflags=32976) at mm/slub.c:1971
1971 void *ret = slab_alloc(s, gfpflags, NUMA_NO_NODE, _RET_IP_);
(gdb) up
#2 0xffffffff81162e5f in kmem_cache_zalloc () at include/linux/slab.h:310
310 return kmem_cache_alloc(k, flags | __GFP_ZERO);
(gdb) up
#3 get_empty_filp () at fs/file_table.c:123
123 f = kmem_cache_zalloc(filp_cachep, GFP_KERNEL);

(gdb) p filp_cachep (/note/ filp_cachep == 's' from slab_alloc())
$37 = (struct kmem_cache *) 0xffff88012b002300
(gdb) p *filp_cachep
$38 = {cpu_slab = 0x15300, flags = 0, min_partial = 7, size = 192,
objsize = 192, offset = 0, oo = {x = 21}, max = {
x = 21}, min = {x = 21}, allocflags = 0, refcount = 13, ctor = 0,
inuse = 192, align = 8, reserved = 0,
name = 0xffff88012b000010 "kmalloc-192", list = {next =
0xffff88012b002268, prev = 0xffff88012b002468}, kobj = {
name = 0xffff880123c7c830 ":t-0000192", entry = {next =
0xffff88012b002280, prev = 0xffff88012b002480},
parent = 0xffff8801249a74f8, kset = 0xffff8801249a74e0, ktype =
0xffffffff82a37790, sd = 0xffff880123ca6fa0,
kref = {refcount = {counter = 1}}, state_initialized = 1,
state_in_sysfs = 1, state_add_uevent_sent = 1,
state_remove_uevent_sent = 0, uevent_suppress = 0},
remote_node_defrag_ratio = 1000, node = {0xffff88012b0010c0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x15320, 0x0, 0x5, 0x800000008,
0x0, 0x200, 0x200, 0x200, 0x100000000, 0x0,
0x800000008, 0x0, 0xffff88012b000020, 0xffff88012b002368,
0xffff88012b002568, 0xffff880123c7c800,
0xffff88012b002380, 0xffff88012b002580, 0xffff8801249a74f8,
0xffff8801249a74e0, 0xffffffff82a37790,
0xffff880123ca65f0, 0x700000001, 0x3e8, 0xffff88012b001100, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x15340, 0x0, 0x5,
0x1000000010, 0x0, 0x100, 0x100, 0x100, 0x500000000, 0x0,
0x800000010, 0x0, 0xffff88012b000030,
0xffff88012b002468, 0xffff88012b002668, 0xffff880123c7c7d0,
0xffff88012b002480, 0xffff88012b002680,
0xffff8801249a74f8, 0xffff8801249a74e0, 0xffffffff82a37790,
0xffff880123ca5c30, 0x700000001, 0x3e8}}

(gdb) p *cred (it's 'killall' running from uid 0)
$46 = {usage = {counter = 116}, uid = 0, gid = 0, suid = 0, sgid = 0,
euid = 0, egid = 0, fsuid = 0, fsgid = 0,
securebits = 0, cap_inheritable = {cap = {0, 0}}, cap_permitted =
{cap = {4294967295, 4294967295}}, cap_effective = {
cap = {4294967295, 4294967295}}, cap_bset = {cap = {4294967295,
4294967295}}, jit_keyring = 0 '\000',
thread_keyring = 0x0, request_key_auth = 0x0, tgcred =
0xffff88011c49f040, security = 0xffff8801076cc0c0,
user = 0xffffffff82a21b20, group_info = 0xffff88009fb61540, rcu =
{next = 0x0, func = 0}}

--
Robert Święcki
--
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