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SubjectRe: Fwd: Oops (bad memory deref) in slab_alloc() due to filp_cachep holding incorrect values
On Mon, May 23, 2011 at 6:35 PM, Christoph Lameter <> wrote:
> On Mon, 23 May 2011, Robert Święcki wrote:
>> Hi, not sure if it's related to SLUB, in any case forwarding.
> Please reboot with the kernel parameter slub_debug or enable
> CONFIG_SLUB_DEBUG_ON. The kernel will then verify all operations of the
> users of the allocator and log important issues to the syslog. This is
> likely some memory corruption.

So, I ran it with CONFIG_SLUB_DEBUG_ON and got Oops in just under a
second - the important thing is that probably that instead of
2.6.39-rc4 I ran 2.6.39

It was

<1>[ 77.662935] BUG: unable to handle kernel NULL pointer
dereference at 0000000000000408
<1>[ 77.663002] IP: [<ffffffff8134cbdf>] cap_capable+0x18/0x67
<4>[ 77.663002] PGD 120428067 PUD 120e35067 PMD 0
<0>[ 77.663002] Oops: 0000 [#1] PREEMPT SMP
<0>[ 77.680925] last sysfs file:

and kdb's backtrace shows:

[1]kdb> bt
Stack traceback for pid 8419
0xffff880120e3aee0 8419 8418 1 1 R 0xffff880120e3b360 *iknowthis2
<c> ffff88011f57deb8<c> 0000000000000000<c> ffff88011f5731a0<c>
<c> ffffffff81378bfb<c> ffff88011f57ded8<c> ffff880120e3aee0<c>
<c> ffff88011f540500<c> 0000000000000000<c> 0000000000000000<c>
Call Trace:
[<ffffffff81378bfb>] ? apparmor_capable+0x27/0x61
[<ffffffff8134d932>] ? security_capable+0x2a/0x2c
[<ffffffff8109d99d>] ? ns_capable+0x3a/0x4f
[<ffffffff8109da2b>] ? nsown_capable+0x24/0x29
[<ffffffff810a6695>] ? sys_setgid+0x43/0x8d
[<ffffffff810c1e11>] ? sys_setgid16+0x1c/0x1e
[<ffffffff81ed6183>] ? ia32_do_call+0x13/0x13

and kgdb shows

(gdb) target remote /dev/ttyS0
Remote debugging using /dev/ttyS0
0xffffffff8134cbdf in cap_capable (tsk=<value optimized out>,
cred=0x0, targ_ns=0x0, cap=6, audit=1) at security/commoncap.c:88
88 if (targ_ns != &init_user_ns && targ_ns->creator == cred->user)

(gdb) p cred
$1 = (const struct cred *) 0x0

(gdb) bt
#0 0xffffffff8134cbdf in cap_capable (tsk=<value optimized out>,
cred=0x0, targ_ns=0x0, cap=6, audit=1) at security/commoncap.c:88
#1 0xffffffff81378bfb in apparmor_capable (task=0xffff880120e3aee0,
cred=0xffff88011f540500, ns=<value optimized out>, cap=6, audit=1) at
#2 0xffffffff8134d932 in security_capable (ns=<value optimized out>,
cred=<value optimized out>, cap=<value optimized out>) at
#3 0xffffffff8109d99d in ns_capable (ns=<value optimized out>,
cap=<value optimized out>) at kernel/capability.c:381
#4 0xffffffff8109da2b in nsown_capable (cap=<value optimized out>) at
#5 0xffffffff810a6695 in sys_setgid (gid=1540) at kernel/sys.c:570
#6 0xffffffff810c1e11 in sys_setgid16 (gid=<value optimized out>) at
#7 <signal handler called>
#8 0x0000000008065952 in __brk_reservation_fn_dmi_alloc__ ()

I looked through apparmor_capable and cap_capable and there's nothing
that could change value of 'cred' from 0xffff880120e3aee0 to 0 between
apparmor_capable() and cap_capable() (i.e. no buffer-overflow causing
functions, in fact no functions or no interesting operations at all).

Out of curiosity I looked at what runs on the other CPU, and it was:

[0]kdb> bt
Stack traceback for pid 23087
0xffff880120df8000 23087 16945 1 0 R 0xffff880120df8480 *iknowthis
<c> ffff880120d61c58<c> 0000000000000018<c>
Call Trace:
<NMI> <<EOE>> [<ffffffff814feb2d>] ? extract_buf+0x5e/0x105
[<ffffffff81ed3f85>] ? retint_restore_args+0x13/0x13
[<ffffffff814fee11>] ? extract_entropy_user+0xb5/0x124
[<ffffffff814fee95>] ? urandom_read+0x15/0x17
[<ffffffff8116200b>] ? vfs_read+0xa9/0xfc
[<ffffffff811633f5>] ? fget_light+0x42/0xa2
[<ffffffff811622a6>] ? sys_read+0x4a/0x6e
[<ffffffff81ed4902>] ? system_call_fastpath+0x16/0x1b

It's very strange, cause in the first case (the problem with SLUB),
the second CPU was also stuck in this routine, and I'm kinda certain
it is the real cause of the problem (it overwrites another CPU's
stack?). My config is here:
- so the question is; could somebody more familiar with kernel
internals look into /dev/urandom sys_read routines. I think they don't
play nicely with SMP PREEMPT kernel (or maybe it just runs out of
kernel stack in there and overwrites another one). Some more data
about the second thread:

(gdb) thread 1
[Switching to thread 1 (Thread 23087)]#0 sha_transform
<incomplete sequence \364>, W=0xffff880120d61ca8) at lib/sha1.c:47
47 W[i+16] = rol32(W[i+13] ^ W[i+8] ^ W[i+2] ^ W[i], 1);

(gdb) bt
#0 sha_transform (digest=0xffff880120d61de8, in=0xffff880120d61ca8
<incomplete sequence \364>,
W=0xffff880120d61ca8) at lib/sha1.c:47
#1 0xffffffff814feb2d in extract_buf (r=0xffffffff82a88670,
out=0xffff880120d61e98 "\320S\032|\201\f7\215\333$") at
#2 0xffffffff814fee11 in extract_entropy_user (r=0xffffffff82a88670,
buf=0x7fec871ced98, nbytes=616) at drivers/char/random.c:910
#3 0xffffffff814fee95 in urandom_read (file=<value optimized out>,
buf=<value optimized out>, nbytes=<value optimized out>, ppos=<value
optimized out>) at drivers/char/random.c:1063
#4 0xffffffff8116200b in vfs_read (file=0xffff880121f13900,
buf=0x7fec871ce000 ",\027\372\061\213L\224
count=<value optimized out>, pos=0xffff880120d61f58) at
#5 0xffffffff811622a6 in sys_read (fd=<value optimized out>,
buf=0x7fec871ce000 ",\027\372\061\213L\224
count=<value optimized out>) at fs/read_write.c:411
#6 <signal handler called>
#7 0x00007fec865e4340 in __brk_reservation_fn_dmi_alloc__ ()
#8 0xffff880120dc1770 in __brk_reservation_fn_dmi_alloc__ ()
#9 0xffffffff82a1bb50 in ?? ()
#10 0x0000000200020000 in __brk_reservation_fn_dmi_alloc__ ()
#11 0x0000000300000001 in __brk_reservation_fn_dmi_alloc__ ()
#12 0x00007ffffffff000 in __brk_reservation_fn_dmi_alloc__ ()
#13 0xffffffff810a4c3d in sys_restart_syscall () at kernel/signal.c:2085
#14 0x0000000000000000 in ?? ()

(gdb) info registers
rax 0xf21ac03a 4061839418
rbx 0xffff880120d61de8 -131936549462552
rcx 0x80 128
rdx 0xffff880120d61ca8 -131936549462872
rsi 0xffff880120d61ca8 -131936549462872
rdi 0xffff880120d61de8 -131936549462552
rbp 0xffff880120d61c98 0xffff880120d61c98
rsp 0xffff880120d61c58 0xffff880120d61c58
r8 0xbcc035f2 3166713330
r9 0x50 80
r10 0xffff880120d61cfc -131936549462788
r11 0x47013d0b 1191263499
r12 0x10 16
r13 0xffffffff82a88670 -2102884752
r14 0xffff880120d61e98 -131936549462376
r15 0xa 10
rip 0xffffffff813d0297 0xffffffff813d0297 <sha_transform+39>
eflags 0x283 [ CF SF IF ]
cs 0x10 16
ss 0x18 24
ds Could not fetch register "ds"; remote failure reply 'E22'

(gdb) disassemble $pc
Dump of assembler code for function sha_transform:
0xffffffff813d0270 <+0>: push rbp
0xffffffff813d0271 <+1>: xor eax,eax
0xffffffff813d0273 <+3>: mov rbp,rsp
0xffffffff813d0276 <+6>: push r15
0xffffffff813d0278 <+8>: push r14
0xffffffff813d027a <+10>: push r13
0xffffffff813d027c <+12>: push r12
0xffffffff813d027e <+14>: push rbx
0xffffffff813d027f <+15>: sub rsp,0x18
0xffffffff813d0283 <+19>: mov ecx,DWORD PTR [rsi+rax*1]
0xffffffff813d0286 <+22>: bswap ecx
0xffffffff813d0288 <+24>: mov DWORD PTR [rdx+rax*1],ecx
0xffffffff813d028b <+27>: add rax,0x4
0xffffffff813d028f <+31>: cmp rax,0x40
0xffffffff813d0293 <+35>: jne 0xffffffff813d0283 <sha_transform+19>
0xffffffff813d0295 <+37>: xor ecx,ecx
=> 0xffffffff813d0297 <+39>: mov eax,DWORD PTR [rdx+rcx*1+0x20]
0xffffffff813d029b <+43>: xor eax,DWORD PTR [rdx+rcx*1+0x34]
0xffffffff813d029f <+47>: xor eax,DWORD PTR [rdx+rcx*1+0x8]
0xffffffff813d02a3 <+51>: xor eax,DWORD PTR [rdx+rcx*1]
0xffffffff813d02a6 <+54>: ror eax,0x1f
0xffffffff813d02a9 <+57>: mov DWORD PTR [rdx+rcx*1+0x40],eax
0xffffffff813d02ad <+61>: add rcx,0x4

Robert Święcki
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