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Subjectmm/sched/net: BUG when running simple code
Hi all,

Okay, I'm really lost. I got the following when fuzzing, and can't really explain what's
going on. It seems that we get a "unable to handle kernel paging request" when running
rather simple code, and I can't figure out how it would cause it.

The code in question is (in net/netlink/af_netlink.c):

static int netlink_getsockopt(struct socket *sock, int level, int optname,
char __user *optval, int __user *optlen)
{
struct sock *sk = sock->sk;
struct netlink_sock *nlk = nlk_sk(sk);
int len, val, err;

if (level != SOL_NETLINK)
return -ENOPROTOOPT;

if (get_user(len, optlen))
return -EFAULT;
if (len < 0) <==== THIS
return -EINVAL;

The disassembly I got shows:

if (get_user(len, optlen))
b1f: e8 00 00 00 00 callq b24 <netlink_getsockopt+0x44>
b20: R_X86_64_PC32 might_fault-0x4
b24: 4c 89 e0 mov %r12,%rax
b27: e8 00 00 00 00 callq b2c <netlink_getsockopt+0x4c>
b28: R_X86_64_PC32 __get_user_4-0x4
b2c: 85 c0 test %eax,%eax
b2e: 74 10 je b40 <netlink_getsockopt+0x60>
return -EFAULT;
b30: bb f2 ff ff ff mov $0xfffffff2,%ebx
b35: e9 06 01 00 00 jmpq c40 <netlink_getsockopt+0x160>
b3a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
if (len < 0)
b40: 85 d2 test %edx,%edx
b42: 0f 88 f0 00 00 00 js c38 <netlink_getsockopt+0x158>
return -EINVAL;

Which agrees with the trace I got:

[ 516.309720] BUG: unable to handle kernel paging request at ffffffffa0f12560
[ 516.309720] IP: netlink_getsockopt (net/netlink/af_netlink.c:2271)
[ 516.309720] PGD 22031067 PUD 22032063 PMD 8000000020e001e1
[ 516.309720] Oops: 0003 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[ 516.309720] Dumping ftrace buffer:
[ 516.309720] (ftrace buffer empty)
[ 516.309720] Modules linked in:
[ 516.309720] CPU: 11 PID: 9212 Comm: trinity-c11 Tainted: G W 3.15.0-next-20140612-sasha-00022-g5e4db85-dirty #645
[ 516.309720] task: ffff8803fc860000 ti: ffff8803fc85c000 task.ti: ffff8803fc85c000
[ 516.309720] RIP: netlink_getsockopt (net/netlink/af_netlink.c:2271)
[ 516.309720] RSP: 0018:ffff8803fc85fed8 EFLAGS: 00010216
[ 516.309720] RAX: ffffffffa0f12560 RBX: 00000000ffffffa4 RCX: 0000000000000003
[ 516.309720] RDX: 00000000ffff9002 RSI: 0000000049908020 RDI: ffff88025c16a100
[ 516.309720] RBP: ffff8803fc85ff18 R08: 0000000000000001 R09: c900000000fd37ff
[ 516.309720] R10: 0000000000000001 R11: 0000000000000000 R12: ffffffffffff9002
[ 516.309720] R13: ffff88025c16a100 R14: 0000000000000001 R15: ffff88025bfa9bd8
[ 516.309720] FS: 00007f54be0a7700(0000) GS:ffff8802c8e00000(0000) knlGS:0000000000000000
[ 516.309720] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 516.309720] CR2: ffffffffa0f12560 CR3: 000000040b1fb000 CR4: 00000000000006a0
[ 516.309720] Stack:
[ 516.309720] ffff8803fc85ff18 ffff8803fc85ff18 ffff8803fc85fef8 8900200549908020
[ 516.309720] ffff8803fc85ff18 ffffffff9ff66470 ffff8803fc85ff18 0000000000000037
[ 516.309720] ffff8803fc85ff78 ffffffff9ff69d26 0000000000000037 0000000000000004
[ 516.309720] Call Trace:
[ 516.309720] ? sockfd_lookup_light (net/socket.c:457)
[ 516.309720] SyS_getsockopt (net/socket.c:1945 net/socket.c:1929)
[ 516.309720] tracesys (arch/x86/kernel/entry_64.S:542)
[ 516.309720] Code: b2 fd 85 c0 74 10 bb f2 ff ff ff e9 06 01 00 00 66 0f 1f 44 00 00 85 d2 0f 88 f0 00 00 00 41 83 fd 04 74 42 41 83 fd 05 0f 84 88 <00> 00 00 41 83 fd 03 0f 85 de 00 00 00 83 fa 03 bb ea ff ff ff
All code
========
0: b2 fd mov $0xfd,%dl
2: 85 c0 test %eax,%eax
4: 74 10 je 0x16
6: bb f2 ff ff ff mov $0xfffffff2,%ebx
b: e9 06 01 00 00 jmpq 0x116
10: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
16: 85 d2 test %edx,%edx
18:* 0f 88 f0 00 00 00 js 0x10e <-- trapping instruction
1e: 41 83 fd 04 cmp $0x4,%r13d
22: 74 42 je 0x66
24: 41 83 fd 05 cmp $0x5,%r13d
28: 0f 84 88 00 00 00 je 0xb6
2e: 41 83 fd 03 cmp $0x3,%r13d
32: 0f 85 de 00 00 00 jne 0x116
38: 83 fa 03 cmp $0x3,%edx
3b: bb ea ff ff ff mov $0xffffffea,%ebx
...

Code starting with the faulting instruction
===========================================
0: 00 00 add %al,(%rax)
2: 00 41 83 add %al,-0x7d(%rcx)
5: fd std
6: 03 0f add (%rdi),%ecx
8: 85 de test %ebx,%esi
a: 00 00 add %al,(%rax)
c: 00 83 fa 03 bb ea add %al,-0x1544fc06(%rbx)
12: ff (bad)
13: ff (bad)
14: ff 00 incl (%rax)
[ 516.309720] RIP netlink_getsockopt (net/netlink/af_netlink.c:2271)
[ 516.309720] RSP <ffff8803fc85fed8>
[ 516.309720] CR2: ffffffffa0f12560

They only theory I had so far is that netlink is a module, and has gone away while the code
was executing, but netlink isn't a module on my kernel.



Thanks,
Sasha


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