kernel BUG at mm/usercopy.c:72!

[Breno Leitao]

Hello,

Kernel 4.12-rc1 is showing a bug when I try it on a POWER8 virtual
machine. Justing SSHing into the machine causes this issue.

	[23.138124] usercopy: kernel memory overwrite attempt detected to d000000003d80030 (mm_struct) (560 bytes)
	[23.138195] ------------[ cut here ]------------
	[23.138229] kernel BUG at mm/usercopy.c:72!
	[23.138252] Oops: Exception in kernel mode, sig: 5 [#3]
	[23.138280] SMP NR_CPUS=2048 
	[23.138280] NUMA 
	[23.138302] pSeries
	[23.138330] Modules linked in:
	[23.138354] CPU: 4 PID: 2215 Comm: sshd Tainted: G      D         4.12.0-rc1+ #9
	[23.138395] task: c0000001e272dc00 task.stack: c0000001e27b0000
	[23.138430] NIP: c000000000342358 LR: c000000000342354 CTR: c0000000006eb060
	[23.138472] REGS: c0000001e27b3a00 TRAP: 0700   Tainted: G      D          (4.12.0-rc1+)
	[23.138513] MSR: 8000000000029033 <SF,EE,ME,IR,DR,RI,LE>
	[23.138517]   CR: 28004222  XER: 20000000
	[23.138565] CFAR: c000000000b34500 SOFTE: 1 
	[23.138565] GPR00: c000000000342354 c0000001e27b3c80 c00000000142a000 000000000000005e 
	[23.138565] GPR04: c0000001ffe0ade8 c0000001ffe21bf8 2920283536302062 79746573290d0a74 
	[23.138565] GPR08: 0000000000000007 c000000000f61864 00000001feeb0000 3064206f74206465 
	[23.138565] GPR12: 0000000000004400 c00000000fb42600 0000000000000015 00000000545bdc40 
	[23.138565] GPR16: 00000000545c49c8 000001000b4b8890 00007ffff78c26f0 00000000545cf000 
	[23.138565] GPR20: 00000000546109c8 000000000000c7e8 0000000054610010 00007ffff78c22e8 
	[23.138565] GPR24: 00000000545c8c40 c0000000ff6bcef0 c0000000001e5220 0000000000000230 
	[23.138565] GPR28: d000000003d80260 0000000000000000 0000000000000230 d000000003d80030 
	[23.138920] NIP [c000000000342358] __check_object_size+0x88/0x2d0
	[23.138956] LR [c000000000342354] __check_object_size+0x84/0x2d0
	[23.138990] Call Trace:
	[23.139006] [c0000001e27b3c80] [c000000000342354] __check_object_size+0x84/0x2d0 (unreliable)
	[23.139056] [c0000001e27b3d00] [c0000000009f5ba8] bpf_prog_create_from_user+0xa8/0x1a0
	[23.139099] [c0000001e27b3d60] [c0000000001e5d30] do_seccomp+0x120/0x720
	[23.139136] [c0000001e27b3dd0] [c0000000000fd53c] SyS_prctl+0x2ac/0x6b0
	[23.139172] [c0000001e27b3e30] [c00000000000af84] system_call+0x38/0xe0
	[23.139218] Instruction dump:
	[23.139240] 60000000 60420000 3c82ff94 3ca2ff9d 38841788 38a5e868 3c62ff95 7fc8f378 
	[23.139283] 7fe6fb78 386310c0 487f2169 60000000 <0fe00000> 60420000 2ba30010 409d018c 
	[23.139328] ---[ end trace 1a1dc952a4b7c4af ]---
	
I found that kernel 4.11 does not have this issue. I also found that, if
I revert 517e1fbeb65f5eade8d14f46ac365db6c75aea9b, I do not see the
problem.

On the other side, if I cherry-pick commit
517e1fbeb65f5eade8d14f46ac365db6c75aea9b into 4.11, I start seeing the
same issue also on 4.11.

FWIW I am using the following .config file[1].

[1] http://paste.ubuntu.com/24582478/