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Subjectmm: derefing NULL vma->vm_mm when unmapping
Hi all,

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

[ 761.704089] BUG: unable to handle kernel NULL pointer dereference at (null)
[ 761.704089] IP: mm_find_pmd (mm/rmap.c:570)
[ 761.704089] PGD 51223067 PUD 50a09067 PMD 0
[ 761.704089] Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[ 761.704089] Dumping ftrace buffer:
[ 761.704089] (ftrace buffer empty)
[ 761.704089] Modules linked in:
[ 761.704089] CPU: 4 PID: 20723 Comm: trinity-c131 Tainted: G W 3.16.0-rc3-next-20140630-sasha-00023-g44434d4-dirty #756
[ 761.704089] task: ffff88004e3c0000 ti: ffff88004e0b8000 task.ti: ffff88004e0b8000
[ 761.704089] RIP: mm_find_pmd (mm/rmap.c:570)
[ 761.704089] RSP: 0000:ffff88004e0bbaa8 EFLAGS: 00010246
[ 761.704089] RAX: 0000000000000000 RBX: 0000000000a65000 RCX: ffff88004e0bbb30
[ 761.704089] RDX: 0000000000000000 RSI: 0000000000a65000 RDI: ffff880000146000
[ 761.704089] RBP: ffff88004e0bbaa8 R08: 0000000000000000 R09: 0000000000000000
[ 761.704089] R10: ffff88004e3c0000 R11: 0000000000000000 R12: ffffea000d766e00
[ 761.704089] R13: ffff88004e0bbb30 R14: ffff880000146000 R15: 0000000000000000
[ 761.704089] FS: 00007f0293c61700(0000) GS:ffff880144e00000(0000) knlGS:0000000000000000
[ 761.704089] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 761.704089] CR2: 0000000000000000 CR3: 000000004e3be000 CR4: 00000000000006a0
[ 761.704089] Stack:
[ 761.704089] ffff88004e0bbae8 ffffffff9c2d0815 800000035d9b8805 ffff880000146000
[ 761.704089] ffffea000d766e00 ffff88000b4c4e58 ffff880034d7d200 0000000000000302
[ 761.704089] ffff88004e0bbb68 ffffffff9c2d1491 ffff88004e0bbb28 ffffffff9f57c58a
[ 761.704089] Call Trace:
[ 761.704089] __page_check_address (mm/rmap.c:618)
[ 761.704089] try_to_unmap_one (mm/rmap.c:1133)
[ 761.704089] ? down_read (kernel/locking/rwsem.c:45 (discriminator 2))
[ 761.704089] ? page_lock_anon_vma_read (./arch/x86/include/asm/atomic.h:118 mm/rmap.c:491)
[ 761.704089] ? page_lock_anon_vma_read (mm/rmap.c:448)
[ 761.704089] rmap_walk (mm/rmap.c:1634 mm/rmap.c:1705)
[ 761.704089] try_to_unmap (mm/rmap.c:1527)
[ 761.704089] ? page_remove_rmap (mm/rmap.c:1124)
[ 761.704089] ? invalid_migration_vma (mm/rmap.c:1483)
[ 761.704089] ? try_to_unmap_one (mm/rmap.c:1391)
[ 761.704089] ? anon_vma_prepare (mm/rmap.c:448)
[ 761.704089] ? invalid_mkclean_vma (mm/rmap.c:1478)
[ 761.704089] ? page_get_anon_vma (mm/rmap.c:405)
[ 761.704089] migrate_pages (mm/migrate.c:912 mm/migrate.c:955 mm/migrate.c:1142)
[ 761.704089] ? perf_trace_mm_numa_migrate_ratelimit (mm/migrate.c:1590)
[ 761.704089] migrate_misplaced_page (mm/migrate.c:1750)
[ 761.704089] __handle_mm_fault (mm/memory.c:3162 mm/memory.c:3212 mm/memory.c:3322)
[ 761.704089] handle_mm_fault (include/linux/memcontrol.h:124 mm/memory.c:3348)
[ 761.704089] ? __do_page_fault (arch/x86/mm/fault.c:1163)
[ 761.704089] __do_page_fault (arch/x86/mm/fault.c:1230)
[ 761.704089] ? vtime_account_user (kernel/sched/cputime.c:687)
[ 761.704089] ? get_parent_ip (kernel/sched/core.c:2550)
[ 761.704089] ? context_tracking_user_exit (include/linux/vtime.h:89 include/linux/jump_label.h:115 include/trace/events/context_tracking.h:47 kernel/context_tracking.c:180)
[ 761.704089] ? preempt_count_sub (kernel/sched/core.c:2606)
[ 761.704089] ? context_tracking_user_exit (kernel/context_tracking.c:184)
[ 761.704089] ? __this_cpu_preempt_check (lib/smp_processor_id.c:63)
[ 761.704089] ? trace_hardirqs_off_caller (kernel/locking/lockdep.c:2638 (discriminator 2))
[ 761.704089] trace_do_page_fault (arch/x86/mm/fault.c:1313 include/linux/jump_label.h:115 include/linux/context_tracking_state.h:27 include/linux/context_tracking.h:45 arch/x86/mm/fault.c:1314)
[ 761.704089] do_async_page_fault (arch/x86/kernel/kvm.c:264)
[ 761.704089] async_page_fault (arch/x86/kernel/entry_64.S:1322)
[ 761.704089] Code: 00 48 8b 5d f0 4c 8b 65 f8 c9 c3 66 0f 1f 44 00 00 66 66 66 66 90 55 48 89 f2 48 8b 47 40 48 c1 ea 27 48 89 e5 81 e2 ff 01 00 00 <48> 8b 3c d0 40 f6 c7 01 75 0c 31 f6 e9 af 00 00 00 0f 1f 44 00
All code
========
0: 00 48 8b add %cl,-0x75(%rax)
3: 5d pop %rbp
4: f0 4c 8b 65 f8 lock mov -0x8(%rbp),%r12
9: c9 leaveq
a: c3 retq
b: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
11: 66 66 66 66 90 data32 data32 data32 xchg %ax,%ax
16: 55 push %rbp
17: 48 89 f2 mov %rsi,%rdx
1a: 48 8b 47 40 mov 0x40(%rdi),%rax
1e: 48 c1 ea 27 shr $0x27,%rdx
22: 48 89 e5 mov %rsp,%rbp
25: 81 e2 ff 01 00 00 and $0x1ff,%edx
2b:* 48 8b 3c d0 mov (%rax,%rdx,8),%rdi <-- trapping instruction
2f: 40 f6 c7 01 test $0x1,%dil
33: 75 0c jne 0x41
35: 31 f6 xor %esi,%esi
37: e9 af 00 00 00 jmpq 0xeb
3c: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)

Code starting with the faulting instruction
===========================================
0: 48 8b 3c d0 mov (%rax,%rdx,8),%rdi
4: 40 f6 c7 01 test $0x1,%dil
8: 75 0c jne 0x16
a: 31 f6 xor %esi,%esi
c: e9 af 00 00 00 jmpq 0xc0
11: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
[ 761.704089] RIP mm_find_pmd (mm/rmap.c:570)
[ 761.704089] RSP <ffff88004e0bbaa8>
[ 761.704089] CR2: 0000000000000000

As I didn't see any code changes around that part I'm thinking that it's a locking
issue that got messed up somewhere rather then a missing '!= NULL' check.


Thanks,
Sasha


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