mm: NULL ptr deref in anon_vma_interval_tree_verify

[Sasha Levin]

Hi all,

While fuzzing with trinity inside a KVM tools (lkvm) guest, on today's linux-next kernel,
I saw the following:

[ 1857.278176] BUG: unable to handle kernel NULL pointer dereference at 0000000000000090
[ 1857.283725] IP: [<ffffffff81229d0f>] anon_vma_interval_tree_verify+0xf/0xa0
[ 1857.283725] PGD 6e19e067 PUD 6e19f067 PMD 0
[ 1857.283725] Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[ 1857.283725] Dumping ftrace buffer:
[ 1857.283725]    (ftrace buffer empty)
[ 1857.283725] CPU 2
[ 1857.283725] Pid: 15637, comm: trinity-child18 Tainted: G        W    3.7.0-rc1-next-20121018-sasha-00002-g60a870d-dirty #61
[ 1857.283725] RIP: 0010:[<ffffffff81229d0f>]  [<ffffffff81229d0f>] anon_vma_interval_tree_verify+0xf/0xa0
[ 1857.283725] RSP: 0018:ffff88007df0fce8  EFLAGS: 00010296
[ 1857.283725] RAX: ffff880089db1000 RBX: ffff880089db0ff0 RCX: ffff8800869e6928
[ 1857.283725] RDX: 0000000000000000 RSI: ffff880089db1008 RDI: ffff880089db0ff0
[ 1857.283725] RBP: ffff88007df0fcf8 R08: ffff88006427d508 R09: ffff88012bb95f20
[ 1857.283725] R10: 0000000000000001 R11: ffff8800c8525c60 R12: ffff88006e199370
[ 1857.283725] R13: ffff88006e199300 R14: 0000000000000000 R15: ffff880089db1000
[ 1857.283725] FS:  00007f322fd4c700(0000) GS:ffff88004d600000(0000) knlGS:0000000000000000
[ 1857.283725] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1857.283725] CR2: 0000000000000090 CR3: 000000006e19d000 CR4: 00000000000406e0
[ 1857.283725] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1857.283725] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 1857.283725] Process trinity-child18 (pid: 15637, threadinfo ffff88007df0e000, task ffff88007ac80000)
[ 1857.283725] Stack:
[ 1857.283725]  ffff88007df0fd38 ffff880089db0ff0 ffff88007df0fd48 ffffffff81233b58
[ 1857.283725]  ffff88007df0fd38 ffff880089db1000 00000000000080d0 ffff880089db1000
[ 1857.283725]  ffff88012bb95f20 ffff8800005d97c8 ffff8800005d97d8 ffff880089db1000
[ 1857.283725] Call Trace:
[ 1857.283725]  [<ffffffff81233b58>] validate_mm+0x58/0x1e0
[ 1857.283725]  [<ffffffff81233da4>] vma_link+0x94/0xe0
[ 1857.283725]  [<ffffffff83a67fd4>] ? _raw_spin_unlock_irqrestore+0x84/0xb0
[ 1857.283725]  [<ffffffff81235f75>] mmap_region+0x3f5/0x5c0
[ 1857.283725]  [<ffffffff812363f7>] do_mmap_pgoff+0x2b7/0x330
[ 1857.283725]  [<ffffffff81220fd1>] ? vm_mmap_pgoff+0x61/0xa0
[ 1857.283725]  [<ffffffff81220fea>] vm_mmap_pgoff+0x7a/0xa0
[ 1857.283725]  [<ffffffff81234c72>] sys_mmap_pgoff+0x182/0x1a0
[ 1857.283725]  [<ffffffff8107dc40>] ? syscall_trace_enter+0x20/0x2e0
[ 1857.283725]  [<ffffffff810738dd>] sys_mmap+0x1d/0x20
[ 1857.283725]  [<ffffffff83a69ad8>] tracesys+0xe1/0xe6
[ 1857.283725] Code: 48 39 ce 77 9e f3 c3 0f 1f 44 00 00 31 c0 c3 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5 53 48 89 fb
48 83 ec 08 48 8b 17 <48> 8b 8a 90 00 00 00 48 39 4f 40 74 34 80 3d a6 82 5b 04 00 75
[ 1857.283725] RIP  [<ffffffff81229d0f>] anon_vma_interval_tree_verify+0xf/0xa0
[ 1857.283725]  RSP <ffff88007df0fce8>
[ 1857.283725] CR2: 0000000000000090
[ 1858.611277] ---[ end trace b51cc425e9b07fc0 ]---

The obvious part is that anon_vma_interval_tree_verify() got called with node == NULL, but when
looking at the caller:

                list_for_each_entry(avc, &vma->anon_vma_chain, same_vma)
                        anon_vma_interval_tree_verify(avc);

How it got called with said NULL becomes less obvious.


Thanks,
Sasha
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