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    SubjectRe: [BUG]NULL pointer dereference at 0000000000000008 __blkdev_put+0x17f/0x1d0
    On 12/30/2013 04:55 PM, Jack Wang wrote:
    > Hi,
    >
    > We saw NULL pointer dereference below:
    >
    > Dec 28 16:24:26 server kernel: [979193.076399] BUG: unable to handle
    > kernel NULL pointer dereference at 0000000000000008
    > Dec 28 16:24:26 server kernel: [979193.076401] IP: [<ffffffff8116952f>]
    > __blkdev_put+0x17f/0x1d0
    > Dec 28 16:24:26 server kernel: [979193.076408] PGD 4bdcaa067 PUD
    > 4bdc43067 PMD 0
    > Dec 28 16:24:26 server kernel: [979193.076410] Oops: 0000 [#1] SMP
    > Dec 28 16:24:26 server kernel: [979193.076412] CPU 6
    > Dec 28 16:24:26 server kernel: [979193.076413] Modules linked in: bridge
    > stp llc nf_conntrack_ipv6 nf_defrag_ipv6 ip6table_filter ip6_tables
    > raid1 md_mod dm_round_robin sd_mod crc_t10dif ib_srp scsi_transport_srp
    > scsi_tgt xt_ETHOIP6(O) x_tables vhost_net(O) macvtap macvlan tun(O)
    > nf_conntrack_ipv4 nf_conntrack nf_defrag_ipv4 rdma_ucm rdma_cm iw_cm
    > ib_addr ib_ipoib ib_cm ib_sa ib_uverbs ib_umad ib_qib mlx4_ib ib_mthca
    > ib_mad ib_core dm_multipath scsi_dh scsi_mod kvm_amd kvm powernow_k8
    > mperf psmouse crc32c_intel microcode tpm_tis tpm tpm_bios serio_raw
    > evdev amd64_edac_mod edac_core edac_mce_amd i2c_piix4 button processor
    > thermal_sys mlx4_core
    > Dec 28 16:24:26 server kernel: [979193.076440]
    > Dec 28 16:24:26 server kernel: [979193.076442] Pid: 56544, comm:
    > multipath Tainted: G O 3.4.71-3-pserver #1 Supermicro BHQGE/BHQGE
    > Dec 28 16:24:26 server kernel: [979193.076445] RIP:
    > 0010:[<ffffffff8116952f>] [<ffffffff8116952f>] __blkdev_put+0x17f/0x1d0
    > Dec 28 16:24:26 server kernel: [979193.076448] RSP:
    > 0018:ffff882802f4beb8 EFLAGS: 00010246
    > Dec 28 16:24:26 server kernel: [979193.076449] RAX: 0000000000000000
    > RBX: ffff881ff78b0d00 RCX: 0000000000000001
    > Dec 28 16:24:26 server kernel: [979193.076451] RDX: 0000000000000000
    > RSI: 000000000000001d RDI: ffff881ff78b0d18
    > Dec 28 16:24:26 server kernel: [979193.076452] RBP: 0000000000000000
    > R08: 0000000000000000 R09: 0000000000000000
    > Dec 28 16:24:26 server kernel: [979193.076453] R10: 0000000000000000
    > R11: 0000000000000246 R12: 000000000000001d
    > Dec 28 16:24:26 server kernel: [979193.076455] R13: ffff881ff78b0d18
    > R14: ffff8807f9e7f400 R15: ffff8804a8d77710
    > Dec 28 16:24:26 server kernel: [979193.076457] FS:
    > 00007ff8c80fe7a0(0000) GS:ffff880807d80000(0000) knlGS:0000000000000000
    > Dec 28 16:24:26 server kernel: [979193.076458] CS: 0010 DS: 0000 ES:
    > 0000 CR0: 0000000080050033
    > Dec 28 16:24:26 server kernel: [979193.076460] CR2: 0000000000000008
    > CR3: 000000064765f000 CR4: 00000000000407e0
    > Dec 28 16:24:26 server kernel: [979193.076461] DR0: 0000000000000000
    > DR1: 0000000000000000 DR2: 0000000000000000
    > Dec 28 16:24:26 server kernel: [979193.076463] DR3: 0000000000000000
    > DR6: 00000000ffff0ff0 DR7: 0000000000000400
    > Dec 28 16:24:26 server kernel: [979193.076464] Process multipath (pid:
    > 56544, threadinfo ffff882802f4a000, task ffff8828020106d0)
    > Dec 28 16:24:26 server kernel: [979193.076466] Stack:
    > Dec 28 16:24:26 server kernel: [979193.076466] 0000000000000000
    > 0000000000000000 ffff880803cf2580 ffff8804a8d77700
    > Dec 28 16:24:26 server kernel: [979193.076468] 0000000000000010
    > ffff88100363eff0 ffff881004609b00 ffff882003c20020
    > Dec 28 16:24:26 server kernel: [979193.076470] ffff8804a8d77710
    > ffffffff81136bad 00007fffbdc8f420 ffff8804a8d77700
    > Dec 28 16:24:26 server kernel: [979193.076472] Call Trace:
    > Dec 28 16:24:26 server kernel: [979193.076477] [<ffffffff81136bad>] ?
    > fput+0xdd/0x270
    > Dec 28 16:24:26 server kernel: [979193.076479] [<ffffffff81132f0c>] ?
    > filp_close+0x5c/0x90
    > Dec 28 16:24:26 server kernel: [979193.076481] [<ffffffff81132fb1>] ?
    > sys_close+0x71/0xc0
    > Dec 28 16:24:26 server kernel: [979193.076484] [<ffffffff816801b9>] ?
    > system_call_fastpath+0x16/0x1b
    > Dec 28 16:24:26 server kernel: [979193.076486] Code: 8b 5c 24 18 48 8b
    > 6c 24 20 4c 8b 64 24 28 4c 8b 6c 24 30 4c 8b 74 24 38 4c 8b 7c 24 40 48
    > 83 c4 48 c3 66 90 49 8b 86 48 03 00 00 <48> 8b 40 08 48 85 c0 0f 84 fc
    > fe ff ff 44 89 e6 4c 89 f7 ff d0
    > Dec 28 16:24:26 server kernel: [979193.076500] RIP [<ffffffff8116952f>]
    > __blkdev_put+0x17f/0x1d0
    > Dec 28 16:24:26 server kernel: [979193.076503] RSP <ffff882802f4beb8>
    > Dec 28 16:24:26 server kernel: [979193.076504] CR2: 0000000000000008
    > Dec 28 16:24:26 server kernel: [979193.077599] ---[ end trace
    > 23f39da823d257f9 ]---
    >
    > disassamble results show:
    > 1465 static int __blkdev_put(struct block_device *bdev, fmode_t mode,
    > int for_part)
    > 1466 {
    > 0xffffffff81162d10 <+0>: sub $0x48,%rsp
    > 0xffffffff81162d14 <+4>: mov %r13,0x30(%rsp)
    > 0xffffffff81162d1d <+13>: mov %rbx,0x18(%rsp)
    > 0xffffffff81162d22 <+18>: mov %rbp,0x20(%rsp)
    > 0xffffffff81162d27 <+23>: mov %r12,0x28(%rsp)
    > 0xffffffff81162d2c <+28>: mov %edx,%ebp
    > 0xffffffff81162d2e <+30>: mov %r14,0x38(%rsp)
    > 0xffffffff81162d33 <+35>: mov %r15,0x40(%rsp)
    > 0xffffffff81162d38 <+40>: mov %rdi,%rbx
    > 0xffffffff81162d45 <+53>: mov %esi,%r12d
    >
    > 1467 int ret = 0;
    > 0xffffffff81162d8e <+126>: xor %ebp,%ebp
    >
    > 1468 struct gendisk *disk = bdev->bd_disk;
    > 0xffffffff81162d3b <+43>: mov 0x90(%rdi),%r14
    >
    > 1469 struct block_device *victim = NULL;
    > 1470
    > 1471 mutex_lock_nested(&bdev->bd_mutex, for_part);
    > 0xffffffff81162d19 <+9>: lea 0x18(%rdi),%r13
    > ---Type <return> to continue, or q <return> to quit---
    > 0xffffffff81162d42 <+50>: mov %r13,%rdi
    > 0xffffffff81162d48 <+56>: callq 0xffffffff8166ece0 <mutex_lock>
    >
    > 1472 if (for_part)
    > 0xffffffff81162d4d <+61>: test %ebp,%ebp
    > 0xffffffff81162d4f <+63>: je 0xffffffff81162d57 <__blkdev_put+71>
    >
    > 1473 bdev->bd_part_count--;
    > 0xffffffff81162d51 <+65>: decl 0x88(%rbx)
    >
    > 1474
    > 1475 if (!--bdev->bd_openers) {
    > 0xffffffff81162d57 <+71>: mov 0x4(%rbx),%eax
    > 0xffffffff81162d5a <+74>: dec %eax
    > 0xffffffff81162d5c <+76>: test %eax,%eax
    > 0xffffffff81162d5e <+78>: mov %eax,0x4(%rbx)
    > 0xffffffff81162d61 <+81>: jne 0xffffffff81162d8e <__blkdev_put+126>
    >
    > 1476 WARN_ON_ONCE(bdev->bd_holders);
    > 0xffffffff81162d63 <+83>: mov 0x58(%rbx),%edx
    > 0xffffffff81162d66 <+86>: test %edx,%edx
    > 0xffffffff81162d68 <+88>: jne 0xffffffff81162e9b <__blkdev_put+395>
    > 0xffffffff81162e9b <+395>: cmpb $0x1,0x936b1e(%rip) #
    > 0xffffffff81a999c0 <__warned.29603>
    > 0xffffffff81162ea2 <+402>: je 0xffffffff81162d6e <__blkdev_put+94>
    > 0xffffffff81162ea8 <+408>: mov $0x5c4,%esi
    > 0xffffffff81162ead <+413>: mov $0xffffffff8193f5a7,%rdi
    > 0xffffffff81162eb4 <+420>: callq 0xffffffff81036ee0 <warn_slowpath_null>
    > 0xffffffff81162eb9 <+425>: movb $0x1,0x936b00(%rip) #
    > 0xffffffff81a999c0 <__warned.29603>
    > 0xffffffff81162ec0 <+432>: jmpq 0xffffffff81162d6e <__blkdev_put+94>
    > 0xffffffff81162ec5: data32 nopw %cs:0x0(%rax,%rax,1)
    >
    > 1484 }
    > 1485 if (bdev->bd_contains == bdev) {
    > 0xffffffff81162d90 <+128>: cmp %rbx,0x70(%rbx)
    > 0xffffffff81162d94 <+132>: je 0xffffffff81162e78 <__blkdev_put+360>
    >
    > 1486 if (disk->fops->release)
    > ---Type <return> to continue, or q <return> to quit---
    > 0xffffffff81162e78 <+360>: mov 0x348(%r14),%raxat
    > 0xffffffff81162e7f <+367>: mov 0x8(%rax),%rax
    > 0xffffffff81162e83 <+371>: test %rax,%rax
    > 0xffffffff81162e86 <+374>: je 0xffffffff81162d9a <__blkdev_put+138>
    >
    > 1487 ret = disk->fops->release(disk, mode);
    > 0xffffffff81162e8c <+380>: mov %r12d,%esi
    > 0xffffffff81162e8f <+383>: mov %r14,%rdi
    > 0xffffffff81162e92 <+386>: callq *%rax
    > 0xffffffff81162e94 <+388>: mov %eax,%ebp
    > 0xffffffff81162e96 <+390>: jmpq 0xffffffff81162d9a <__blkdev_put+138>
    > snip
    >
    > Bug happened at line 1486, looks disk->fops is NULL here for some
    > reason, is it reasonable to add a check like:
    >
    > if (disk->fops)
    > if (disk->fops->release)
    > ret = disk->fops->release(disk, mode);
    >
    >
    > Happy New Year and Best regards:)
    > Jack
    >

    Ping, could you share opnions on this, attached with patch I proposaled.

    Jack
    From 153918f99e45c685700e919a92384395dc18fd5d Mon Sep 17 00:00:00 2001
    From: Jack Wang <jinpu.wang@profitbricks.com>
    Date: Thu, 2 Jan 2014 10:24:29 +0100
    Subject: [PATCH] fix null pointer dereference in __blkdev_put We were hit by
    bug below: Dec 28 16:24:26 pserver1812 kernel: [979193.076399] BUG: unable to
    handle kernel NULL pointer dereference at 0000000000000008 Dec 28 16:24:26
    pserver1812 kernel: [979193.076401] IP: [<ffffffff8116952f>]
    __blkdev_put+0x17f/0x1d0 Dec 28 16:24:26 pserver1812 kernel: [979193.076442]
    Pid: 56544, comm: multipath Tainted: G O 3.4.71-3-pserver #1
    Supermicro BHQGE/BHQGE Dec 28 16:24:26 pserver1812 kernel: [979193.076445]
    RIP: 0010:[<ffffffff8116952f>] [<ffffffff8116952f>] __blkdev_put+0x17f/0x1d0
    Dec 28 16:24:26 pserver1812 kernel: [979193.076472] Call Trace: Dec 28
    16:24:26 pserver1812 kernel: [979193.076477] [<ffffffff81136bad>] ?
    fput+0xdd/0x270 Dec 28 16:24:26 pserver1812 kernel: [979193.076479]
    [<ffffffff81132f0c>] ? filp_close+0x5c/0x90 Dec 28 16:24:26 pserver1812
    kernel: [979193.076481] [<ffffffff81132fb1>] ? sys_close+0x71/0xc0 Dec 28
    16:24:26 pserver1812 kernel: [979193.076484] [<ffffffff816801b9>] ?
    system_call_fastpath+0x16/0x1b Dec 28 16:24:26 pserver1812 kernel:
    [979193.076486] Code: 8b 5c 24 18 48 8b 6c 24 20 4c 8b 64 24 28 4c 8b 6c 24
    30 4c 8b 74 24 38 4c 8b 7c 24 40 48 83 c4 48 c3 66 90 49 8b 86 48 03 00 00
    <48> 8b 40 08 48 85 c0 0f 84 fc fe ff ff 44 89 e6 4c 89 f7 ff d0 Dec 28
    16:24:26 pserver1812 kernel: [979193.076500] RIP [<ffffffff8116952f>]
    __blkdev_put+0x17f/0x1d0 Dec 28 16:24:26 pserver1812 kernel: [979193.076503]
    RSP <ffff882802f4beb8> Dec 28 16:24:26 pserver1812 kernel: [979193.076504]
    CR2: 0000000000000008 Dec 28 16:24:26 pserver1812 kernel: [979193.077599]
    ---[ end trace 23f39da823d257f9 ]---

    Disamble code show null pointer happened in fops, fix by check before use it.

    Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
    ---
    fs/block_dev.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    diff --git a/fs/block_dev.c b/fs/block_dev.c
    index 319d9c7..d3c45b4 100644
    --- a/fs/block_dev.c
    +++ b/fs/block_dev.c
    @@ -1483,8 +1483,9 @@ static int __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
    &default_backing_dev_info);
    }
    if (bdev->bd_contains == bdev) {
    - if (disk->fops->release)
    - ret = disk->fops->release(disk, mode);
    + if (disk->fops)
    + if (disk->fops->release)
    + ret = disk->fops->release(disk, mode);
    }
    if (!bdev->bd_openers) {
    struct module *owner = disk->fops->owner;
    --
    1.8.4
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