netdev - Re: [PATCH net v2] ip6mr: fix stale iterator

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Message-ID: <CACT4Y+YjmOJ8Zw2j0BooBMfeh5FP2s8__C373mX7O-YfTY+PZA@mail.gmail.com>
Date:   Wed, 31 Jan 2018 15:49:15 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
Cc:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        syzkaller-bugs@...glegroups.com,
        bot+eceb3204562c41a438fa1f2335e0fe4f6886d669@...kaller.appspotmail.com,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        roopa@...ulusnetworks.com, Eric Biggers <ebiggers3@...il.com>
Subject: Re: [PATCH net v2] ip6mr: fix stale iterator

On Wed, Jan 31, 2018 at 3:29 PM, Nikolay Aleksandrov
<nikolay@...ulusnetworks.com> wrote:
> When we dump the ip6mr mfc entries via proc, we initialize an iterator
> with the table to dump but we don't clear the cache pointer which might
> be initialized from a prior read on the same descriptor that ended. This
> can result in lock imbalance (an unnecessary unlock) leading to other
> crashes and hangs. Clear the cache pointer like ipmr does to fix the issue.
> Thanks for the reliable reproducer.
>
> Here's syzbot's trace:
>  WARNING: bad unlock balance detected!
>  4.15.0-rc3+ #128 Not tainted
>  syzkaller971460/3195 is trying to release lock (mrt_lock) at:
>  [<000000006898068d>] ipmr_mfc_seq_stop+0xe1/0x130 net/ipv6/ip6mr.c:553
>  but there are no more locks to release!
>
>  other info that might help us debug this:
>  1 lock held by syzkaller971460/3195:
>   #0:  (&p->lock){+.+.}, at: [<00000000744a6565>] seq_read+0xd5/0x13d0
>  fs/seq_file.c:165
>
>  stack backtrace:
>  CPU: 1 PID: 3195 Comm: syzkaller971460 Not tainted 4.15.0-rc3+ #128
>  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>  Google 01/01/2011
>  Call Trace:
>   __dump_stack lib/dump_stack.c:17 [inline]
>   dump_stack+0x194/0x257 lib/dump_stack.c:53
>   print_unlock_imbalance_bug+0x12f/0x140 kernel/locking/lockdep.c:3561
>   __lock_release kernel/locking/lockdep.c:3775 [inline]
>   lock_release+0x5f9/0xda0 kernel/locking/lockdep.c:4023
>   __raw_read_unlock include/linux/rwlock_api_smp.h:225 [inline]
>   _raw_read_unlock+0x1a/0x30 kernel/locking/spinlock.c:255
>   ipmr_mfc_seq_stop+0xe1/0x130 net/ipv6/ip6mr.c:553
>   traverse+0x3bc/0xa00 fs/seq_file.c:135
>   seq_read+0x96a/0x13d0 fs/seq_file.c:189
>   proc_reg_read+0xef/0x170 fs/proc/inode.c:217
>   do_loop_readv_writev fs/read_write.c:673 [inline]
>   do_iter_read+0x3db/0x5b0 fs/read_write.c:897
>   compat_readv+0x1bf/0x270 fs/read_write.c:1140
>   do_compat_preadv64+0xdc/0x100 fs/read_write.c:1189
>   C_SYSC_preadv fs/read_write.c:1209 [inline]
>   compat_SyS_preadv+0x3b/0x50 fs/read_write.c:1203
>   do_syscall_32_irqs_on arch/x86/entry/common.c:327 [inline]
>   do_fast_syscall_32+0x3ee/0xf9d arch/x86/entry/common.c:389
>   entry_SYSENTER_compat+0x51/0x60 arch/x86/entry/entry_64_compat.S:125
>  RIP: 0023:0xf7f73c79
>  RSP: 002b:00000000e574a15c EFLAGS: 00000292 ORIG_RAX: 000000000000014d
>  RAX: ffffffffffffffda RBX: 000000000000000f RCX: 0000000020a3afb0
>  RDX: 0000000000000001 RSI: 0000000000000067 RDI: 0000000000000000
>  RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
>  R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
>  R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>  BUG: sleeping function called from invalid context at lib/usercopy.c:25
>  in_atomic(): 1, irqs_disabled(): 0, pid: 3195, name: syzkaller971460
>  INFO: lockdep is turned off.
>  CPU: 1 PID: 3195 Comm: syzkaller971460 Not tainted 4.15.0-rc3+ #128
>  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>  Google 01/01/2011
>  Call Trace:
>   __dump_stack lib/dump_stack.c:17 [inline]
>   dump_stack+0x194/0x257 lib/dump_stack.c:53
>   ___might_sleep+0x2b2/0x470 kernel/sched/core.c:6060
>   __might_sleep+0x95/0x190 kernel/sched/core.c:6013
>   __might_fault+0xab/0x1d0 mm/memory.c:4525
>   _copy_to_user+0x2c/0xc0 lib/usercopy.c:25
>   copy_to_user include/linux/uaccess.h:155 [inline]
>   seq_read+0xcb4/0x13d0 fs/seq_file.c:279
>   proc_reg_read+0xef/0x170 fs/proc/inode.c:217
>   do_loop_readv_writev fs/read_write.c:673 [inline]
>   do_iter_read+0x3db/0x5b0 fs/read_write.c:897
>   compat_readv+0x1bf/0x270 fs/read_write.c:1140
>   do_compat_preadv64+0xdc/0x100 fs/read_write.c:1189
>   C_SYSC_preadv fs/read_write.c:1209 [inline]
>   compat_SyS_preadv+0x3b/0x50 fs/read_write.c:1203
>   do_syscall_32_irqs_on arch/x86/entry/common.c:327 [inline]
>   do_fast_syscall_32+0x3ee/0xf9d arch/x86/entry/common.c:389
>   entry_SYSENTER_compat+0x51/0x60 arch/x86/entry/entry_64_compat.S:125
>  RIP: 0023:0xf7f73c79
>  RSP: 002b:00000000e574a15c EFLAGS: 00000292 ORIG_RAX: 000000000000014d
>  RAX: ffffffffffffffda RBX: 000000000000000f RCX: 0000000020a3afb0
>  RDX: 0000000000000001 RSI: 0000000000000067 RDI: 0000000000000000
>  RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
>  R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
>  R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>  WARNING: CPU: 1 PID: 3195 at lib/usercopy.c:26 _copy_to_user+0xb5/0xc0
>  lib/usercopy.c:26
>
> Reported-by: syzbot <bot+eceb3204562c41a438fa1f2335e0fe4f6886d669@...kaller.appspotmail.com>
> Signed-off-by: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
> ---
> v2: make sure the trace doesn't ruin the patch
> No fixes tag because it seems this has been there forever.

Don't we need to Cc stable 2.6 in this case or something like this. We
want it to be backported.

>
>  net/ipv6/ip6mr.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
> index a2e1a864eb46..4fc566ec7e79 100644
> --- a/net/ipv6/ip6mr.c
> +++ b/net/ipv6/ip6mr.c
> @@ -495,6 +495,7 @@ static void *ipmr_mfc_seq_start(struct seq_file *seq, loff_t *pos)
>                 return ERR_PTR(-ENOENT);
>
>         it->mrt = mrt;
> +       it->cache = NULL;
>         return *pos ? ipmr_mfc_seq_idx(net, seq->private, *pos - 1)
>                 : SEQ_START_TOKEN;
>  }
> --
> 2.1.4
>
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