On Wed, Jan 31, 2018 at 3:29 PM, Nikolay Aleksandrov
<niko...@cumulusnetworks.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+eceb3204562c41a438fa1f2335e0fe4f6886d...@syzkaller.appspotmail.com>
> Signed-off-by: Nikolay Aleksandrov <niko...@cumulusnetworks.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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