netdev - [PATCH net] ip6mr: fix stale iterator cache pointer

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Date:   Wed, 31 Jan 2018 15:39:49 +0200
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, yoshfuji@...ux-ipv6.org,
        syzkaller-bugs@...glegroups.com,
        bot+eceb3204562c41a438fa1f2335e0fe4f6886d669@...kaller.appspotmail.com,
        kuznet@....inr.ac.ru, roopa@...ulusnetworks.com,
        ebiggers3@...il.com,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
Subject: [PATCH net] ip6mr: fix stale iterator cache pointer

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>
---
No fixes tag because it seems this has been there forever.

 net/ipv6/ip6mr.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index 754ef84cf354..9f6cace9c817 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -494,6 +494,7 @@ 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