[PATCH net v2] ipv6: fix suspecious RCU usage warning

[PATCH net v2] ipv6: fix suspecious RCU usage warning

[Wei Wang]

Syzbot reported the suspecious RCU usage in nexthop_fib6_nh() when
called from ipv6_route_seq_show(). The reason is ipv6_route_seq_start()
calls rcu_read_lock_bh(), while nexthop_fib6_nh() calls
rcu_dereference_rtnl().
The fix proposed is to add a variant of nexthop_fib6_nh() to use
rcu_dereference_bh_rtnl() for ipv6_route_seq_show().

The reported trace is as follows:
./include/net/nexthop.h:416 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
2 locks held by syz-executor.0/17895:
     at: seq_read+0x71/0x12a0 fs/seq_file.c:169
     at: seq_file_net include/linux/seq_file_net.h:19 [inline]
     at: ipv6_route_seq_start+0xaf/0x300 net/ipv6/ip6_fib.c:2616

stack backtrace:
CPU: 1 PID: 17895 Comm: syz-executor.0 Not tainted 4.15.0-syzkaller #0
Call Trace:
 [<ffffffff849edf9e>] __dump_stack lib/dump_stack.c:17 [inline]
 [<ffffffff849edf9e>] dump_stack+0xd8/0x147 lib/dump_stack.c:53
 [<ffffffff8480b7fa>] lockdep_rcu_suspicious+0x153/0x15d kernel/locking/lockdep.c:5745
 [<ffffffff8459ada6>] nexthop_fib6_nh include/net/nexthop.h:416 [inline]
 [<ffffffff8459ada6>] ipv6_route_native_seq_show net/ipv6/ip6_fib.c:2488 [inline]
 [<ffffffff8459ada6>] ipv6_route_seq_show+0x436/0x7a0 net/ipv6/ip6_fib.c:2673
 [<ffffffff81c556df>] seq_read+0xccf/0x12a0 fs/seq_file.c:276
 [<ffffffff81dbc62c>] proc_reg_read+0x10c/0x1d0 fs/proc/inode.c:231
 [<ffffffff81bc28ae>] do_loop_readv_writev fs/read_write.c:714 [inline]
 [<ffffffff81bc28ae>] do_loop_readv_writev fs/read_write.c:701 [inline]
 [<ffffffff81bc28ae>] do_iter_read+0x49e/0x660 fs/read_write.c:935
 [<ffffffff81bc81ab>] vfs_readv+0xfb/0x170 fs/read_write.c:997
 [<ffffffff81c88847>] kernel_readv fs/splice.c:361 [inline]
 [<ffffffff81c88847>] default_file_splice_read+0x487/0x9c0 fs/splice.c:416
 [<ffffffff81c86189>] do_splice_to+0x129/0x190 fs/splice.c:879
 [<ffffffff81c86f66>] splice_direct_to_actor+0x256/0x890 fs/splice.c:951
 [<ffffffff81c8777d>] do_splice_direct+0x1dd/0x2b0 fs/splice.c:1060
 [<ffffffff81bc4747>] do_sendfile+0x597/0xce0 fs/read_write.c:1459
 [<ffffffff81bca205>] SYSC_sendfile64 fs/read_write.c:1520 [inline]
 [<ffffffff81bca205>] SyS_sendfile64+0x155/0x170 fs/read_write.c:1506
 [<ffffffff81015fcf>] do_syscall_64+0x1ff/0x310 arch/x86/entry/common.c:305
 [<ffffffff84a00076>] entry_SYSCALL_64_after_hwframe+0x42/0xb7

Fixes: f88d8ea67fbdb ("ipv6: Plumb support for nexthop object in a fib6_info")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Wei Wang <weiwan@google.com>
Cc: David Ahern <dsahern@kernel.org>
Cc: Ido Schimmel <idosch@idosch.org>
Cc: Petr Machata <petrm@nvidia.com>
Cc: Eric Dumazet <edumazet@google.com>
---
 include/net/nexthop.h | 24 ++++++++++++++++++++++++
 net/ipv6/ip6_fib.c    |  2 +-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/include/net/nexthop.h b/include/net/nexthop.h
index 7bc057aee40b..a10a319d7eb2 100644
--- a/include/net/nexthop.h
+++ b/include/net/nexthop.h
@@ -410,6 +410,7 @@ static inline struct fib_nh *fib_info_nh(struct fib_info *fi, int nhsel)
 int fib6_check_nexthop(struct nexthop *nh, struct fib6_config *cfg,
 		       struct netlink_ext_ack *extack);
 
+/* Caller should either hold rcu_read_lock(), or RTNL. */
 static inline struct fib6_nh *nexthop_fib6_nh(struct nexthop *nh)
 {
 	struct nh_info *nhi;
@@ -430,6 +431,29 @@ static inline struct fib6_nh *nexthop_fib6_nh(struct nexthop *nh)
 	return NULL;
 }
 
+/* Variant of nexthop_fib6_nh().
+ * Caller should either hold rcu_read_lock_bh(), or RTNL.
+ */
+static inline struct fib6_nh *nexthop_fib6_nh_bh(struct nexthop *nh)
+{
+	struct nh_info *nhi;
+
+	if (nh->is_group) {
+		struct nh_group *nh_grp;
+
+		nh_grp = rcu_dereference_bh_rtnl(nh->nh_grp);
+		nh = nexthop_mpath_select(nh_grp, 0);
+		if (!nh)
+			return NULL;
+	}
+
+	nhi = rcu_dereference_bh_rtnl(nh->nh_info);
+	if (nhi->family == AF_INET6)
+		return &nhi->fib6_nh;
+
+	return NULL;
+}
+
 static inline struct net_device *fib6_info_nh_dev(struct fib6_info *f6i)
 {
 	struct fib6_nh *fib6_nh;
diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index ef9d022e693f..679699e953f1 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -2486,7 +2486,7 @@ static int ipv6_route_native_seq_show(struct seq_file *seq, void *v)
 	const struct net_device *dev;
 
 	if (rt->nh)
-		fib6_nh = nexthop_fib6_nh(rt->nh);
+		fib6_nh = nexthop_fib6_nh_bh(rt->nh);
 
 	seq_printf(seq, "%pi6 %02x ", &rt->fib6_dst.addr, rt->fib6_dst.plen);
 
-- 
2.30.1.766.gb4fecdf3b7-goog

Re: [PATCH net v2] ipv6: fix suspecious RCU usage warning

[Ido Schimmel]

On Tue, Mar 09, 2021 at 06:20:35PM -0800, Wei Wang wrote:
> Syzbot reported the suspecious RCU usage in nexthop_fib6_nh() when
> called from ipv6_route_seq_show(). The reason is ipv6_route_seq_start()
> calls rcu_read_lock_bh(), while nexthop_fib6_nh() calls
> rcu_dereference_rtnl().
> The fix proposed is to add a variant of nexthop_fib6_nh() to use
> rcu_dereference_bh_rtnl() for ipv6_route_seq_show().
> 
> The reported trace is as follows:
> ./include/net/nexthop.h:416 suspicious rcu_dereference_check() usage!
> 
> other info that might help us debug this:
> 
> rcu_scheduler_active = 2, debug_locks = 1
> 2 locks held by syz-executor.0/17895:
>      at: seq_read+0x71/0x12a0 fs/seq_file.c:169
>      at: seq_file_net include/linux/seq_file_net.h:19 [inline]
>      at: ipv6_route_seq_start+0xaf/0x300 net/ipv6/ip6_fib.c:2616

[...]

> 
> Fixes: f88d8ea67fbdb ("ipv6: Plumb support for nexthop object in a fib6_info")
> Reported-by: syzbot <syzkaller@googlegroups.com>
> Signed-off-by: Wei Wang <weiwan@google.com>
> Cc: David Ahern <dsahern@kernel.org>
> Cc: Ido Schimmel <idosch@idosch.org>
> Cc: Petr Machata <petrm@nvidia.com>
> Cc: Eric Dumazet <edumazet@google.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>

Re: [PATCH net v2] ipv6: fix suspecious RCU usage warning

[David Ahern]

On 3/9/21 7:20 PM, Wei Wang wrote:
> Syzbot reported the suspecious RCU usage in nexthop_fib6_nh() when
> called from ipv6_route_seq_show(). The reason is ipv6_route_seq_start()
> calls rcu_read_lock_bh(), while nexthop_fib6_nh() calls
> rcu_dereference_rtnl().
> The fix proposed is to add a variant of nexthop_fib6_nh() to use
> rcu_dereference_bh_rtnl() for ipv6_route_seq_show().
> 

...

> 
> Fixes: f88d8ea67fbdb ("ipv6: Plumb support for nexthop object in a fib6_info")
> Reported-by: syzbot <syzkaller@googlegroups.com>
> Signed-off-by: Wei Wang <weiwan@google.com>
> Cc: David Ahern <dsahern@kernel.org>
> Cc: Ido Schimmel <idosch@idosch.org>
> Cc: Petr Machata <petrm@nvidia.com>
> Cc: Eric Dumazet <edumazet@google.com>
> ---
>  include/net/nexthop.h | 24 ++++++++++++++++++++++++
>  net/ipv6/ip6_fib.c    |  2 +-
>  2 files changed, 25 insertions(+), 1 deletion(-)
> 


Reviewed-by: David Ahern <dsahern@kernel.org>

Thanks, Wei.

Re: [PATCH net v2] ipv6: fix suspecious RCU usage warning

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (refs/heads/master):

On Tue,  9 Mar 2021 18:20:35 -0800 you wrote:
> Syzbot reported the suspecious RCU usage in nexthop_fib6_nh() when
> called from ipv6_route_seq_show(). The reason is ipv6_route_seq_start()
> calls rcu_read_lock_bh(), while nexthop_fib6_nh() calls
> rcu_dereference_rtnl().
> The fix proposed is to add a variant of nexthop_fib6_nh() to use
> rcu_dereference_bh_rtnl() for ipv6_route_seq_show().
> 
> [...]

Here is the summary with links:
  - [net,v2] ipv6: fix suspecious RCU usage warning
    https://git.kernel.org/netdev/net/c/28259bac7f1d

You are awesome, thank you!
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