Sliver server crashes with panic when canaries are resolved

[lorenzog]

Sliver crashes when DNS canaries are resolved.

To Reproduce
Steps to reproduce the behavior:

1. Install Sliver 1.5.41 on a Ubuntu 22.04 server

2. Set up a DNS redirector on a different server, using socat to route packets: ulimit -u unlimited ; socat udp4-listen:53,fork,reuseaddr udp4:10.0.0.2:5353

3. Set up a DNS attack domain and a DNS canary domain, with NS zone pointing at the same DNS redirector public IP

4. On the Sliver server, set up a DNS listener:

   [server] sliver > dns -d t.attackdomain.com -l 5353 -p
   [*] Starting DNS listener with parent domain(s) [t.dubleclick.net.] ...
   [*] Successfully started job #2
   
   

5. Generate a timed implant with a canary: [server] sliver > generate -c foo.a.canary.com -G -e -f exe -b http.attackdomain.com -n bar.attackdomain.com -w 2023-10-17

6. Check the canary is set up:

   [server] sliver > canaries
   
   Sliver Name          Domain                          Triggered   First Trigger                   Latest Trigger
   HUNGRY_CHERRY        dwgk7c5.foo.a.canary.com.   false       Never                           Never
   

7. Trigger the canary from a different machine

8. Sliver crashes

Expected behavior
Sliver does not crash

Screenshots

[server] sliver > panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0xe335d8]

goroutine 40 [running]:
github.com/bishopfox/sliver/server/core.(*ImplantConnection).GetLastMessage(0xc0003c3700?)
        github.com/bishopfox/sliver/server/core/connnection.go:44 +0x38
github.com/bishopfox/sliver/server/core.(*Session).LastCheckin(...)
        github.com/bishopfox/sliver/server/core/sessions.go:81
github.com/bishopfox/sliver/server/core.(*Session).ToProtobuf(0xc0003f8500)
        github.com/bishopfox/sliver/server/core/sessions.go:134 +0x2c
github.com/bishopfox/sliver/server/rpc.(*Server).Events(0x6?, 0x18ab1c0?, {0x98590f0, 0xc000112b70})
        github.com/bishopfox/sliver/server/rpc/rpc-events.go:45 +0x4e5
github.com/bishopfox/sliver/protobuf/rpcpb._SliverRPC_Events_Handler({0x1a1f200?, 0xa50b620}, {0x9856c20, 0xc0003163e0})
        github.com/bishopfox/sliver/protobuf/rpcpb/services_grpc.pb.go:4650 +0xd3
github.com/grpc-ecosystem/go-grpc-middleware/logging/logrus.PayloadStreamServerInterceptor.func1({0x1a1f200, 0xa50b620}, {0x9856c20?, 0xc0003163e0?}, 0xc000429590, 0x966cfd0)
        github.com/grpc-ecosystem/go-grpc-middleware@v1.4.0/logging/logrus/payload_interceptors.go:49 +0x15f
google.golang.org/grpc.getChainStreamHandler.func1({0x1a1f200, 0xa50b620}, {0x9856c20, 0xc0003163e0})
        google.golang.org/grpc@v1.55.0/server.go:1483 +0xb9
github.com/grpc-ecosystem/go-grpc-middleware/logging/logrus.StreamServerInterceptor.func1({0x1a1f200, 0xa50b620}, {0x9856cb0, 0xc0003a04c0}, 0xc000429590, 0xc0003a0500)
        github.com/grpc-ecosystem/go-grpc-middleware@v1.4.0/logging/logrus/server_interceptors.go:61 +0x153
google.golang.org/grpc.getChainStreamHandler.func1({0x1a1f200, 0xa50b620}, {0x9856cb0, 0xc0003a04c0})
        google.golang.org/grpc@v1.55.0/server.go:1483 +0xb9
github.com/grpc-ecosystem/go-grpc-middleware/tags.StreamServerInterceptor.func1({0x1a1f200, 0xa50b620}, {0x9856c20?, 0xc0003163c0?}, 0xc000429590, 0xc0003a0440)
        github.com/grpc-ecosystem/go-grpc-middleware@v1.4.0/tags/interceptors.go:39 +0x135
google.golang.org/grpc.getChainStreamHandler.func1({0x1a1f200, 0xa50b620}, {0x9856c20, 0xc0003163c0})
        google.golang.org/grpc@v1.55.0/server.go:1483 +0xb9
github.com/grpc-ecosystem/go-grpc-middleware/auth.StreamServerInterceptor.func1({0x1a1f200, 0xa50b620}, {0x9856fc8?, 0xc0001ea1e0?}, 0xc000429590, 0xc0003a0400)
        github.com/grpc-ecosystem/go-grpc-middleware@v1.4.0/auth/auth.go:66 +0x146
google.golang.org/grpc.chainStreamInterceptors.func1({0x1a1f200, 0xa50b620}, {0x9856fc8, 0xc0001ea1e0}, 0x1748d40?, 0xc000112b10?)
        google.golang.org/grpc@v1.55.0/server.go:1474 +0x8f
google.golang.org/grpc.(*Server).processStreamingRPC(0xc0003403c0, {0x985c6e0, 0xc0003a4340}, 0xc00016ab40, 0xc00037ff50, 0xa41b120, 0x0)
        google.golang.org/grpc@v1.55.0/server.go:1638 +0x1363
google.golang.org/grpc.(*Server).handleStream(0xc0003403c0, {0x985c6e0, 0xc0003a4340}, 0xc00016ab40, 0x0)
        google.golang.org/grpc@v1.55.0/server.go:1718 +0x9f0
google.golang.org/grpc.(*Server).serveStreams.func1.1()
        google.golang.org/grpc@v1.55.0/server.go:959 +0x98
created by google.golang.org/grpc.(*Server).serveStreams.func1
        google.golang.org/grpc@v1.55.0/server.go:957 +0x18c

Desktop (please complete the following information):

• OS: Ubuntu 22.04
• Version 1.5.41

Notes

• The same happens when the DNS listener is set up to work with the canary domain only
• It does not crash when looking up a canary that has been already looked up and is marked as trigggered