Race condition in LinuxInterfaceProvider::load()

[yoannd]

Brief description

When loading scapy, it is possible that the initialization fails if interfaces are being deleted. This is because we're reading /proc/net/dev once and then iterating over all interfaces; however, if interfaces are disappearing during this operation, ioctl() will fail with errno 19.

Scapy version

2.4.5rc1.dev108

Python version

2.7.6

Operating system

4.15.0-45

Additional environment information

No response

How to reproduce

1. Create 1000 tap interfaces

for i in $(seq 1 1000); do ip tuntap add tap$i mode tap; done

2. Open a python prompt and prepare yourself to initialize scapy (without hitting return yet)

Python 2.7.6 (default, Nov 23 2017, 15:49:48) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from scapy.all import *

3. Open a bash prompt and prepare yourself to delete the 1000 interfaces (without hitting return yet)

for i in $(seq 1 1000); do ip link del tap$i; done

4. Hit return on the python prompt, and quickly after on the bash prompt
5. The following exception will occur

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "scapy/all.py", line 16, in <module>
    from scapy.arch import *
  File "scapy/arch/__init__.py", line 141, in <module>
    _set_conf_sockets()  # Apply config
  File "scapy/config.py", line 622, in _set_conf_sockets
    conf.ifaces.reload()
  File "scapy/interfaces.py", line 238, in reload
    self._reload_provs()
  File "scapy/interfaces.py", line 234, in _reload_provs
    self._load(prov.reload(), prov)
  File "scapy/interfaces.py", line 53, in reload
    return self.load()
  File "scapy/arch/linux.py", line 417, in load
    ifflags = struct.unpack("16xH14x", get_if(i, SIOCGIFFLAGS))[0]
  File "scapy/arch/common.py", line 65, in get_if
    return ioctl(sck, cmd, struct.pack("16s16x", iff.encode("utf8")))
IOError: [Errno 19] No such device

Actual result

No response

Expected result

The expected result is that scapy continues to load without failing.

Related resources

No response

[guedou]

Nice catch! Could you test the fix from #3371?

[yoannd]

Thanks for the quick fix! I just tested it and it works :) Just a note that there's a typo, the warning is in the try clause instead of the except clause

(Also, when I was doing my internal testing prior to reporting, I had extended the try clause until after setting ip (because there are a few other calls to get_if_index, get_if_raw_addr, get_if_raw_hwaddr. But this might have been paranoid as I couldn't repro the race with these calls)

[guedou]

I moved the warning to the except clause and extended the try / except block.

[yoannd]

Thanks a lot! Works for me :)