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Commit | Line | Data |
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1 | NetWatch! | |
2 | ||
3 | This is NetWatch, a system for remote system-management-mode-based | |
4 | control of a machine without support from or awareness by the OS. It works by | |
5 | taking over a second network card to provide a standard VNC server, such that | |
6 | a machine elsewhere on the network can see the text or graphics console of the | |
7 | machine and inject keystrokes as needed. | |
8 | ||
9 | System management mode, introduced with the 386SL, essentially allows | |
10 | system driver code to run outside of OS control, caused by a special interrupt | |
11 | pin on the CPU. This was originally intended for applications such as laptop | |
12 | fan control; it is also the mechansim by which USB legacy keyboard emulation | |
13 | occurs. When a system management interrupt occurs, the northbridge remaps | |
14 | portions of memory to expose previously-hidden code, and asserts an SMI# signal, | |
15 | causing the CPU to save all its state into system management RAM and vector to | |
16 | a magic entry point. | |
17 | ||
18 | This is somewhat slow, and so there is a moderate performance impact | |
19 | caused by running NetWatch, more significant when a VNC session is open. | |
20 | Because NetWatch is invisible to the OS, its CPU usage is difficult to monitor; | |
21 | we do so by comparing the MD5 throughput of the system with NetWatch | |
22 | running versus without. The only way that the OS could detect this performance | |
23 | drain is by spinning tightly and watching for a sudden jump in the CPU's time | |
24 | stamp counters. | |
25 | ||
26 | Although it would be possible to start up NetWatch after an OS kernel | |
27 | has already loaded, it is easier and more useful to load it from GRUB before | |
28 | the OS boots, such that even the bootloader itself can be controlled over the | |
29 | network. We do this by providing a stub loader (grubload/) which can be invoked | |
30 | from GRUB, and takes care of loading the main NetWatch ELF image. Once this is | |
31 | done and NetWatch is up and running, the loader returns to real mode and | |
32 | reinvokes GRUB via the BIOS. | |
33 | ||
34 | Our current development platform, the Intel ICH2, does not allow SMM | |
35 | traps on arbitrary PCI accesses. This makes stealing the network card from the | |
36 | OS somewhat difficult, since there is nothing SMM code can do to cleanly block | |
37 | access. NetWatch simply chooses its desired network card, and then repeatedly | |
38 | clobbers the PCI base address registers. Although Linux resets the BARs to sane | |
39 | values when it probes the PCI bus, by the time it attempts to actually load | |
40 | the network driver, the card will no longer be accessible; fortunately, the | |
41 | driver quickly gives up, and Linux no longer attempts to access the card. | |
42 | ||
43 | The northbridge can be configured to invoke a system management | |
44 | interrupt every 64 milliseconds, and so the bulk of NetWatch's work is done | |
45 | from this interrupt: checking the network card for incoming packets, invoking | |
46 | lwIP, and sending any response packets necessary. SMM entry also occurs when | |
47 | when the OS reads from the keyboard I/O ports, to inject scan codes as needed. | |
48 | ||
49 | Much of NetWatch is very hardware-dependent, and although we've tried | |
50 | to maintain clean interface separation to allow for easy porting, the current | |
51 | implementation requires: | |
52 | ||
53 | - Intel ICH2 system chipset | |
54 | - 3C509 Ethernet card to be used by NetWatch, plus another card of | |
55 | any type for the OS | |
56 | - BIOS which does not set the D_LCK bit. Any system old enough to be | |
57 | based on the ICH2 is very likely to have a suitable BIOS. | |
58 | ||
59 | Current open issues are listed in the TODO file. See GUIDE for an | |
60 | overview of which source files do what. |