X-Git-Url: http://git.joshuawise.com/netwatch.git/blobdiff_plain/c00c1c15f28f3d926d9ce500417265f8afe72eca..93e304e362865d8e41a0c75bb799ac8418895e3d:/README diff --git a/README b/README index 2977c05..6610c47 100644 --- a/README +++ b/README @@ -1,52 +1,61 @@ NetWatch! - This is NetWatch, a system for remote system-management-mode-based -control of a machine without support from or awareness by the OS. It works by -taking over a second network card to provide a standard VNC server, such that -a machine elsewhere on the network can see the text or graphics console of the -machine and inject keystrokes as needed. - - . - - . - - Because NetWatch is invisible to the OS, its CPU usage is difficult to -monitor; we do so by comparing the MD5 throughput of the system with NetWatch -running versus without. The only way that the OS could detect this performance -drain is by spinning tightly and watching for a sudden jump in the CPU's time -stamp counters. - - Although it would be possible to start up NetWatch after an OS kernel -has already loaded, it is easier and more useful to load it from GRUB before -the OS boots, such that even the bootloader itself can be controlled over the -network. We do this by providing a stub loader (grubload/) which can be invoked -from GRUB, and takes care of loading the main NetWatch ELF image. Once this is -done and NetWatch is up and running, the loader returns to real mode and -reinvokes GRUB via the BIOS. - - Our current development platform, the Intel ICH2, does not allow SMM -traps on arbitrary PCI accesses. This makes stealing the network card from the -OS somewhat difficult, since there is nothing SMM code can do to cleanly block -access. NetWatch simply chooses its desired network card, and then repeatedly -clobbers the PCI base address registers. Although Linux resets the BARs to sane -values when it probes the PCI bus, by the time it attempts to actually load -the network driver, the card will no longer be accessible; fortunately, the -driver quickly gives up, and Linux no longer attempts to access the card. - - The northbridge can be configured to invoke a system management -interrupt every 64 milliseconds, and so the bulk of NetWatch's work is done -from this interrupt: checking the network card for incoming packets, invoking -lwIP, and sending any response packets necessary. SMM entry also occurs when -when the OS reads from the keyboard I/O ports, to inject scan codes as needed. - - Much of NetWatch is very hardware-dependent, and although we've tried -to maintain clean interface separation to allow for easy porting, the current +This is NetWatch, a system for remote system-management-mode-based control +of a machine without support from or awareness by the OS. It works by +taking over a second network card to provide a standard VNC server, such +that a machine elsewhere on the network can see the text or graphics console +of the machine and inject keystrokes as needed. + +System management mode, introduced with the 386SL, essentially allows system +driver code to run outside of OS control, caused by a special interrupt pin +on the CPU. This was originally intended for applications such as laptop +fan control; it is also the mechanism by which USB legacy keyboard emulation +occurs. When a system management interrupt occurs, the northbridge remaps +portions of memory to expose previously-hidden code, and asserts an SMI# +signal, causing the CPU to save all its state into system management RAM and +vector to a magic entry point. + +This is somewhat slow, and so there is a moderate performance impact caused +by running NetWatch, more significant when a VNC session is open. Because +NetWatch is invisible to the OS, its CPU usage is difficult to monitor; we +do so by comparing the MD5 throughput of the system with NetWatch running +versus without. The only way that the OS could detect this performance +drain is by spinning tightly and watching for a sudden jump in the CPU's +time stamp counters. + +Although it would be possible to start up NetWatch after an OS kernel has +already loaded, it is easier and more useful to load it from GRUB before the +OS boots, such that even the bootloader itself can be controlled over the +network. We do this by providing a stub loader (grubload/) which can be +invoked from GRUB, and takes care of loading the main NetWatch ELF image. +Once this is done and NetWatch is up and running, the loader returns to real +mode and reinvokes GRUB via the BIOS. + +Our current development platform, the Intel ICH2, does not allow SMM traps +on arbitrary PCI accesses. This makes stealing the network card from the OS +somewhat difficult, since there is nothing SMM code can do to cleanly block +access. NetWatch simply chooses its desired network card, and then +repeatedly clobbers the PCI base address registers. Although Linux resets +the BARs to sane values when it probes the PCI bus, by the time it attempts +to actually load the network driver, the card will no longer be accessible; +fortunately, the driver quickly gives up, and Linux no longer attempts to +access the card. + +The northbridge can be configured to invoke a system management interrupt +every 64 milliseconds, and so the bulk of NetWatch's work is done from this +interrupt: checking the network card for incoming packets, invoking lwIP, +and sending any response packets necessary. SMM entry also occurs when when +the OS reads from the keyboard I/O ports, to inject scan codes as needed. + +Much of NetWatch is very hardware-dependent, and although we've tried to +maintain clean interface separation to allow for easy porting, the current implementation requires: - - Intel ICH2 system chipset - - 3C509 Ethernet card to be used by NetWatch, plus another card of + * Intel ICH2 system chipset + * 3C509 Ethernet card to be used by NetWatch, plus another card of any type for the OS - - BIOS which does not set the D_LCK bit. Any system old enough to be + * BIOS which does not set the D_LCK bit. Any system old enough to be based on the ICH2 is very likely to have a suitable BIOS. - Current open issues are listed in the TODO file. +Current open issues are listed in the TODO file. See GUIDE for an overview +of which source files do what.