On Thu, Mar 07, 2019 at 07:05:20PM +0100, Jesús Diéguez Fernández wrote:
[...] > I've sorted out the MSR support check, but I'm struggling a bit with Great! > handling the exception. Yeah, this can be difficult. > I've read the Intel SDM and other docs, and I think I understand the > reason why the system reboots: when the rdmsr or wrmsr access a > restricted area, a general protection exception is raised; and because > there's no interrupt 13 (GP#) handler installed, it double faults and > then triple faults, resulting in a cpu halt and motherboard reboot. > > The linux kernel uses the following function to read and write to the msr: > > https://elixir.bootlin.com/linux/latest/source/arch/x86/include/asm/msr.h#L137 > > If I understood it correctly, it uses some gcc syntax to setup the > handler of an exception in a different section, but I think that it > relies on the IDT that the linux kernel has already installed, so I > can't use that code. > > https://github.com/torvalds/linux/blob/master/Documentation/x86/exception-tables.txt > > Apart from setting up the IDT, I've found the opcodes CLI and STI, but > I'm quite sure that trying to use them to ignore the interrupt is a dead > end. > > I've looked around in the grub code and there are only a few files that > reference CLI, STI, LIDT or IRET, mainly the code that makes a stage > change. > I also found that the file grub-core/commands/i386/pc/drivemap.c sets up > a int13h trap to handle the disk mappings. > > Since I'm not very familiar with this area (and maybe I'm > overcomplicating it), it seems that the approach of handling the > exception could take a lot more effort than the whole module itself. > > Now, the questions I have are about what direction I should take: > > - Do I really need to make a custom handler (something like drivemap.c > does), or is there any simpler way? > > - I assume that I'm not the first one to need to handle a general > protection exception in grub, but I couldn't find any example in other > modules/commands. Any reference would be appreciated. I think that we should have generic #GP handler which catches general protection faults. > - And from a practical point of view, in case that I need to setup a > custom handler for this, is it mandatory to ship the patch with it or > could it be added later (maybe adding a TODO now)? I can accept this patch series without #GP handler if: - you add warning to the GRUB doc that #GP for rdmsr/wrmsr are not handled and system reboots; so, potential users have to be careful using these modules, - you add TODO notice to the code saying that generic #GP handling should be added to the GRUB code, - you promise me that at some point, let's say after GRUB release, you will try to implement generic #GP handler. Is this acceptable for you? Daniel _______________________________________________ Grub-devel mailing list Grub-devel@gnu.org https://lists.gnu.org/mailman/listinfo/grub-devel
