On Fri, Jan 6, 2017 at 10:02 AM, Thomas Garnier <thgar...@google.com> wrote: > On Thu, Jan 5, 2017 at 6:34 PM, Andy Lutomirski <l...@amacapital.net> wrote: >> On Thu, Jan 5, 2017 at 3:05 PM, Linus Torvalds >> <torva...@linux-foundation.org> wrote: >>> On Thu, Jan 5, 2017 at 12:18 PM, Andy Lutomirski <l...@kernel.org> wrote: >>>> >>>> Hmm. I bet that if we preset the accessed bits in all the segments >>>> then we don't need it to be writable in general. >>> >>> I'm not sure that this is architecturally safe. >>> >> >> Hmm. Last time I looked, I couldn't find *anything* in the SDM >> explaining what happened if a GDT access resulted in a page fault. I >> did discover that Xen intentionally (!) lazily populates and maps LDT >> pages. An attempt to access a not-present page results in #PF with >> the error cod e indicating kernel access even if the access came from >> user mode. >> >> SDM volume 3 7.2.2 says "Pages corresponding to the previous task’s >> TSS, the current task’s TSS, and the descriptor table entries for >> each all should be marked as read/write." But I don't see how a CPU >> implementation could possibly care what the page table for the TSS >> descriptor table entries says after LTR is done because the CPU isn't >> even supposed to *read* that memory. >> >> OTOH a valid implementation could easily require that the page table >> says that the page is writable merely to load a segment, especially in >> weird cases (IRET?). That being said, this is all quite easy to test. >> >> Also, Thomas, why are you creating a new memory region? I don't see >> any benefit to randomizing the GDT address. How about just putting it >> in the fixmap? This would be NR_CPUS * 4 pages if do my limit=0xffff >> idea. I'm not sure if the fixmap code knows how to handle this much >> space. > > When I looked at the fixmap, you had to define the space you need > ahead of time and I am not sure there was enough space as you said.
Can you try it and see if anything goes wrong? Even if something does go wrong, I think we should fix *that* rather than making the memory layout more complicated.
