On Tue, Mar 21, 2017 at 4:51 PM, Andy Lutomirski <l...@kernel.org> wrote: > On Tue, Mar 21, 2017 at 3:32 PM, Andy Lutomirski <l...@amacapital.net> wrote: >> On Tue, Mar 21, 2017 at 2:11 PM, Linus Torvalds >> <torva...@linux-foundation.org> wrote: >>> On Tue, Mar 21, 2017 at 1:25 PM, Thomas Garnier <thgar...@google.com> wrote: >>>> The issue seems to be related to exceptions happening in close pages >>>> to the fixmap GDT remapping. >>>> >>>> The original page fault happen in do_test_wp_bit which set a fixmap >>>> entry to test WP flag. If I grow the number of processors supported >>>> increasing the distance between the remapped GDT page and the WP test >>>> page, the error does not reproduce. >>>> >>>> I am still looking at the exact distance between repro and no-repro as >>>> well as the exact root cause. >>> >>> Hmm. Have we set the GDT limit incorrectly, somehow? The GDT *can* >>> cover 8k entries, which at 8 bytes each would be 64kB. >> >> The QEMU barf says the GDT limit is 0xff, for better or for worse. >> >>> >>> So somebody trying to load an invalid segment (say, 0xffff) might end >>> up causing an access to the GDT base + 64k - 8. >>> >>> It is also possible that the CPU might do a page table writability >>> check *before* it does the limit check. That would sound odd, though. >>> Might be a CPU errata. >>> >> > >> There's presumably something genuinely wrong with our GDT. > > This is suspicious. I added this code in test_wp_bit: > > if (memcmp(get_current_gdt_ro(), get_current_gdt_rw(), 4096) != 0) { > pr_err("Oh crap\n"); > BUG_ON(1); > } > > It printed "Oh crap" and blew up. Methinks something's wrong with the > fixmap. Is it possible that we're crossing a PMD boundary and failing > to translate the addresses right?
I might be that. We crash when the PKMAP_BASE is just after the FIX_WP_TEST. I will continue testing couple scenarios and design a fix. Moving the GDT FIXMAP at the beginning or align the base (or pad the end). -- Thomas