> On Jul 20, 2018, at 11:37 AM, Joerg Roedel <jroe...@suse.de> wrote: > >> On Fri, Jul 20, 2018 at 12:32:10PM -0700, Andy Lutomirski wrote: >> I'm just reading your changelog, and you said the PMDs are no longer >> shared between the page tables. So this presumably means that >> vmalloc_fault() no longer actually works correctly on PTI systems. I >> didn't read the code to figure out *why* it doesn't work, but throwing >> random vmalloc_sync_all() calls around is wrong. > > Hmm, so the whole point of vmalloc_fault() fault is to sync changes from > swapper_pg_dir to process page-tables when the relevant parts of the > kernel page-table are not shared, no? > > That is also the reason we don't see this on 64 bit, because there these > parts *are* shared. > > So with that reasoning vmalloc_fault() works as designed, except that > a warning is issued when it's happens in the NMI path. That warning comes > from > > ebc8827f75954 x86: Barf when vmalloc and kmemcheck faults happen in NMI > > which went into 2.6.37 and was added because the NMI handler were not > nesting-safe back then. Reason probably was that the handler on 64 bit > has to use an IST stack and a nested NMI would overwrite the stack of > the upper handler. We don't have this problem on 32 bit as a nested NMI > will not do another stack-switch there. >
Thanks for digging! The problem was presumably that vmalloc_fault() will IRET and re-enable NMIs on the way out. But we’ve supported page faults on user memory in NMI handlers on 32-bit and 64-bit for quite a while, and it’s fine now. I would remove the warning, re-test, and revert the other patch. The one case we can’t handle in vmalloc_fault() is a fault on a stack access. I don’t expect this to be a problem for PTI. It was a problem for CONFIG_VMAP_STACK, though. > I am not sure about 64 bit, but there is a lot of assembly magic to make > NMIs nesting-safe, so I guess the problem should be gone there too. > > > Regards, > > Joerg
