Messages in this thread | | | Date | Tue, 31 Mar 2020 18:54:12 +0200 (CEST) | From | Richard Weinberger <> | Subject | Re: [PATCH] UML: add support for KASAN under x86_64 |
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Patricia,
----- Ursprüngliche Mail ----- > Von: "Patricia Alfonso" <trishalfonso@google.com> > An: "Johannes Berg" <johannes@sipsolutions.net> > CC: "Dmitry Vyukov" <dvyukov@google.com>, "Jeff Dike" <jdike@addtoit.com>, "richard" <richard@nod.at>, "anton ivanov" > <anton.ivanov@cambridgegreys.com>, "Andrey Ryabinin" <aryabinin@virtuozzo.com>, "Brendan Higgins" > <brendanhiggins@google.com>, "davidgow" <davidgow@google.com>, "linux-um" <linux-um@lists.infradead.org>, > "linux-kernel" <linux-kernel@vger.kernel.org>, "kasan-dev" <kasan-dev@googlegroups.com> > Gesendet: Dienstag, 31. März 2020 18:39:21 > Betreff: Re: [PATCH] UML: add support for KASAN under x86_64
> On Mon, Mar 30, 2020 at 1:41 AM Johannes Berg <johannes@sipsolutions.net> wrote: >> >> On Mon, 2020-03-30 at 10:38 +0200, Dmitry Vyukov wrote: >> > On Mon, Mar 30, 2020 at 9:44 AM Johannes Berg <johannes@sipsolutions.net> wrote: >> > > On Fri, 2020-03-20 at 16:18 +0100, Dmitry Vyukov wrote: >> > > > > Wait ... Now you say 0x7fbfffc000, but that is almost fine? I think you >> > > > > confused the values - because I see, on userspace, the following: >> > > > >> > > > Oh, sorry, I copy-pasted wrong number. I meant 0x7fff8000. >> > > >> > > Right, ok. >> > > >> > > > Then I would expect 0x1000 0000 0000 to work, but you say it doesn't... >> > > >> > > So it just occurred to me - as I was mentioning this whole thing to >> > > Richard - that there's probably somewhere some check about whether some >> > > space is userspace or not. >> > > > > Yeah, it seems the "Kernel panic - not syncing: Segfault with no mm", > "Kernel mode fault at addr...", and "Kernel tried to access user > memory at addr..." errors all come from segv() in > arch/um/kernel/trap.c due to what I think is this type of check > whether the address is > in userspace or not.
Segfault with no mm means that a (not fixable) pagefault happened while kernel code ran.
>> > > I'm beginning to think that we shouldn't just map this outside of the >> > > kernel memory system, but properly treat it as part of the memory that's >> > > inside. And also use KASAN_VMALLOC. >> > > >> > > We can probably still have it at 0x7fff8000, just need to make sure we >> > > actually map it? I tried with vm_area_add_early() but it didn't really >> > > work once you have vmalloc() stuff... >> > > > What x86 does when KASAN_VMALLOC is disabled is make all vmalloc > region accesses succeed by default > by using the early shadow memory to have completely unpoisoned and > unpoisonable read-only pages for all of vmalloc (which includes > modules). When KASAN_VMALLOC is enabled in x86, the shadow memory is not > allocated for the vmalloc region at startup. New chunks of shadow > memory are allocated and unpoisoned every time there's a vmalloc() > call. A similar thing might have to be done here by mprotect()ing > the vmalloc space as read only, unpoisoned without KASAN_VMALLOC. This > issue here is that > kasan_init runs so early in the process that the vmalloc region for > uml is not setup yet. > > >> > But we do mmap it, no? See kasan_init() -> kasan_map_memory() -> mmap. >> >> Of course. But I meant inside the UML PTE system. We end up *unmapping* >> it when loading modules, because it overlaps vmalloc space, and then we >> vfree() something again, and unmap it ... because of the overlap. >> >> And if it's *not* in the vmalloc area, then the kernel doesn't consider >> it valid, and we seem to often just fault when trying to determine >> whether it's valid kernel memory or not ... Though I'm not really sure I >> understand the failure part of this case well yet. >> > > I have been testing this issue in a multitude of ways and have only > been getting more confused. It's still very unclear where exactly the > problem occurs, mostly because the errors I found most frequently were > reported in segv(), but the stack traces never contained segv. > > Does anyone know if/how UML determines if memory being accessed is > kernel or user memory?
In contrast to classic x86, without KPTI and SMAP/SMEP, UML has a strong separation between user- and kernel-memory. This is also why copy_from/to_user() is so expensive.
In arch/um/kernel/trap.c segv() you can see the logic. Also see UPT_IS_USER().
Thanks, //richard
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