Re: [PATCH] UML: add support for KASAN under x86_64

Dmitry Vyukov
2020-03-20T15:18:45Z

On Fri, Mar 20, 2020 at 2:39 PM Johannes Berg <johannes@sipsolutions.net> wrote:
>
> On Wed, 2020-03-11 at 18:34 +0100, Dmitry Vyukov wrote:
>
> > > $ gdb -p ...
> > > (gdb) p/x task_size
> > > $1 = 0x7fc0000000
> > > (gdb) p/x __end_of_fixed_addresses
> > > $2 = 0x0
> > > (gdb) p/x end_iomem
> > > $3 = 0x70000000
> > > (gdb) p/x __va_space
> > >
> > > #define TASK_SIZE (task_size)
> > > #define FIXADDR_TOP (TASK_SIZE - 2 * PAGE_SIZE)
> > >
> > > #define FIXADDR_START (FIXADDR_TOP - FIXADDR_SIZE)
> > > #define FIXADDR_SIZE (__end_of_fixed_addresses << PAGE_SHIFT)
> > >
> > > #define VMALLOC_END (FIXADDR_START-2*PAGE_SIZE)
> > >
> > > #define MODULES_VADDR VMALLOC_START
> > > #define MODULES_END VMALLOC_END
> > > #define VMALLOC_START ((end_iomem + VMALLOC_OFFSET) & ~(VMALLOC_OFFSET-1))
> > > #define VMALLOC_OFFSET (__va_space)
> > > #define __va_space (8*1024*1024)
> > >
> > >
> > > So from that, it would look like the UML vmalloc area is from
> > > 0x 70800000 all the way to
> > > 0x7fbfffc000, which obviously clashes with the KASAN_SHADOW_OFFSET being
> > > just 0x7fff8000.
> > >
> > >
> > > I'm guessing that basically the module loading overwrote the kasan
> > > shadow then?
> >
> > Well, ok, this is definitely not going to fly :)
>
> Yeah, not with vmalloc/modules at least, but you can't really prevent
> vmalloc :)
>
> > I don't know if it's easy to move modules to a different location.
>
> We'd have to not just move modules, but also vmalloc space. They're one
> and the same in UML.
>
> > It
> > would be nice because 0x7fbfffc000 is the shadow start that's used in
> > userspace asan and it allows to faster instrumentation (if offset is
> > within first 2 gigs, the instruction encoding is much more compact,
> > for >2gigs it will require several instructions).
>
> 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. Here is the
user-space mapping that uses it:
https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/asan/asan_mapping.h#L25

> || `[0x10007fff8000, 0x7fffffffffff]` || HighMem ||
> || `[0x02008fff7000, 0x10007fff7fff]` || HighShadow ||
> || `[0x00008fff7000, 0x02008fff6fff]` || ShadowGap ||
> || `[0x00007fff8000, 0x00008fff6fff]` || LowShadow ||
> || `[0x000000000000, 0x00007fff7fff]` || LowMem ||
>
>
> Now, I also don't really understand what UML is doing here -
> os_get_top_address() determines some sort of "top address"? But all that
> is only on 32-bit, on 64-bit, that's always 0x7fc0000000.

Then I would expect 0x1000 0000 0000 to work, but you say it doesn't...

> So basically that means it's just _slightly_ higher than what you
> suggested as the KASAN_SHADOW_OFFSET now (even if erroneously?), and
> shouldn't actually clash (and we can just change the top address value
> to be slightly lower anyway to prevent clashing).
>
> > But if it's not really easy, I guess we go with a large shadow start
> > (at least initially). A slower but working KASAN is better than fast
> > non-working KASAN :)
>
> Indeed, but I can't even get it to work regardless of the offset.
>
> Note that I have lockdep enabled, and at least some crashes appear to be
> because of the stack unwinding code that is called by lockdep in various
> situations...

This is something new, right? The previous stacks you posted did not
mention lockdep.

> > > I tried changing it
> > >
> > > config KASAN_SHADOW_OFFSET
> > > hex
> > > depends on KASAN
> > > - default 0x7fff8000
> > > + default 0x8000000000
> > >
> > >
> > > and also put a check in like this:
> > >
> > > +++ b/arch/um/kernel/um_arch.c
> > > @@ -13,6 +13,7 @@
> > > #include <linux/sched.h>
> > > #include <linux/sched/task.h>
> > > #include <linux/kmsg_dump.h>
> > > +#include <linux/kasan.h>
> > >
> > > #include <asm/pgtable.h>
> > > #include <asm/processor.h>
> > > @@ -267,9 +268,11 @@ int __init linux_main(int argc, char **argv)
> > > /*
> > > * TASK_SIZE needs to be PGDIR_SIZE aligned or else exit_mmap craps
> > > * out
> > > */
> > > task_size = host_task_size & PGDIR_MASK;
> > >
> > > + if (task_size > KASAN_SHADOW_OFFSET)
> > > + panic("KASAN shadow offset must be bigger than task size");
> > >
> > >
> > > but now I just crash accessing the shadow even though it was mapped fine?
> >
> > Yes, this is puzzling.
> > I noticed that RIP is the same in both cases and it relates to vmap code.
> > A support for shadow for vmalloced-memory was added to KASAN recently
> > and I suspect it may conflict with UML.
>
> This can't be it - HAVE_ARCH_KASAN_VMALLOC isn't selected, so
> KASAN_VMALLOC isn't set.
>
> > What does pte-manipulation code even do under UML?
>
> No idea.
>
> > Looking at the code around, kasan_mem_notifier may be a problem too,
> > or at least excessive and confusing. We already have shadow for
> > everything, we don't need _any_ of dynamic/lazy shadow mapping.
>
> CONFIG_MEMORY_HOTPLUG is also not supported in ARCH=um, or at least not
> used in my config.

Ack.

Maybe if you dump /proc/self/maps for the process, it will shed some light.
Or is it possible to run it under strace? If we get all
mmap/munmap/mprotect, we will maybe see the offender that messes the
shadow...