Re: [PATCH v10 12/22] kasan, arm64: fix up fault handling logic

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Nov 14, 2018 at 09:06:23PM +0100, Andrey Konovalov wrote:
> On Tue, Nov 13, 2018 at 11:07 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> > On Tue, Nov 13, 2018 at 04:01:27PM +0100, Andrey Konovalov wrote:
> >> On Thu, Nov 8, 2018 at 1:22 PM, Mark Rutland <mark.rutland@arm.com> wrote:
> >> > On Tue, Nov 06, 2018 at 06:30:27PM +0100, Andrey Konovalov wrote:
> >> >> show_pte in arm64 fault handling relies on the fact that the top byte of
> >> >> a kernel pointer is 0xff, which isn't always the case with tag-based
> >> >> KASAN.
> >> >
> >> > That's for the TTBR1 check, right?
> >> >
> >> > i.e. for the following to work:
> >> >
> >> >         if (addr >= VA_START)
> >> >
> >> > ... we need the tag bits to be an extension of bit 55...
> >> >
> >> >>
> >> >> This patch resets the top byte in show_pte.
> >> >>
> >> >> Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> >> >> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> >> >> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> >> >> ---
> >> >>  arch/arm64/mm/fault.c | 3 +++
> >> >>  1 file changed, 3 insertions(+)
> >> >>
> >> >> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> >> >> index 7d9571f4ae3d..d9a84d6f3343 100644
> >> >> --- a/arch/arm64/mm/fault.c
> >> >> +++ b/arch/arm64/mm/fault.c
> >> >> @@ -32,6 +32,7 @@
> >> >>  #include <linux/perf_event.h>
> >> >>  #include <linux/preempt.h>
> >> >>  #include <linux/hugetlb.h>
> >> >> +#include <linux/kasan.h>
> >> >>
> >> >>  #include <asm/bug.h>
> >> >>  #include <asm/cmpxchg.h>
> >> >> @@ -141,6 +142,8 @@ void show_pte(unsigned long addr)
> >> >>       pgd_t *pgdp;
> >> >>       pgd_t pgd;
> >> >>
> >> >> +     addr = (unsigned long)kasan_reset_tag((void *)addr);
> >> >
> >> > ... but this ORs in (0xffUL << 56), which is not correct for addresses
> >> > which aren't TTBR1 addresses to begin with, where bit 55 is clear, and
> >> > throws away useful information.
> >> >
> >> > We could use untagged_addr() here, but that wouldn't be right for
> >> > kernels which don't use TBI1, and we'd erroneously report addresses
> >> > under the TTBR1 range as being in the TTBR1 range.
> >> >
> >> > I also see that the entry assembly for el{1,0}_{da,ia} clears the tag
> >> > for EL0 addresses.
> >> >
> >> > So we could have:
> >> >
> >> > static inline bool is_ttbr0_addr(unsigned long addr)
> >> > {
> >> >         /* entry assembly clears tags for TTBR0 addrs */
> >> >         return addr < TASK_SIZE_64;
> >> > }
> >> >
> >> > static inline bool is_ttbr1_addr(unsigned long addr)
> >> > {
> >> >         /* TTBR1 addresses may have a tag if HWKASAN is in use */
> >> >         return arch_kasan_reset_tag(addr) >= VA_START;
> >> > }
> >> >
> >> > ... and use those in the conditionals, leaving the addr as-is for
> >> > reporting purposes.
> >>
> >> Actually it looks like 276e9327 ("arm64: entry: improve data abort
> >> handling of tagged pointers") already takes care of both user and
> >> kernel fault addresses and correctly removes tags from them. So I
> >> think we need to drop this patch.
> >
> > The clear_address_tag macro added in that commit only removes tags from TTBR0
> > addresses, so that's not sufficient if the kernel is used tagged addresses
> > (which will be in the TTBR1 range).
> 
> Do I understand correctly that TTBR0 means user space addresses and
> TTBR1 means kernel addresses? 

Effectively, yes. The address space is split into two halves (with a gap in the
middle). The high half (where we map the kernel) is covered by TTBR1, and the
low half (where we map userspace) is covered by TTBR0.

The TTBRs are the Translation Table Base Registers -- the two halves have
separate page tables.

> In that commit I see that the clear_address_tag() macro is used in el0_da and
> in el1_da, which means that it untags both user and kernel addresses (on data
> aborts). Do I misunderstand something?

It's called for faults taken from EL0 and EL1, but it only removes the tags
from addresses covered by TTBR0. The logic is:

	.macro  clear_address_tag, dst, addr
	tst     \addr, #(1 << 55)
	bic     \dst, \addr, #(0xff << 56)
	csel    \dst, \dst, \addr, eq
	.endm

... which in C would be:

	if (!(addr & (1UL << 55))) {
		addr &= ~(0xffUL << 56);
	}

... and therefore does not affect TTBR1 addresses.

Thanks,
Mark.