Re: [PATCH 2/2] kasan, arm64: fix pointer tags in KASAN reports

[Andrey Konovalov]

On Fri, Jan 15, 2021 at 6:06 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
> On Fri, Jan 15, 2021 at 06:00:36PM +0100, Andrey Konovalov wrote:
> > On Fri, Jan 15, 2021 at 5:56 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> > >
> > > On Fri, Jan 15, 2021 at 05:30:40PM +0100, Andrey Konovalov wrote:
> > > > On Wed, Jan 13, 2021 at 5:54 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> > > > > On Wed, Jan 13, 2021 at 05:03:30PM +0100, Andrey Konovalov wrote:
> > > > > > As of the "arm64: expose FAR_EL1 tag bits in siginfo" patch, the address
> > > > > > that is passed to report_tag_fault has pointer tags in the format of 0x0X,
> > > > > > while KASAN uses 0xFX format (note the difference in the top 4 bits).
> > > > > >
> > > > > > Fix up the pointer tag before calling kasan_report.
> > > > > >
> > > > > > Link: https://linux-review.googlesource.com/id/I9ced973866036d8679e8f4ae325de547eb969649
> > > > > > Fixes: dceec3ff7807 ("arm64: expose FAR_EL1 tag bits in siginfo")
> > > > > > Fixes: 4291e9ee6189 ("kasan, arm64: print report from tag fault handler")
> > > > > > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> > > > > > ---
> > > > > >  arch/arm64/mm/fault.c | 2 ++
> > > > > >  1 file changed, 2 insertions(+)
> > > > > >
> > > > > > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> > > > > > index 3c40da479899..a218f6f2fdc8 100644
> > > > > > --- a/arch/arm64/mm/fault.c
> > > > > > +++ b/arch/arm64/mm/fault.c
> > > > > > @@ -304,6 +304,8 @@ static void report_tag_fault(unsigned long addr, unsigned int esr,
> > > > > >  {
> > > > > >       bool is_write  = ((esr & ESR_ELx_WNR) >> ESR_ELx_WNR_SHIFT) != 0;
> > > > > >
> > > > > > +     /* The format of KASAN tags is 0xF<x>. */
> > > > > > +     addr |= (0xF0UL << MTE_TAG_SHIFT);
> > > > >
> > > > > Ah, I see, that top 4 bits are zeroed by do_tag_check_fault(). When this
> > > > > was added, the only tag faults were generated for user addresses.
> > > > >
> > > > > Anyway, I'd rather fix it in there based on bit 55, something like (only
> > > > > compile-tested):
> > > > >
> > > > > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> > > > > index 3c40da479899..2b71079d2d32 100644
> > > > > --- a/arch/arm64/mm/fault.c
> > > > > +++ b/arch/arm64/mm/fault.c
> > > > > @@ -709,10 +709,11 @@ static int do_tag_check_fault(unsigned long far, unsigned int esr,
> > > > >                               struct pt_regs *regs)
> > > > >  {
> > > > >         /*
> > > > > -        * The architecture specifies that bits 63:60 of FAR_EL1 are UNKNOWN for tag
> > > > > -        * check faults. Mask them out now so that userspace doesn't see them.
> > > > > +        * The architecture specifies that bits 63:60 of FAR_EL1 are UNKNOWN
> > > > > +        * for tag check faults. Set them to the corresponding bits in the
> > > > > +        * untagged address.
> > > > >          */
> > > > > -       far &= (1UL << 60) - 1;
> > > > > +       far = (untagged_addr(far) & ~MTE_TAG_MASK) | (far & MTE_TAG_MASK) ;
> > > > >         do_bad_area(far, esr, regs);
> > > > >         return 0;
> > > > >  }
> > > >
> > > > BTW, we can do "untagged_addr(far) | (far & MTE_TAG_MASK)" here, as
> > > > untagged_addr() doesn't change kernel pointers.
> > >
> > > untagged_addr() does change tagged kernel pointers, it sign-extends from
> > > bit 55. So the top byte becomes 0xff and you can no longer or the tag
> > > bits in.
> >
> > That's __untagged_addr(), untagged_addr() keeps the bits for kernel
> > pointers as of  597399d0cb91.
>
> Ah, you are right. In this case I think we should use __untagged_addr()
> above. Even if the tag check fault happened on a kernel address, bits
> 63:60 are still unknown.

Yeah, I keep forgetting about [__]untagged_addr() too. Maybe we need
better names? Like untagged_addr() and untagged_addr_ttbr0()?

Anyway, I'll do the explicit calculation with __untagged_addr() in the
next version.

Thanks!