[PATCH 2/2] kasan, arm64: fix pointer tags in KASAN reports
Andrey Konovalov
andreyknvl at google.com
Fri Jan 15 11:30:40 EST 2021
On Wed, Jan 13, 2021 at 5:54 PM Catalin Marinas <catalin.marinas at arm.com> 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 at google.com>
> > ---
> > 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.
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