Re: [PATCH] riscv: asid: Fixup stale TLB entry cause application crash

[Guo Ren]

On Tue, Nov 8, 2022 at 5:25 PM Guo Ren <guoren@xxxxxxxxxx> wrote:
>
> On Tue, Nov 8, 2022 at 5:02 PM <guoren@xxxxxxxxxx> wrote:
> >
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > When use_asid_allocator is enabled, the userspace application would
> > crash by stale tlb entry. Because only using cpumask_clear_cpu without
> > local_flush_tlb_all couldn't guarantee CPU's tlb contains fresh mapping
> > entry. Then set_mm_asid would cause user space application get a stale
> > value by stale tlb entry, but set_mm_noasid is okay.
> >
> > Here is the symptom of the bug:
> > unhandled signal 11 code 0x1 (coredump)
> >    0x0000003fd6d22524 <+4>:     auipc   s0,0x70
> >    0x0000003fd6d22528 <+8>:     ld      s0,-148(s0) # 0x3fd6d92490
> > => 0x0000003fd6d2252c <+12>:    ld      a5,0(s0)
> > (gdb) i r s0
> > s0          0x8082ed1cc3198b21       0x8082ed1cc3198b21
> > (gdb) x/16 0x3fd6d92490
> > 0x3fd6d92490:   0xd80ac8a8      0x0000003f
> > The core dump file show us the value of register s0 is wrong, but the
> > value in memory is right.
> >
> > When task run on CPU0, the task loaded/speculative-loaded the value of
> > address-0x3fd6d92490, the first version tlb mapping enter in CPU0's tlb.
> > When the task switched from CPU0 to CPU1 without local_tlb_flush_all
> > (because of asid), the task happened to write a value on address:
> > 0x3fd6d92490 that caused do_page_fault -> wp_page_copy ->
> > ptep_clear_flush -> ptep_get_and_clear & flush_tlb_page.
> > The flush_tlb_page would use mm_cpumask(mm) to determine which CPUs need
> > tlb flush, but CPU0 cleared the CPU0's mm_cpumask in previous switch_mm.
> > So we only flushed the CPU1's tlb entry, and setted second version mapping
> > of the pte. When the task switch from CPU1 to CPU0 again, it still used a
> > stale tlb entry on CPU0 which contained a wrong target physical address.
> > When the task happened to read that value, the bug would be raised.
> >
> > Fixes: 65d4b9c53017 ("RISC-V: Implement ASID allocator")
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> > Cc: Anup Patel <apatel@xxxxxxxxxxxxxxxx>
> > Cc: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> > ---
> >  arch/riscv/mm/context.c | 1 -
> >  1 file changed, 1 deletion(-)
> >
> > diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> > index 7acbfbd14557..843e86b63532 100644
> > --- a/arch/riscv/mm/context.c
> > +++ b/arch/riscv/mm/context.c
> > @@ -317,7 +317,6 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> >          */
> >         cpu = smp_processor_id();
> >
> > -       cpumask_clear_cpu(cpu, mm_cpumask(prev));
> It should be:
>
> -       cpumask_clear_cpu(cpu, mm_cpumask(prev));
> +       if (static_branch_unlikely(&use_asid_allocator))
Sorry,
        if (!static_branch_unlikely(&use_asid_allocator))

I would RESEND the patch.

> +               cpumask_clear_cpu(cpu, mm_cpumask(prev));
> +
>
> No bug on noasid part.
>
> >         cpumask_set_cpu(cpu, mm_cpumask(next));
> >
> >         set_mm(next, cpu);
> > --
> > 2.36.1
> >
>
>
> --
> Best Regards
>  Guo Ren



-- 
Best Regards
 Guo Ren