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

[Guo Ren]

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))
+               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