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

Conor Dooley
2022-11-08T10:28:15Z

On 8 November 2022 10:20:44 GMT, guoren@kernel.org wrote:
>From: Guo Ren <guoren@linux.alibaba.com>
>
>After use_asid_allocator enabled, the userspace application will
>crash for stale tlb entry. Because only using cpumask_clear_cpu without
>local_flush_tlb_all couldn't guarantee CPU's tlb entries fresh. Then
>set_mm_asid would cause user space application get a stale value by
>the 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 shows that the value of register s0 is wrong, but the
>value in memory is right. This is because 'ld s0, -148(s0)' use a stale
>mapping entry in TLB and got a wrong value from a stale physical
>address.
>
>When task run on CPU0, the task loaded/speculative-loaded the value of
>address(0x3fd6d92490), and the first version of tlb mapping entry was
>PTWed into 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). It caused do_page_fault -> wp_page_copy ->
>ptep_clear_flush -> ptep_get_and_clear & flush_tlb_page.
>The flush_tlb_page used mm_cpumask(mm) to determine which CPUs need
>tlb flush, but CPU0 had cleared the CPU0's mm_cpumask in previous switch_mm.
>So we only flushed the CPU1 tlb, and setted second version mapping
>of the pte. When the task switch from CPU1 to CPU0 again, CPU0 still used a
>stale tlb mapping entry 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@linux.alibaba.com>
>Signed-off-by: Guo Ren <guoren@kernel.org>
>Cc: Anup Patel <apatel@ventanamicro.com>
>Cc: Palmer Dabbelt <palmer@rivosinc.com>
>---
> arch/riscv/mm/context.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
>diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
>index 7acbfbd14557..8ad6c2493e93 100644
>--- a/arch/riscv/mm/context.c
>+++ b/arch/riscv/mm/context.c
>@@ -317,7 +317,9 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
> */
> cpu = smp_processor_id();
>
>- cpumask_clear_cpu(cpu, mm_cpumask(prev));
>+ if (!static_branch_unlikely(&use_asid_allocator))
>+ cpumask_clear_cpu(cpu, mm_cpumask(prev));
>+
> cpumask_set_cpu(cpu, mm_cpumask(next));
>
> set_mm(next, cpu);

This is a completely different patch to what you already sent. Why have you marked it RESEND rather than v2?