Re: [PATCH V2] riscv: asid: Fixup stale TLB entry cause application crash

[Andrew Jones]

On Wed, Nov 09, 2022 at 12:40:56AM -0500, guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> 
> After use_asid_allocator is enabled, the userspace application will
> crash by stale TLB entries. Because only using cpumask_clear_cpu without
> local_flush_tlb_all couldn't guarantee CPU's TLB entries were fresh.
> Then set_mm_asid would cause the user space application to 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 /2x 0x3fd6d92490
> 0x3fd6d92490:   0xd80ac8a8      0x0000003f
> The core dump file shows that register s0 is wrong, but the value in
> memory is correct. Because 'ld s0, -148(s0)' used a stale mapping entry
> in TLB and got a wrong result from an incorrect physical address.
> 
> When the task ran on CPU0, which loaded/speculative-loaded the value of
> address(0x3fd6d92490), then the first version of the mapping entry was
> PTWed into CPU0's TLB.
> When the task switched from CPU0 to CPU1 (No local_tlb_flush_all here by
> asid), it happened to write a value on the 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 the previous
> switch_mm. So we only flushed the CPU1 TLB and set the second version
> mapping of the PTE. When the task switched from CPU1 to CPU0 again, CPU0
> still used a stale TLB mapping entry which contained a wrong target
> physical address. It raised a bug when the task happened to read that
> value.
> 
> The solution is to keep all CPUs' footmarks of cpumask(mm) in switch_mm,
> which could prevent losing pieces of stuff during TLB flush.
> 
> 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>
> ---
> Changes in v2:
>  - Fixup nommu compile problem (Thx Conor, Also Reported-by: kernel
>    test robot <lkp@xxxxxxxxx>)
>  - Keep cpumask_clear_cpu for noasid
> ---
>  arch/riscv/mm/context.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> index 7acbfbd14557..f58e4b211595 100644
> --- a/arch/riscv/mm/context.c
> +++ b/arch/riscv/mm/context.c
> @@ -317,7 +317,11 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>  	 */
>  	cpu = smp_processor_id();
>  
> -	cpumask_clear_cpu(cpu, mm_cpumask(prev));
> +#ifdef CONFIG_MMU
> +	if (!static_branch_unlikely(&use_asid_allocator))
> +#endif

That's not very pretty. Can't we just do the following, instead?

diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
index 7acbfbd14557..ace419761e31 100644
--- a/arch/riscv/mm/context.c
+++ b/arch/riscv/mm/context.c
@@ -16,10 +16,11 @@
 #include <asm/cacheflush.h>
 #include <asm/mmu_context.h>

-#ifdef CONFIG_MMU

 DEFINE_STATIC_KEY_FALSE(use_asid_allocator);

+#ifdef CONFIG_MMU
+
 static unsigned long asid_bits;
 static unsigned long num_asids;
 static unsigned long asid_mask;


Thanks,
drew

> +		cpumask_clear_cpu(cpu, mm_cpumask(prev));
> +
>  	cpumask_set_cpu(cpu, mm_cpumask(next));
>  
>  	set_mm(next, cpu);
> -- 
> 2.36.1
> 
> 
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