Re: [PATCH V3] riscv: asid: Fixup stale TLB entry cause application crash

[Sergey Matyukevich]

> 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.
> 
>    CPU0                               CPU1
>    - switch 'task' in
>    - read addr (Fill stale mapping
>      entry into TLB)
>    - switch 'task' out (no tlb_flush)
>                                       - switch 'task' in (no tlb_flush)
>                                       - write addr cause pagefault
>                                         do_page_fault() (change to
>                                         new addr mapping)
>                                           wp_page_copy()
>                                             ptep_clear_flush()
>                                               ptep_get_and_clear()
>                                               & flush_tlb_page()
>                                         write new value into addr
>                                       - switch 'task' out (no tlb_flush)
>    - switch 'task' in (no tlb_flush)
>    - read addr again (Use stale
>      mapping entry in TLB)
>      get wrong value from old phyical
>      addr, BUG!
> 
> The solution is to keep all CPUs' footmarks of cpumask(mm) in switch_mm,
> which could guarantee to invalidate all stale TLB entries 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 v3:
>  - Move set/clear cpumask(mm) into set_mm (Make code more pretty
>    with Andrew's advice)
>  - Optimize comment description
> 
> 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 | 30 ++++++++++++++++++++----------
>  1 file changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> index 7acbfbd14557..0f784e3d307b 100644
> --- a/arch/riscv/mm/context.c
> +++ b/arch/riscv/mm/context.c
> @@ -205,12 +205,24 @@ static void set_mm_noasid(struct mm_struct *mm)
>  	local_flush_tlb_all();
>  }
>  
> -static inline void set_mm(struct mm_struct *mm, unsigned int cpu)
> +static inline void set_mm(struct mm_struct *prev,
> +			  struct mm_struct *next, unsigned int cpu)
>  {
> -	if (static_branch_unlikely(&use_asid_allocator))
> -		set_mm_asid(mm, cpu);
> -	else
> -		set_mm_noasid(mm);
> +	/*
> +	 * The mm_cpumask indicates which harts' TLBs contain the virtual
> +	 * address mapping of the mm. Compared to noasid, using asid
> +	 * can't guarantee that stale TLB entries are invalidated because
> +	 * the asid mechanism wouldn't flush TLB for every switch_mm for
> +	 * performance. So when using asid, keep all CPUs footmarks in
> +	 * cpumask() until mm reset.
> +	 */
> +	cpumask_set_cpu(cpu, mm_cpumask(next));
> +	if (static_branch_unlikely(&use_asid_allocator)) {
> +		set_mm_asid(next, cpu);
> +	} else {
> +		cpumask_clear_cpu(cpu, mm_cpumask(prev));
> +		set_mm_noasid(next);
> +	}
>  }
>  
>  static int __init asids_init(void)
> @@ -264,7 +276,8 @@ static int __init asids_init(void)
>  }
>  early_initcall(asids_init);
>  #else
> -static inline void set_mm(struct mm_struct *mm, unsigned int cpu)
> +static inline void set_mm(struct mm_struct *prev,
> +			  struct mm_struct *next, unsigned int cpu)
>  {
>  	/* Nothing to do here when there is no MMU */
>  }
> @@ -317,10 +330,7 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>  	 */
>  	cpu = smp_processor_id();
>  
> -	cpumask_clear_cpu(cpu, mm_cpumask(prev));
> -	cpumask_set_cpu(cpu, mm_cpumask(next));
> -
> -	set_mm(next, cpu);
> +	set_mm(prev, next, cpu);
>  
>  	flush_icache_deferred(next, cpu);
>  }

Tested-by: Sergey Matyukevich <sergey.matyukevich@xxxxxxxxxxxxx>

Thanks,
Sergey