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

[Zong Li]

Lad, Prabhakar <prabhakar.csengg@xxxxxxxxx> 於 2022年12月23日 週五 下午8:54寫道：
>
> Hi Guo,
>
> Thank you for the patch.
>
> On Fri, Nov 11, 2022 at 8:00 AM <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.
> >
> >    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(-)
> >
> As reported on the patch [0] I was seeing consistent failures on the
> RZ/Five SoC while running bonnie++ utility. After applying this patch
> on top of Palmer's for-next branch (eb67d239f3aa) I am no longer
> seeing this issue.
>
> Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
>
> [0] https://patchwork.kernel.org/project/linux-riscv/patch/20220829205219.283543-1-geomatsi@xxxxxxxxx/
>

Hi all,
I got the same situation (i.e. unhandle signal 11) on our internal
multi-core system, I tried the patch[0] & [1], but it still doesn't
work, I guess there are still some potential problems. After applying
this patch, the situation disappeared, I took some time to look at
other arches' implementations, such as arc, they don't clear the
mm_cpumask due to the similar issue. I can't say which approach might
be better, but I'd like to point out that this patch works to me.
Thanks.

Tested-by: Zong Li <zong.li@xxxxxxxxxx>

[0] https://lore.kernel.org/linux-riscv/20220829205219.283543-1-geomatsi@xxxxxxxxx/
[1] https://lore.kernel.org/linux-riscv/20230129211818.686557-1-geomatsi@xxxxxxxxx/

> Cheers,
> Prabhakar
>
> > 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);
> >  }
> > --
> > 2.36.1
> >
> >
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>
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