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Date: Mon, 21 Nov 2022 22:59:50 +0300
From: Sergey Matyukevich <geomatsi@...il.com>
To: guoren@...nel.org
Cc: anup@...infault.org, paul.walmsley@...ive.com, palmer@...belt.com,
conor.dooley@...rochip.com, heiko@...ech.de,
philipp.tomsich@...ll.eu, alex@...ti.fr, hch@....de,
ajones@...tanamicro.com, gary@...yguo.net, jszhang@...nel.org,
linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
Guo Ren <guoren@...ux.alibaba.com>,
Anup Patel <apatel@...tanamicro.com>,
Palmer Dabbelt <palmer@...osinc.com>
Subject: Re: [PATCH V3] riscv: asid: Fixup stale TLB entry cause application
crash
> 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@...ux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@...nel.org>
> Cc: Anup Patel <apatel@...tanamicro.com>
> Cc: Palmer Dabbelt <palmer@...osinc.com>
> ---
> 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@...el.com>)
> - 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@...tacore.com>
Thanks,
Sergey
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