Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a separate file

[Guo Ren]

Hello Catalin,

Thanks for sharing about CnP assid experience. See my comment below.

On Mon, Jul 1, 2019 at 5:17 PM Catalin Marinas
> From the ASID reservation/allocation perspective, the mechanism is the
> same between multi-threaded with a shared TLB and multi-core. On arm64,
> a local_flush_tlb_all() on a thread invalidates the TLB for the other
> threads of the same core.
>
> The actual problem with multi-threaded CPUs is a lot more subtle.
> Digging some internal email from 1.5 years ago and pasting it below
> (where "current ASID algorithm" refers to the one prior to the fix and
> CnP - Common Not Private - means shared TLBs on a multi-threaded CPU):
>
>
> The current ASID roll-over algorithm allows for a small window where
> active_asids for a CPU (P1) is different from the actual ASID in TTBR0.
> This can lead to a roll-over on a different CPU (P2) allocating an ASID
> (for a different task) which is still hardware-active on P1.
>
> A TLBI on a CPU (or a peer CPU with CnP) does not guarantee that all the
> entries corresponding to a valid TTBRx are removed as they can still be
> speculatively loaded immediately after TLBI.
>
> While having two different page tables with the same ASID on different
> CPUs should be fine without CnP, it becomes problematic when CnP is
> enabled:
>
> P1                                      P2
> --                                      --
> TTBR0.BADDR = T1
> TTBR0.ASID = A1
> check_and_switch_context(T2,A2)
>   asid_maps[P1] = A2
>   goto fastpath
>                                         check_and_switch_context(T3,A0)
>                                           new_context
>                                             ASID roll-over allocates A1
>                                               since it is not active
>                                           TLBI ALL
> speculate TTBR0.ASID = A1 entry
>                                           TTBR0.BADDR = T3
>                                           TTBR0.ASID = A1
>   TTBR0.BADDR = T2
>   TTBR0.ASID = A2
>
> After this, the common TLB on P1 and P2 (CnP) contains entries
> corresponding to the old T1 and A1. Task T3 using the same ASID A1 can
> hit such entries. (T1,A1) will eventually be removed from the TLB on the
> next context switch on P1 since tlb_flush_pending was set but this is
> not guaranteed to happen.
>
>
> The fix on arm64 (as part of 5ffdfaedfa0a - "arm64: mm: Support Common
> Not Private translations") was to set the reserved TTBR0 in
> check_and_switch_context(), preventing speculative loads into the TLB
> being tagged with the wrong ASID. So this is specific to the ARM CPUs
> behaviour w.r.t. speculative TLB loads, it may not be the case (yet) for
> your architecture.

The most important thing is that TLBI ALL occurs between
"asid_maps[P1] = A2" and "TTBR0.BADDR = T2", then speculative
execution after TLBI which access to user space code/data will result
in a valid asid entry which re-filled into the TLB by PTW.

A similar problem should exist if C-SKY ISA supports SMT. Although the
C-SKY kernel prohibits the kernel from speculating on user space code
directly, ld/st can access user space memory in csky kernel mode.
Therefore, a similar problem occurs when it speculatively executes
copy_from / to_user codes in that window.

RISC-V ISA has a SUM setting bit that prevents the kernel from
speculating access to user space. So this problem has been bypassed
from the design.

I saw arm64 to prevent speculation by temporarily setting TTBR0.el1 to
a zero page table. Is that used to prevent speculative execution user
space code or just prevent ld/st in copy_use_* ?

-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/