Date:   Fri, 12 Apr 2019 17:49:16 +0000
From:   Nadav Amit <namit@...are.com>
To:     Andy Lutomirski <luto@...capital.net>
CC:     Peter Zijlstra <peterz@...radead.org>,
        kernel test robot <lkp@...el.com>, LKP <lkp@...org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: 1808d65b55 ("asm-generic/tlb: Remove arch_tlb*_mmu()"):  BUG:
 KASAN: stack-out-of-bounds in __change_page_attr_set_clr

> On Apr 12, 2019, at 10:14 AM, Andy Lutomirski <luto@...capital.net> wrote:
> 
> 
> 
> On Apr 12, 2019, at 10:05 AM, Nadav Amit <namit@...are.com> wrote:
> 
>>> On Apr 12, 2019, at 8:11 AM, Nadav Amit <namit@...are.com> wrote:
>>> 
>>>> On Apr 12, 2019, at 4:17 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>>> 
>>>> On Fri, Apr 12, 2019 at 12:56:33PM +0200, Peter Zijlstra wrote:
>>>>>> On Thu, Apr 11, 2019 at 11:13:48PM +0200, Peter Zijlstra wrote:
>>>>>>> On Thu, Apr 11, 2019 at 09:54:24PM +0200, Peter Zijlstra wrote:
>>>>>>>> On Thu, Apr 11, 2019 at 09:39:06PM +0200, Peter Zijlstra wrote:
>>>>>>>> I think this bisect is bad. If you look at your own logs this patch
>>>>>>>> merely changes the failure, but doesn't make it go away.
>>>>>>>> 
>>>>>>>> Before this patch (in fact, before tip/core/mm entirely) the errror
>>>>>>>> reads like the below, which suggests there is memory corruption
>>>>>>>> somewhere, and the fingered patch just makes it trigger differently.
>>>>>>>> 
>>>>>>>> It would be very good to find the source of this corruption, but I'm
>>>>>>>> fairly certain it is not here.
>>>>>>> 
>>>>>>> I went back to v4.20 to try and find a time when the below error did not
>>>>>>> occur, but even that reliably triggers the warning.
>>>>>> 
>>>>>> So I also tested v4.19 and found that that was good, which made me
>>>>>> bisect v4.19..v4.20
>>>>>> 
>>>>>> # bad: [8fe28cb58bcb235034b64cbbb7550a8a43fd88be] Linux 4.20
>>>>>> # good: [84df9525b0c27f3ebc2ebb1864fa62a97fdedb7d] Linux 4.19
>>>>>> git bisect start 'v4.20' 'v4.19'
>>>>>> # bad: [ec9c166434595382be3babf266febf876327774d] Merge tag 'mips_fixes_4.20_1' of git://git.kernel.org/pub/scm/linux/kernel/git/mips/linux
>>>>>> git bisect bad ec9c166434595382be3babf266febf876327774d
>>>>>> # bad: [50b825d7e87f4cff7070df6eb26390152bb29537] Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next
>>>>>> git bisect bad 50b825d7e87f4cff7070df6eb26390152bb29537
>>>>>> # good: [99e9acd85ccbdc8f5785f9e961d4956e96bd6aa5] Merge tag 'mlx5-updates-2018-10-17' of git://git.kernel.org/pub/scm/linux/kernel/git/saeed/linux
>>>>>> git bisect good 99e9acd85ccbdc8f5785f9e961d4956e96bd6aa5
>>>>>> # good: [c403993a41d50db1e7d9bc2d43c3c8498162312f] Merge tag 'for-linus-4.20' of https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fcminyard%2Flinux-ipmi&amp;data=02%7C01%7Cnamit%40vmware.com%7Ca1c3ea5d4bc34cfc785508d6bf388ff3%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636906647013777573&amp;sdata=3VSR3VdE5rxOitAdkqFNPpAnAtLgDmYLzJtoUrs5v9Y%3D&amp;reserved=0
>>>>>> git bisect good c403993a41d50db1e7d9bc2d43c3c8498162312f
>>>>>> # good: [c05f3642f4304dd081876e77a68555b6aba4483f] Merge branch 'perf-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
>>>>>> git bisect good c05f3642f4304dd081876e77a68555b6aba4483f
>>>>>> # bad: [44786880df196a4200c178945c4d41675faf9fb7] Merge branch 'parisc-4.20-1' of git://git.kernel.org/pub/scm/linux/kernel/git/deller/parisc-linux
>>>>>> git bisect bad 44786880df196a4200c178945c4d41675faf9fb7
>>>>>> # bad: [99792e0cea1ed733cdc8d0758677981e0cbebfed] Merge branch 'x86-mm-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
>>>>>> git bisect bad 99792e0cea1ed733cdc8d0758677981e0cbebfed
>>>>>> # good: [fec98069fb72fb656304a3e52265e0c2fc9adf87] Merge branch 'x86-cpu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
>>>>>> git bisect good fec98069fb72fb656304a3e52265e0c2fc9adf87
>>>>>> # bad: [a31acd3ee8f7dbc0370bdf4a4bfef7a8c13c7542] x86/mm: Page size aware flush_tlb_mm_range()
>>>>>> git bisect bad a31acd3ee8f7dbc0370bdf4a4bfef7a8c13c7542
>>>>>> # good: [a7295fd53c39ce781a9792c9dd2c8747bf274160] x86/mm/cpa: Use flush_tlb_kernel_range()
>>>>>> git bisect good a7295fd53c39ce781a9792c9dd2c8747bf274160
>>>>>> # good: [9cf38d5559e813cccdba8b44c82cc46ba48d0896] kexec: Allocate decrypted control pages for kdump if SME is enabled
>>>>>> git bisect good 9cf38d5559e813cccdba8b44c82cc46ba48d0896
>>>>>> # good: [5b12904065798fee8b153a506ac7b72d5ebbe26c] x86/mm/doc: Clean up the x86-64 virtual memory layout descriptions
>>>>>> git bisect good 5b12904065798fee8b153a506ac7b72d5ebbe26c
>>>>>> # good: [cf089611f4c446285046fcd426d90c18f37d2905] proc/vmcore: Fix i386 build error of missing copy_oldmem_page_encrypted()
>>>>>> git bisect good cf089611f4c446285046fcd426d90c18f37d2905
>>>>>> # good: [a5b966ae42a70b194b03eaa5eaea70d8b3790c40] Merge branch 'tlb/asm-generic' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux into x86/mm
>>>>>> git bisect good a5b966ae42a70b194b03eaa5eaea70d8b3790c40
>>>>>> # first bad commit: [a31acd3ee8f7dbc0370bdf4a4bfef7a8c13c7542] x86/mm: Page size aware flush_tlb_mm_range()
>>>>>> 
>>>>>> And 'funnily' the bad patch is one of mine too :/
>>>>>> 
>>>>>> I'll go have a look at that tomorrow, because currrently I'm way past
>>>>>> tired.
>>>>> 
>>>>> OK, so the below patchlet makes it all good. It turns out that the
>>>>> provided config has:
>>>>> 
>>>>> CONFIG_X86_L1_CACHE_SHIFT=7
>>>>> 
>>>>> which then, for some obscure raisin, results in flush_tlb_mm_range()
>>>>> compiling to use 320 bytes of stack:
>>>>> 
>>>>> sub    $0x140,%rsp
>>>>> 
>>>>> Where a 'defconfig' build results in:
>>>>> 
>>>>> sub    $0x58,%rsp
>>>>> 
>>>>> The thing that pushes it over the edge in the above fingered patch is
>>>>> the addition of a field to struct flush_tlb_info, which grows if from 32
>>>>> to 36 bytes.
>>>>> 
>>>>> So my proposal is to basically revert that, unless we can come up with
>>>>> something that GCC can't screw up.
>>>> 
>>>> To clarify, 'that' is Nadav's patch:
>>>> 
>>>> 515ab7c41306 ("x86/mm: Align TLB invalidation info")
>>>> 
>>>> which turns out to be the real problem.
>>> 
>>> Sorry for that. I still think it should be aligned, especially with all the
>>> effort the Intel puts around to avoid bus-locking on unaligned atomic
>>> operations.
>>> 
>>> So the right solution seems to me as putting this data structure off stack.
>>> It would prevent flush_tlb_mm_range() from being reentrant, so we can keep a
>>> few entries for this matter and atomically increase the entry number every
>>> time we enter flush_tlb_mm_range().
>>> 
>>> But my question is - should flush_tlb_mm_range() be reentrant, or can we
>>> assume no TLB shootdowns are initiated in interrupt handlers and #MC
>>> handlers?
>> 
>> Peter, what do you say about this one? I assume there are no nested TLB
>> flushes, but the code can easily be adapted (assuming there is a limit on
>> the nesting level).
> 
> You need IRQs on to flush, right?  So as long as preemption is off, it won’t nest.

Yes. I figured, but it still had all kind of theoretical scenarios in my mind
(IRQs are conditionally enabled in #MC handler, etc.)

> But is there really any measurable performance benefit to aligning it like
> this? There shouldn’t actually be any atomically — it’s just a little data
> structure telling everyone what to do.

At the time I measured (I hacked the code to force misalignment), and it was
marginal (i.e., hard to say).

Having said that, it seems to me as a more scalable design choice. From a
brief look, the vast majority of on_each_cpu() arguments are off the stack.

Correct me if I am wrong, but keeping them off the stack should help, not
only by preventing unalignment, but also by preventing some TLB misses:
right now tlb_flush_info instances of different threads are in different
virtual addresses (and because the kernel stack is vmalloc’d, kernel stack
virtual-address mappings do not share the same TLB-entry).

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