linux-kernel - Re: [lkp-robot] [x86/cpu_entry_area] 10043e02db: kernel_BUG_at

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Date:   Wed, 27 Dec 2017 19:12:47 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     kernel test robot <xiaolong.ye@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        LKML <linux-kernel@...r.kernel.org>, tipbuild@...or.com,
        LKP <lkp@...org>, Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [lkp-robot] [x86/cpu_entry_area] 10043e02db: kernel_BUG_at_arch/x86/mm/physaddr.c

On Wed, Dec 27, 2017 at 7:05 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 26 Dec 2017, kernel test robot wrote:
>
>>
>> FYI, we noticed the following commit (built with gcc-6):
>>
>> commit: 10043e02db7f8a4161f76434931051e7d797a5f6 ("x86/cpu_entry_area: Add debugstore entries to cpu_entry_area")
>> https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git WIP.x86/pti
>
> ...
>
>> [    0.000000] kernel BUG at arch/x86/mm/physaddr.c:27!
>> PANIC: early exception 0x06 IP 10:ffffffff8115586f error 0 cr2 0xffff88000e468000
>> [    0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.14.0-00160-g10043e02 #1
>> [    0.000000] task: ffffffff8a4683c0 task.stack: ffffffff8a400000
>> [    0.000000] RIP: 0010:__phys_addr+0x268/0x276
>> [    0.000000] RSP: 0000:ffffffff8a407bd8 EFLAGS: 00010002 ORIG_RAX: 0000000000000000
>> [    0.000000] RAX: 0000000000000000 RBX: 0000780000000000 RCX: 1ffffffff17a9a01
>> [    0.000000] RDX: dffffc0000000000 RSI: 0000000000000003 RDI: ffffffff8bd4d340
>> [    0.000000] RBP: ffffffff8a407bf8 R08: 0000000000000001 R09: ffffffff8a407a48
>> [    0.000000] R10: ffff880000010000 R11: ffff880000010fff R12: 0000000000000001
>> [    0.000000] R13: 0000000000000001 R14: 0000000000000000 R15: fffffbd00c401000
>> [    0.000000] FS:  0000000000000000(0000) GS:ffffffff8cb4d000(0000) knlGS:0000000000000000
>> [    0.000000] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [    0.000000] CR2: ffff88000e468000 CR3: 000000000cde8000 CR4: 00000000000406b0
>> [    0.000000] Call Trace:
>> [    0.000000]  kasan_populate_shadow+0x3f2/0x497
>
> So this dies simply because kasan_populate_shadow() runs out of memory and
> has no sanity check whatsoever.
>
> static __init void *early_alloc(size_t size, int nid)
> {
>         return memblock_virt_alloc_try_nid_nopanic(size, size,
>                 __pa(MAX_DMA_ADDRESS), BOOTMEM_ALLOC_ACCESSIBLE, nid);
> }
>
> kasan_populate_pmd()
> {
>         .....
>
>                 p = early_alloc(PAGE_SIZE, nid);
>                 entry = pfn_pte(PFN_DOWN(__pa(p)), PAGE_KERNEL);
>
> I've instrumented the whole thing and early_alloc() returns NULL at some
> point and then __pa(NULL) dies in the VIRTUAL_DEBUG code. Well, it would
> die with VIRTUAL_DEBUG=n as well at some other place.
>
> Not really a problem caused by the patch above, it's merily exposing a code
> path which relies blindly on "enough memory available" assumptions.
>
> Throwing more memory at the VM makes the problem go away...

Hi Thomas,

We just need a check inside of early_alloc() to properly diagnose such
situation, right?
KASAN allocates 1/8 of physical memory, so the code has _some_ logic
behind it. But I guess it runs into a corner case where 1/8 is somehow
larger than 1 :)