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From: Steven Price <steven.price@arm.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"alex@ghiti.fr" <alex@ghiti.fr>,
	"aou@eecs.berkeley.edu" <aou@eecs.berkeley.edu>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Qian Cai <cai@lca.pw>, Catalin Marinas <Catalin.Marinas@arm.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"dave.jiang@intel.com" <dave.jiang@intel.com>,
	David Miller <davem@davemloft.net>,
	Dmitry Vyukov <dvyukov@google.com>,
	Alexander Potapenko <glider@google.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Peter Anvin <hpa@zytor.com>, James Morse <James.Morse@arm.com>,
	James Hogan <jhogan@kernel.org>,
	Kan Liang <kan.liang@linux.intel.com>,
	Linux-MM <linux-mm@kvack.org>,
	Russell King - ARM Linux <linux@armlinux.org.uk>,
	Andrew Lutomirski <luto@kernel.org>,
	Mark Rutland <Mark.Rutland@arm.com>,
	"mawilcox@microsoft.com" <mawilcox@microsoft.com>,
	Ingo Molnar <mingo@elte.hu>,
	"mm-commits@vger.kernel.org" <mm-commits@vger.kernel.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	"n-horiguchi@ah.jp.nec.com" <n-horiguchi@ah.jp.nec.com>,
	Palmer Dabbelt <palmer@sifive.com>,
	Paul Burton <paul.burton@mips.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Paul Mackerras <paulus@samba.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"ralf@linux-mips.org" <ralf@linux-mips.org>,
	"shashim@codeaurora.org" <shashim@codeaurora.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"vgupta@synopsys.com" <vgupta@synopsys.com>,
	Will Deacon <will@kernel.org>,
	"zong.li@sifive.com" <zong.li@sifive.com>
Subject: Re: [patch 064/158] mm: add generic ptdump
Date: Mon, 2 Dec 2019 09:09:24 +0000
Message-ID: <20191202090924.GA46592@arm.com> (raw)
In-Reply-To: <20191201154553.GE6629@zn.tnic>

On Sun, Dec 01, 2019 at 03:45:54PM +0000, Borislav Petkov wrote:
> On Sun, Dec 01, 2019 at 04:21:19PM +0100, Borislav Petkov wrote:
> > On Sun, Dec 01, 2019 at 04:10:11PM +0100, Borislav Petkov wrote:
> > > So lemme first confirm it really is caused by those patches.
> > 
> > Yeah, those patches are causing it. Tried your current master - it is OK
> > - and then applied Andrew's patches I was CCed on, ontop, and I got in a
> > VM:
> > 
> > VFS: Mounted root (ext4 filesystem) readonly on device 8:2.
> > devtmpfs: mounted
> > Freeing unused kernel image (initmem) memory: 664K
> > Write protecting kernel text and read-only data: 18164k
> > NX-protecting the kernel data: 7416k
> > BUG: kernel NULL pointer dereference, address: 00000014
> > #PF: supervisor read access in kernel mode
> > #PF: error_code(0x0000) - not-present page
> > *pdpt = 0000000000000000 *pde = f000ff53f000ff53 
> > Oops: 0000 [#1] PREEMPT SMP PTI
> > CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.4.0+ #3
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.1-1 04/01/2014
> > EIP: __lock_acquire.isra.0+0x2e8/0x4e0
> > Code: e8 bd a1 2f 00 85 c0 74 11 8b 1d 08 8f 26 c5 85 db 0f 84 05 1a 00 00 8d 76 00 31 db 8d 65 f4 89 d8 5b 5e 5f 5d c3 8d 74 26 00 <8b> 44 90 04 85 c0 0f 85 4c fd ff ff e9 33 fd ff ff 8d b4 26 00 00
> > EAX: 00000010 EBX: 00000010 ECX: 00000001 EDX: 00000000
> > ESI: f1070040 EDI: f1070040 EBP: f1073e04 ESP: f1073de0
> > DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068 EFLAGS: 00010097
> > CR0: 80050033 CR2: 00000014 CR3: 05348000 CR4: 001406b0
> > Call Trace:
> >  lock_acquire+0x42/0x60
> >  ? __walk_page_range+0x4d9/0x590
> >  _raw_spin_lock+0x22/0x40
> >  ? __walk_page_range+0x4d9/0x590
> >  __walk_page_range+0x4d9/0x590
> 

Thanks for looking into this. I've been able to reproduce it locally
with that config and I can see what's going wrong here.

walk_pte_range() is being called with end=0xffffffff, but the comparison
in the function is:

	if (addr == end)
		break;

So addr never actually equals end, it skips from 0xfffff000 to 0x0. This
means the function continues walking straight off the end and
dereferencing 'random' ptes. As a quick hack I modified the condition
to:

	if (addr == end || !addr)
		break;

and I can then boot the VM. Clearly that's not the correct solution -
I'll go away and have a think about the cleanest way of handling this
case and also do some more testing before I resubmit for 5.6.

Sorry for the trouble and thanks again for investigating.

Steve



  reply index

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-01  1:53 akpm
2019-12-01  9:07 ` Borislav Petkov
2019-12-01 14:45   ` Linus Torvalds
2019-12-01 15:10     ` Borislav Petkov
2019-12-01 15:21       ` Borislav Petkov
2019-12-01 15:45         ` Borislav Petkov
2019-12-02  9:09           ` Steven Price [this message]
2019-12-02 15:42             ` Borislav Petkov
2019-12-03 10:47 ` David Hildenbrand
2019-12-03 11:00   ` David Hildenbrand

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