Re: [PATCH 2/2] /proc/kpageflags: do not use uninitialized struct pages

From: Michal Hocko <mhocko@kernel.org>
To: Toshiki Fukasawa <t-fukasawa@vx.jp.nec.com>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"dan.j.williams@intel.com" <dan.j.williams@intel.com>,
	"adobriyan@gmail.com" <adobriyan@gmail.com>,
	"hch@lst.de" <hch@lst.de>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	Junichi Nomura <j-nomura@ce.jp.nec.com>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: [PATCH 2/2] /proc/kpageflags: do not use uninitialized struct pages
Date: Fri, 26 Jul 2019 09:06:15 +0200	[thread overview]
Message-ID: <20190726070615.GB6142@dhcp22.suse.cz> (raw)
In-Reply-To: <40b3078e-fb8b-87ef-5c4e-6321956cc940@vx.jp.nec.com>

On Fri 26-07-19 06:25:49, Toshiki Fukasawa wrote:
> 
> 
> On 2019/07/25 18:03, Michal Hocko wrote:
> > On Thu 25-07-19 02:31:18, Toshiki Fukasawa wrote:
> >> A kernel panic was observed during reading /proc/kpageflags for
> >> first few pfns allocated by pmem namespace:
> >>
> >> BUG: unable to handle page fault for address: fffffffffffffffe
> >> [  114.495280] #PF: supervisor read access in kernel mode
> >> [  114.495738] #PF: error_code(0x0000) - not-present page
> >> [  114.496203] PGD 17120e067 P4D 17120e067 PUD 171210067 PMD 0
> >> [  114.496713] Oops: 0000 [#1] SMP PTI
> >> [  114.497037] CPU: 9 PID: 1202 Comm: page-types Not tainted 5.3.0-rc1 #1
> >> [  114.497621] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
> >> [  114.498706] RIP: 0010:stable_page_flags+0x27/0x3f0
> >> [  114.499142] Code: 82 66 90 66 66 66 66 90 48 85 ff 0f 84 d1 03 00 00 41 54 55 48 89 fd 53 48 8b 57 08 48 8b 1f 48 8d 42 ff 83 e2 01 48 0f 44 c7 <48> 8b 00 f6 c4 02 0f 84 57 03 00 00 45 31 e4 48 8b 55 08 48 89 ef
> >> [  114.500788] RSP: 0018:ffffa5e601a0fe60 EFLAGS: 00010202
> >> [  114.501373] RAX: fffffffffffffffe RBX: ffffffffffffffff RCX: 0000000000000000
> >> [  114.502009] RDX: 0000000000000001 RSI: 00007ffca13a7310 RDI: ffffd07489000000
> >> [  114.502637] RBP: ffffd07489000000 R08: 0000000000000001 R09: 0000000000000000
> >> [  114.503270] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000240000
> >> [  114.503896] R13: 0000000000080000 R14: 00007ffca13a7310 R15: ffffa5e601a0ff08
> >> [  114.504530] FS:  00007f0266c7f540(0000) GS:ffff962dbbac0000(0000) knlGS:0000000000000000
> >> [  114.505245] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >> [  114.505754] CR2: fffffffffffffffe CR3: 000000023a204000 CR4: 00000000000006e0
> >> [  114.506401] Call Trace:
> >> [  114.506660]  kpageflags_read+0xb1/0x130
> >> [  114.507051]  proc_reg_read+0x39/0x60
> >> [  114.507387]  vfs_read+0x8a/0x140
> >> [  114.507686]  ksys_pread64+0x61/0xa0
> >> [  114.508021]  do_syscall_64+0x5f/0x1a0
> >> [  114.508372]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> >> [  114.508844] RIP: 0033:0x7f0266ba426b
> >>
> >> The reason for the panic is that stable_page_flags() which parses
> >> the page flags uses uninitialized struct pages reserved by the
> >> ZONE_DEVICE driver.
> > 
> > Why pmem hasn't initialized struct pages? 
> 
> We proposed to initialize in previous approach but that wasn't merged.
> (See https://marc.info/?l=linux-mm&m=152964792500739&w=2)
> 
> > Isn't that a bug that should be addressed rather than paper over it like this?
> 
> I'm not sure. What do you think, Dan?

Yeah, I am really curious about details. Why do we keep uninitialized
struct pages at all? What is a random pfn walker supposed to do? What
kind of metadata would be clobbered? In other words much more details
please.
-- 
Michal Hocko
SUSE Labs