Re: [PATCH v1] mm: Fix access of uninitialized memmaps in fs/proc/page.c

[David Hildenbrand]

On 11.10.19 02:50, Naoya Horiguchi wrote:
> On Fri, Oct 11, 2019 at 12:11:25AM +0000, Horiguchi Naoya(堀口 直也) wrote:
> > On Thu, Oct 10, 2019 at 09:30:01AM +0200, David Hildenbrand wrote:
> > > On 09.10.19 11:57, Naoya Horiguchi wrote:
> > > > Hi David,
> > > >
> > > > On Wed, Oct 09, 2019 at 11:12:04AM +0200, David Hildenbrand wrote:
> > > > > There are various places where we access uninitialized memmaps, namely:
> > > > > - /proc/kpagecount
> > > > > - /proc/kpageflags
> > > > > - /proc/kpagecgroup
> > > > > - memory_failure() - which reuses stable_page_flags() from fs/proc/page.c
> > > >
> > > > Ah right, memory_failure is another victim of this bug.
> > > >
> > > > > We have initialized memmaps either when the section is online or when
> > > > > the page was initialized to the ZONE_DEVICE. Uninitialized memmaps contain
> > > > > garbage and in the worst case trigger kernel BUGs, especially with
> > > > > CONFIG_PAGE_POISONING.
> > > > >
> > > > > For example, not onlining a DIMM during boot and calling /proc/kpagecount
> > > > > with CONFIG_PAGE_POISONING:
> > > > > :/# cat /proc/kpagecount > tmp.test
> > > > > [   95.600592] BUG: unable to handle page fault for address: fffffffffffffffe
> > > > > [   95.601238] #PF: supervisor read access in kernel mode
> > > > > [   95.601675] #PF: error_code(0x0000) - not-present page
> > > > > [   95.602116] PGD 114616067 P4D 114616067 PUD 114618067 PMD 0
> > > > > [   95.602596] Oops: 0000 [#1] SMP NOPTI
> > > > > [   95.602920] CPU: 0 PID: 469 Comm: cat Not tainted 5.4.0-rc1-next-20191004+ #11
> > > > > [   95.603547] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.4
> > > > > [   95.604521] RIP: 0010:kpagecount_read+0xce/0x1e0
> > > > > [   95.604917] Code: e8 09 83 e0 3f 48 0f a3 02 73 2d 4c 89 e7 48 c1 e7 06 48 03 3d ab 51 01 01 74 1d 48 8b 57 08 480
> > > > > [   95.606450] RSP: 0018:ffffa14e409b7e78 EFLAGS: 00010202
> > > > > [   95.606904] RAX: fffffffffffffffe RBX: 0000000000020000 RCX: 0000000000000000
> > > > > [   95.607519] RDX: 0000000000000001 RSI: 00007f76b5595000 RDI: fffff35645000000
> > > > > [   95.608128] RBP: 00007f76b5595000 R08: 0000000000000001 R09: 0000000000000000
> > > > > [   95.608731] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000140000
> > > > > [   95.609327] R13: 0000000000020000 R14: 00007f76b5595000 R15: ffffa14e409b7f08
> > > > > [   95.609924] FS:  00007f76b577d580(0000) GS:ffff8f41bd400000(0000) knlGS:0000000000000000
> > > > > [   95.610599] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > > [   95.611083] CR2: fffffffffffffffe CR3: 0000000078960000 CR4: 00000000000006f0
> > > > > [   95.611686] Call Trace:
> > > > > [   95.611906]  proc_reg_read+0x3c/0x60
> > > > > [   95.612228]  vfs_read+0xc5/0x180
> > > > > [   95.612505]  ksys_read+0x68/0xe0
> > > > > [   95.612785]  do_syscall_64+0x5c/0xa0
> > > > > [   95.613092]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > > > >
> > > > > Note that there are still two possible races as far as I can see:
> > > > > - pfn_to_online_page() succeeding but the memory getting offlined and
> > > > >    removed. get_online_mems() could help once we run into this.
> > > > > - pfn_zone_device() succeeding but the memmap not being fully
> > > > >    initialized yet. As the memmap is initialized outside of the memory
> > > > >    hoptlug lock, get_online_mems() can't help.
> > > > >
> > > > > Let's keep the existing interfaces working with ZONE_DEVICE memory. We
> > > > > can later come back and fix these rare races and eventually speed-up the
> > > > > ZONE_DEVICE detection.
> > > >
> > > > Actually, Toshiki is writing code to refactor and optimize the pfn walking
> > > > part, where we find the pfn ranges covered by zone devices by running over
> > > > xarray pgmap_array and use the range info to reduce pointer dereferences
> > > > to speed up pfn walk. I hope he will share it soon.
> > >
> > > AFAIKT, Michal is not a friend of special-casing PFN walkers in that
> > > way. We should have a mechanism to detect if a memmap was initialized
> > > without having to go via pgmap, special-casing. See my other mail where
> > > I draft one basic approach.
> >
> > OK, so considering your v2 approach, we could have another pfn_to_page()
> > variant like pfn_to_zone_device_page(), where we check that a given pfn
> > belongs to the memory section backed by zone memory, then another check if
> > the pfn has initialized memmap or not, and return NULL if memmap not
> > initialied.  We'll try this approach then, but if you find problems/concerns,
> > please let me know.
>
> Sorry, you already mentioned detail here,
> https://lore.kernel.org/lkml/c6198acd-8ff7-c40c-cb4e-f0f12f841b38@xxxxxxxxxx/

I'm planning on sending a proper writeup of the overall approach and  
pitfalls maybe next week. Feel free to ping me in case I forget.

--

Thanks,

David / dhildenb