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

[David Hildenbrand]

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.


-- 

Thanks,

David / dhildenb