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SubjectRe: [PATCH v1] mm: Fix access of uninitialized memmaps in fs/proc/page.c
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On 09.10.19 11:37, Michal Hocko wrote:
> On Wed 09-10-19 11:12:04, 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
>>
>> 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. This patch now also makes sure we don't dump data
>> about memory blocks that are already offline again.
>
> As I've already expressed already I am not really happy to have explicit
> checks for zone device pages in pfn walkers. This is simply too fragile.
>
> pfn_to_online_page makes sense because offline pages are not really in a
> defined state. This would be worth a patch of its own. I remember there

The issue is, once I check for pfn_to_online_page(), these functions
can't handle ZONE_DEVICE at all anymore. Especially in regards to
memory_failure() I don't think this is acceptable. So while I
(personally) only care about adding pfn_to_online_page() checks, the
in-this-sense-fragile-subsection ZONE_DEVICE implementation requires me
to introduce a temporary check for initialized memmaps.

> was a discussion about the uninitialized zone device memmaps. It would
> be really good to summarize this discussion in the changelog and
> conclude why the explicit check is really good and what were other
> alternatives considered.

Yeah, I also expressed my feelings and the issues to be solved by
ZONE_DEVICE people in https://lkml.org/lkml/2019/9/6/114. However, the
discussion stalled and nobody really proposed a solution or followed up.

I want to have this fixed now, as it is much easier to trigger with
"[PATCH v6 00/10] mm/memory_hotplug: Shrink zones before removing
memory" for !ZONE_DEVICE.

As we already do have a similar check (pfn_zone_device_reserved()) that
I merely convert for now, I will summarize and make it clearer that this
is something that should be reworked by ZONE_DEVICE people rather sooner
than later.

Thanks!

>
> Thanks!
>
>> Reported-by: Qian Cai <cai@lca.pw>
>> Cc: Alexey Dobriyan <adobriyan@gmail.com>
>> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Stephen Rothwell <sfr@canb.auug.org.au>
>> Cc: Michal Hocko <mhocko@kernel.org>
>> Cc: Toshiki Fukasawa <t-fukasawa@vx.jp.nec.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Konstantin Khlebnikov <koct9i@gmail.com>
>> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
>> Cc: Anthony Yznaga <anthony.yznaga@oracle.com>
>> Cc: Jason Gunthorpe <jgg@ziepe.ca>
>> Cc: Dan Williams <dan.j.williams@intel.com>
>> Cc: Logan Gunthorpe <logang@deltatee.com>
>> Cc: Ira Weiny <ira.weiny@intel.com>
>> Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
>> Cc: linux-fsdevel@vger.kernel.org
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>> fs/proc/page.c | 12 ++++++------
>> include/linux/memremap.h | 11 +++++++++--
>> mm/memory-failure.c | 22 ++++++++++++++++------
>> mm/memremap.c | 19 +++++++++++--------
>> 4 files changed, 42 insertions(+), 22 deletions(-)
>>
>> diff --git a/fs/proc/page.c b/fs/proc/page.c
>> index decd3fe39674..76502af461e2 100644
>> --- a/fs/proc/page.c
>> +++ b/fs/proc/page.c
>> @@ -42,7 +42,8 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
>> return -EINVAL;
>>
>> while (count > 0) {
>> - if (pfn_valid(pfn))
>> + if (pfn_valid(pfn) &&
>> + (pfn_to_online_page(pfn) || pfn_zone_device(pfn)))
>> ppage = pfn_to_page(pfn);
>> else
>> ppage = NULL;
>> @@ -97,9 +98,6 @@ u64 stable_page_flags(struct page *page)
>> if (!page)
>> return BIT_ULL(KPF_NOPAGE);
>>
>> - if (pfn_zone_device_reserved(page_to_pfn(page)))
>> - return BIT_ULL(KPF_RESERVED);
>> -
>> k = page->flags;
>> u = 0;
>>
>> @@ -218,7 +216,8 @@ static ssize_t kpageflags_read(struct file *file, char __user *buf,
>> return -EINVAL;
>>
>> while (count > 0) {
>> - if (pfn_valid(pfn))
>> + if (pfn_valid(pfn) &&
>> + (pfn_to_online_page(pfn) || pfn_zone_device(pfn)))
>> ppage = pfn_to_page(pfn);
>> else
>> ppage = NULL;
>> @@ -263,7 +262,8 @@ static ssize_t kpagecgroup_read(struct file *file, char __user *buf,
>> return -EINVAL;
>>
>> while (count > 0) {
>> - if (pfn_valid(pfn))
>> + if (pfn_valid(pfn) &&
>> + (pfn_to_online_page(pfn) || pfn_zone_device(pfn)))
>> ppage = pfn_to_page(pfn);
>> else
>> ppage = NULL;
>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
>> index c676e33205d3..c076bb163c2f 100644
>> --- a/include/linux/memremap.h
>> +++ b/include/linux/memremap.h
>> @@ -123,7 +123,8 @@ static inline struct vmem_altmap *pgmap_altmap(struct dev_pagemap *pgmap)
>> }
>>
>> #ifdef CONFIG_ZONE_DEVICE
>> -bool pfn_zone_device_reserved(unsigned long pfn);
>> +bool pfn_zone_device(unsigned long pfn);
>> +bool __pfn_zone_device(unsigned long pfn, struct dev_pagemap *pgmap);
>> void *memremap_pages(struct dev_pagemap *pgmap, int nid);
>> void memunmap_pages(struct dev_pagemap *pgmap);
>> void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
>> @@ -134,7 +135,13 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
>> unsigned long vmem_altmap_offset(struct vmem_altmap *altmap);
>> void vmem_altmap_free(struct vmem_altmap *altmap, unsigned long nr_pfns);
>> #else
>> -static inline bool pfn_zone_device_reserved(unsigned long pfn)
>> +static inline bool pfn_zone_device(unsigned long pfn)
>> +{
>> + return false;
>> +}
>> +
>> +static inline bool __pfn_zone_device(unsigned long pfn,
>> + struct dev_pagemap *pgmap)
>> {
>> return false;
>> }
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index 7ef849da8278..2b4cc6b67720 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -1161,6 +1161,14 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
>> loff_t start;
>> dax_entry_t cookie;
>>
>> + /* memmaps of driver reserved memory is not initialized */
>> + if (!__pfn_zone_device(pfn, pgmap)) {
>> + pr_err("Memory failure: %#lx: driver reserved memory\n",
>> + pfn);
>> + rc = -ENXIO;
>> + goto out;
>> + }
>> +
>> /*
>> * Prevent the inode from being freed while we are interrogating
>> * the address_space, typically this would be handled by
>> @@ -1253,17 +1261,19 @@ int memory_failure(unsigned long pfn, int flags)
>> if (!sysctl_memory_failure_recovery)
>> panic("Memory failure on page %lx", pfn);
>>
>> - if (!pfn_valid(pfn)) {
>> + p = pfn_to_online_page(pfn);
>> + if (!p) {
>> + if (pfn_valid(pfn)) {
>> + pgmap = get_dev_pagemap(pfn, NULL);
>> + if (pgmap)
>> + return memory_failure_dev_pagemap(pfn, flags,
>> + pgmap);
>> + }
>> pr_err("Memory failure: %#lx: memory outside kernel control\n",
>> pfn);
>> return -ENXIO;
>> }
>>
>> - pgmap = get_dev_pagemap(pfn, NULL);
>> - if (pgmap)
>> - return memory_failure_dev_pagemap(pfn, flags, pgmap);
>> -
>> - p = pfn_to_page(pfn);
>> if (PageHuge(p))
>> return memory_failure_hugetlb(pfn, flags);
>> if (TestSetPageHWPoison(p)) {
>> diff --git a/mm/memremap.c b/mm/memremap.c
>> index 7fed8bd32a18..9f3bb223aec7 100644
>> --- a/mm/memremap.c
>> +++ b/mm/memremap.c
>> @@ -73,21 +73,24 @@ static unsigned long pfn_next(unsigned long pfn)
>> return pfn + 1;
>> }
>>
>> +bool __pfn_zone_device(unsigned long pfn, struct dev_pagemap *pgmap)
>> +{
>> + return pfn >= pfn_first(pgmap) && pfn <= pfn_end(pgmap);
>> +}
>> +
>> /*
>> - * This returns true if the page is reserved by ZONE_DEVICE driver.
>> + * Returns true if the page was initialized to the ZONE_DEVICE (especially,
>> + * is not reserved for driver usage).
>> */
>> -bool pfn_zone_device_reserved(unsigned long pfn)
>> +bool pfn_zone_device(unsigned long pfn)
>> {
>> struct dev_pagemap *pgmap;
>> - struct vmem_altmap *altmap;
>> - bool ret = false;
>> + bool ret;
>>
>> pgmap = get_dev_pagemap(pfn, NULL);
>> if (!pgmap)
>> - return ret;
>> - altmap = pgmap_altmap(pgmap);
>> - if (altmap && pfn < (altmap->base_pfn + altmap->reserve))
>> - ret = true;
>> + return false;
>> + ret = __pfn_zone_device(pfn, pgmap);
>> put_dev_pagemap(pgmap);
>>
>> return ret;
>> --
>> 2.21.0
>


--

Thanks,

David / dhildenb

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