Re: [PATCH v2] fs/proc/page: Skip uninitialized page when iterating page structures

[Waiman Long]

On 8/28/19 4:00 AM, Michal Hocko wrote:
> On Tue 27-08-19 16:22:38, Michal Hocko wrote:
>> Dan, isn't this something we have discussed recently?
> This was http://lkml.kernel.org/r/20190725023100.31141-3-t-fukasawa@xxxxxxxxxxxxx
> and talked about /proc/kpageflags but this is essentially the same thing
> AFAIU. I hope we get a consistent solution for both issues.
>
Yes, it is the same problem. The uninitialized page structure problem
affects all the 3 /proc/kpage{cgroup,count,flags) files.

Toshiki's patch seems to fix it just for /proc/kpageflags, though.

-Longman

>> On Mon 26-08-19 08:43:36, Waiman Long wrote:
>>> It was found that on a dual-socket x86-64 system with nvdimm, reading
>>> /proc/kpagecount may cause the system to panic:
>>>
>>> ===================
>>> [   79.917682] BUG: unable to handle page fault for address: fffffffffffffffe
>>> [   79.924558] #PF: supervisor read access in kernel mode
>>> [   79.929696] #PF: error_code(0x0000) - not-present page
>>> [   79.934834] PGD 87b60d067 P4D 87b60d067 PUD 87b60f067 PMD 0
>>> [   79.940494] Oops: 0000 [#1] SMP NOPTI
>>> [   79.944157] CPU: 89 PID: 3455 Comm: cp Not tainted 5.3.0-rc5-test+ #14
>>> [   79.950682] Hardware name: Dell Inc. PowerEdge R740/07X9K0, BIOS 2.2.11 06/13/2019
>>> [   79.958246] RIP: 0010:kpagecount_read+0xdb/0x1a0
>>> [   79.962859] Code: e8 09 83 e0 3f 48 0f a3 02 73 2d 4c 89 f7 48 c1 e7 06 48 03 3d fe da de 00 74 1d 48 8b 57 08 48 8d 42 ff 83 e2 01 48 0f 44 c7 <48> 8b 00 f6 c4 02 75 06 83 7f 30 80 7d 62 31 c0 4c 89 f9 e8 5d c9
>>> [   79.981603] RSP: 0018:ffffb0d9c950fe70 EFLAGS: 00010202
>>> [   79.986830] RAX: fffffffffffffffe RBX: ffff8beebe5383c0 RCX: ffffb0d9c950ff00
>>> [   79.993963] RDX: 0000000000000001 RSI: 00007fd85b29e000 RDI: ffffe77a22000000
>>> [   80.001095] RBP: 0000000000020000 R08: 0000000000000001 R09: 0000000000000000
>>> [   80.008226] R10: 0000000000000000 R11: 0000000000000001 R12: 00007fd85b29e000
>>> [   80.015358] R13: ffffffff893f0480 R14: 0000000000880000 R15: 00007fd85b29e000
>>> [   80.022491] FS:  00007fd85b312800(0000) GS:ffff8c359fb00000(0000) knlGS:0000000000000000
>>> [   80.030576] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> [   80.036321] CR2: fffffffffffffffe CR3: 0000004f54a38001 CR4: 00000000007606e0
>>> [   80.043455] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>> [   80.050586] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>>> [   80.057718] PKRU: 55555554
>>> [   80.060428] Call Trace:
>>> [   80.062877]  proc_reg_read+0x39/0x60
>>> [   80.066459]  vfs_read+0x91/0x140
>>> [   80.069686]  ksys_read+0x59/0xd0
>>> [   80.072922]  do_syscall_64+0x59/0x1e0
>>> [   80.076588]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>> [   80.081637] RIP: 0033:0x7fd85a7f5d75
>>> ===================
>>>
>>> It turns out the panic was caused by the kpagecount_read() function
>>> hitting an uninitialized page structure at PFN 0x880000 where all its
>>> fields were set to -1. The compound_head value of -1 will mislead the
>>> kernel to treat -2 as a pointer to the head page of the compound page
>>> leading to the crash.
>>>
>>> The system have 12 GB of nvdimm ranging from PFN 0x880000-0xb7ffff.
>>> However, only PFN 0x88c200-0xb7ffff are released by the nvdimm
>>> driver to the kernel and initialized. IOW, PFN 0x880000-0x88c1ff
>>> remain uninitialized. Perhaps these 196 MB of nvdimm are reserved for
>>> internal use.
>>>
>>> To fix the panic, we need to find out if a page structure has been
>>> initialized. This is done now by checking if the PFN is in the range
>>> of a memory zone assuming that pages in a zone is either correctly
>>> marked as not present in the mem_section structure or have their page
>>> structures initialized.
>>>
>>> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
>>> ---
>>>  fs/proc/page.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++---
>>>  1 file changed, 65 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/fs/proc/page.c b/fs/proc/page.c
>>> index 544d1ee15aee..fee55ad95893 100644
>>> --- a/fs/proc/page.c
>>> +++ b/fs/proc/page.c
>>> @@ -21,6 +21,64 @@
>>>  #define KPMMASK (KPMSIZE - 1)
>>>  #define KPMBITS (KPMSIZE * BITS_PER_BYTE)
>>>  
>>> +/*
>>> + * It is possible a page structure is contained in a mem_section that is
>>> + * regarded as valid but the page structure itself is not properly
>>> + * initialized. For example, portion of the device memory may be used
>>> + * internally by device driver or firmware without being managed by the
>>> + * kernel and hence their page structures may not be initialized.
>>> + *
>>> + * An uninitialized page structure may cause the PFN iteration code
>>> + * in this file to panic the system. To safe-guard against this
>>> + * possibility, an additional check of the PFN is done to make sure
>>> + * that it is in a valid range in one of the memory zones:
>>> + *
>>> + *	[zone_start_pfn, zone_start_pfn + spanned_pages)
>>> + *
>>> + * It is possible that some of the PFNs within a zone is not present.
>>> + * In this case, it will have to rely on the current mem_section check
>>> + * as well as the affected page structures are still properly initialized.
>>> + */
>>> +struct zone_range {
>>> +	unsigned long pfn_start;
>>> +	unsigned long pfn_end;
>>> +};
>>> +
>>> +static void find_next_zone_range(struct zone_range *range)
>>> +{
>>> +	unsigned long start, end;
>>> +	pg_data_t *pgdat;
>>> +	struct zone *zone;
>>> +	int i;
>>> +
>>> +	/*
>>> +	 * Scan all the zone structures to find the next closest one.
>>> +	 */
>>> +	start = end = -1UL;
>>> +	for (pgdat = first_online_pgdat(); pgdat;
>>> +	     pgdat = next_online_pgdat(pgdat)) {
>>> +		for (zone = pgdat->node_zones, i = 0; i < MAX_NR_ZONES;
>>> +		     zone++, i++) {
>>> +			if (!zone->spanned_pages)
>>> +				continue;
>>> +			if ((zone->zone_start_pfn >= range->pfn_end) &&
>>> +			    (zone->zone_start_pfn < start)) {
>>> +				start = zone->zone_start_pfn;
>>> +				end   = start + zone->spanned_pages;
>>> +			}
>>> +		}
>>> +	}
>>> +	range->pfn_start = start;
>>> +	range->pfn_end   = end;
>>> +}
>>> +
>>> +static inline bool pfn_in_zone(unsigned long pfn, struct zone_range *range)
>>> +{
>>> +	if (pfn >= range->pfn_end)
>>> +		find_next_zone_range(range);
>>> +	return pfn >= range->pfn_start && pfn < range->pfn_end;
>>> +}
>>> +
>>>  /* /proc/kpagecount - an array exposing page counts
>>>   *
>>>   * Each entry is a u64 representing the corresponding
>>> @@ -31,6 +89,7 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
>>>  {
>>>  	u64 __user *out = (u64 __user *)buf;
>>>  	struct page *ppage;
>>> +	struct zone_range range = { 0, 0 };
>>>  	unsigned long src = *ppos;
>>>  	unsigned long pfn;
>>>  	ssize_t ret = 0;
>>> @@ -42,10 +101,11 @@ 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_in_zone(pfn, &range))
>>>  			ppage = pfn_to_page(pfn);
>>>  		else
>>>  			ppage = NULL;
>>> +
>>>  		if (!ppage || PageSlab(ppage) || page_has_type(ppage))
>>>  			pcount = 0;
>>>  		else
>>> @@ -206,6 +266,7 @@ static ssize_t kpageflags_read(struct file *file, char __user *buf,
>>>  {
>>>  	u64 __user *out = (u64 __user *)buf;
>>>  	struct page *ppage;
>>> +	struct zone_range range = { 0, 0 };
>>>  	unsigned long src = *ppos;
>>>  	unsigned long pfn;
>>>  	ssize_t ret = 0;
>>> @@ -216,7 +277,7 @@ 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_in_zone(pfn, &range))
>>>  			ppage = pfn_to_page(pfn);
>>>  		else
>>>  			ppage = NULL;
>>> @@ -250,6 +311,7 @@ static ssize_t kpagecgroup_read(struct file *file, char __user *buf,
>>>  {
>>>  	u64 __user *out = (u64 __user *)buf;
>>>  	struct page *ppage;
>>> +	struct zone_range range = { 0, 0 };
>>>  	unsigned long src = *ppos;
>>>  	unsigned long pfn;
>>>  	ssize_t ret = 0;
>>> @@ -261,7 +323,7 @@ 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_in_zone(pfn, &range))
>>>  			ppage = pfn_to_page(pfn);
>>>  		else
>>>  			ppage = NULL;
>>> -- 
>>> 2.18.1
>> -- 
>> Michal Hocko
>> SUSE Labs