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

[Michal Hocko]

Dan, isn't this something we have discussed recently?

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