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

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

[Waiman Long]

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@redhat.com>
---
 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..ef697eb42065 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->start && pfn < range->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

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

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 3064 bytes --]

Hi Waiman,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on linus/master]
[cannot apply to v5.3-rc6 next-20190823]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Waiman-Long/fs-proc-page-Skip-uninitialized-page-when-iterating-page-structures/20190826-105836
config: x86_64-lkp (attached as .config)
compiler: gcc-7 (Debian 7.4.0-10) 7.4.0
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   fs/proc/page.c: In function 'find_next_zone_range':
>> fs/proc/page.c:58:2: error: expected ';' before 'for'
     for (pgdat = first_online_pgdat(); pgdat;
     ^~~
   fs/proc/page.c:52:6: warning: unused variable 'i' [-Wunused-variable]
     int i;
         ^
   fs/proc/page.c:51:15: warning: unused variable 'zone' [-Wunused-variable]
     struct zone *zone;
                  ^~~~
   fs/proc/page.c:50:13: warning: unused variable 'pgdat' [-Wunused-variable]
     pg_data_t *pgdat;
                ^~~~~
   fs/proc/page.c: In function 'pfn_in_zone':
>> fs/proc/page.c:79:23: error: 'struct zone_range' has no member named 'start'; did you mean 'pfn_start'?
     return pfn >= range->start && pfn < range->end;
                          ^~~~~
                          pfn_start
>> fs/proc/page.c:79:43: error: 'struct zone_range' has no member named 'end'
     return pfn >= range->start && pfn < range->end;
                                              ^~
   fs/proc/page.c:80:1: warning: control reaches end of non-void function [-Wreturn-type]
    }
    ^

vim +58 fs/proc/page.c

    46	
    47	static void find_next_zone_range(struct zone_range *range)
    48	{
    49		unsigned long start, end;
  > 50		pg_data_t *pgdat;
  > 51		struct zone *zone;
    52		int i;
    53	
    54		/*
    55		 * Scan all the zone structures to find the next closest one.
    56		 */
    57		start = end = -1UL
  > 58		for (pgdat = first_online_pgdat(); pgdat;
    59		     pgdat = next_online_pgdat(pgdat)) {
    60			for (zone = pgdat->node_zones, i = 0; i < MAX_NR_ZONES;
    61			     zone++, i++) {
    62				if (!zone->spanned_pages)
    63					continue;
    64				if ((zone->zone_start_pfn >= range->pfn_end) &&
    65				    (zone->zone_start_pfn < start)) {
    66					start = zone->zone_start_pfn;
    67					end   = start + zone->spanned_pages;
    68				}
    69			}
    70		}
    71		range->pfn_start = start;
    72		range->pfn_end   = end;
    73	}
    74	
    75	static inline bool pfn_in_zone(unsigned long pfn, struct zone_range *range)
    76	{
    77		if (pfn >= range->pfn_end)
    78			find_next_zone_range(range);
  > 79		return pfn >= range->start && pfn < range->end;
    80	}
    81	

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

[-- Attachment #2: .config.gz --]
[-- Type: application/gzip, Size: 27705 bytes --]

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

[Waiman Long]

On 8/26/19 12:08 AM, kbuild test robot wrote:
> Hi Waiman,
>
> Thank you for the patch! Yet something to improve:
>
> [auto build test ERROR on linus/master]
> [cannot apply to v5.3-rc6 next-20190823]
> [if your patch is applied to the wrong git tree, please drop us a note to help improve the system]
>
> url:    https://github.com/0day-ci/linux/commits/Waiman-Long/fs-proc-page-Skip-uninitialized-page-when-iterating-page-structures/20190826-105836
> config: x86_64-lkp (attached as .config)
> compiler: gcc-7 (Debian 7.4.0-10) 7.4.0
> reproduce:
>         # save the attached .config to linux build tree
>         make ARCH=x86_64 
>
> If you fix the issue, kindly add following tag
> Reported-by: kbuild test robot <lkp@intel.com>
>
> All errors (new ones prefixed by >>):
>
>    fs/proc/page.c: In function 'find_next_zone_range':
>>> fs/proc/page.c:58:2: error: expected ';' before 'for'
>      for (pgdat = first_online_pgdat(); pgdat;
>      ^~~
>    fs/proc/page.c:52:6: warning: unused variable 'i' [-Wunused-variable]
>      int i;
>          ^
>    fs/proc/page.c:51:15: warning: unused variable 'zone' [-Wunused-variable]
>      struct zone *zone;
>                   ^~~~
>    fs/proc/page.c:50:13: warning: unused variable 'pgdat' [-Wunused-variable]
>      pg_data_t *pgdat;
>                 ^~~~~
>    fs/proc/page.c: In function 'pfn_in_zone':
>>> fs/proc/page.c:79:23: error: 'struct zone_range' has no member named 'start'; did you mean 'pfn_start'?
>      return pfn >= range->start && pfn < range->end;
>                           ^~~~~
>                           pfn_start
>>> fs/proc/page.c:79:43: error: 'struct zone_range' has no member named 'end'
>      return pfn >= range->start && pfn < range->end;
>                                               ^~
>    fs/proc/page.c:80:1: warning: control reaches end of non-void function [-Wreturn-type]
>     }
>     ^
>
> vim +58 fs/proc/page.c
>
>     46	
>     47	static void find_next_zone_range(struct zone_range *range)
>     48	{
>     49		unsigned long start, end;
>   > 50		pg_data_t *pgdat;
>   > 51		struct zone *zone;
>     52		int i;
>     53	
>     54		/*
>     55		 * Scan all the zone structures to find the next closest one.
>     56		 */
>     57		start = end = -1UL
>   > 58		for (pgdat = first_online_pgdat(); pgdat;
>     59		     pgdat = next_online_pgdat(pgdat)) {
>     60			for (zone = pgdat->node_zones, i = 0; i < MAX_NR_ZONES;
>     61			     zone++, i++) {
>     62				if (!zone->spanned_pages)
>     63					continue;
>     64				if ((zone->zone_start_pfn >= range->pfn_end) &&
>     65				    (zone->zone_start_pfn < start)) {
>     66					start = zone->zone_start_pfn;
>     67					end   = start + zone->spanned_pages;
>     68				}
>     69			}
>     70		}
>     71		range->pfn_start = start;
>     72		range->pfn_end   = end;
>     73	}
>     74	
>     75	static inline bool pfn_in_zone(unsigned long pfn, struct zone_range *range)
>     76	{
>     77		if (pfn >= range->pfn_end)
>     78			find_next_zone_range(range);
>   > 79		return pfn >= range->start && pfn < range->end;
>     80	}
>     81	
>
> ---
> 0-DAY kernel test infrastructure                Open Source Technology Center
> https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

Sorry, I have accidentally sent the old version with syntax error out.
Has posted the right one in v2.

Cheers,
Longman