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

Waiman Long
2019-08-25T16:38:42Z

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