[PATCH 2/2] /proc/kpageflags: do not use uninitialized struct pages

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A kernel panic was observed during reading /proc/kpageflags for
first few pfns allocated by pmem namespace:

BUG: unable to handle page fault for address: fffffffffffffffe
[  114.495280] #PF: supervisor read access in kernel mode
[  114.495738] #PF: error_code(0x0000) - not-present page
[  114.496203] PGD 17120e067 P4D 17120e067 PUD 171210067 PMD 0
[  114.496713] Oops: 0000 [#1] SMP PTI
[  114.497037] CPU: 9 PID: 1202 Comm: page-types Not tainted 5.3.0-rc1 #1
[  114.497621] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
[  114.498706] RIP: 0010:stable_page_flags+0x27/0x3f0
[  114.499142] Code: 82 66 90 66 66 66 66 90 48 85 ff 0f 84 d1 03 00 00 41 54 55 48 89 fd 53 48 8b 57 08 48 8b 1f 48 8d 42 ff 83 e2 01 48 0f 44 c7 <48> 8b 00 f6 c4 02 0f 84 57 03 00 00 45 31 e4 48 8b 55 08 48 89 ef
[  114.500788] RSP: 0018:ffffa5e601a0fe60 EFLAGS: 00010202
[  114.501373] RAX: fffffffffffffffe RBX: ffffffffffffffff RCX: 0000000000000000
[  114.502009] RDX: 0000000000000001 RSI: 00007ffca13a7310 RDI: ffffd07489000000
[  114.502637] RBP: ffffd07489000000 R08: 0000000000000001 R09: 0000000000000000
[  114.503270] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000240000
[  114.503896] R13: 0000000000080000 R14: 00007ffca13a7310 R15: ffffa5e601a0ff08
[  114.504530] FS:  00007f0266c7f540(0000) GS:ffff962dbbac0000(0000) knlGS:0000000000000000
[  114.505245] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  114.505754] CR2: fffffffffffffffe CR3: 000000023a204000 CR4: 00000000000006e0
[  114.506401] Call Trace:
[  114.506660]  kpageflags_read+0xb1/0x130
[  114.507051]  proc_reg_read+0x39/0x60
[  114.507387]  vfs_read+0x8a/0x140
[  114.507686]  ksys_pread64+0x61/0xa0
[  114.508021]  do_syscall_64+0x5f/0x1a0
[  114.508372]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  114.508844] RIP: 0033:0x7f0266ba426b

The reason for the panic is that stable_page_flags() which parses
the page flags uses uninitialized struct pages reserved by the
ZONE_DEVICE driver.

Earlier approach to fix this was discussed here:
https://marc.info/?l=linux-mm&m=152964770000672&w=2

This is another approach. To avoid using the uninitialized struct page,
immediately return with KPF_RESERVED at the beginning of
stable_page_flags() if the page is reserved by ZONE_DEVICE driver.

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Toshiki Fukasawa <t-fukasawa@xxxxxxxxxxxxx>
---
 fs/proc/page.c           |  3 +++
 include/linux/memremap.h |  6 ++++++
 kernel/memremap.c        | 20 ++++++++++++++++++++
 3 files changed, 29 insertions(+)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 69064ad..decd3fe 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -97,6 +97,9 @@ 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;
 
diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index f8a5b2a..2cfc3c2 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -124,6 +124,7 @@ static inline struct vmem_altmap *pgmap_altmap(struct dev_pagemap *pgmap)
 }
 
 #ifdef CONFIG_ZONE_DEVICE
+bool pfn_zone_device_reserved(unsigned long pfn);
 void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
 void devm_memunmap_pages(struct device *dev, struct dev_pagemap *pgmap);
 struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
@@ -132,6 +133,11 @@ 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)
+{
+	return false;
+}
+
 static inline void *devm_memremap_pages(struct device *dev,
 		struct dev_pagemap *pgmap)
 {
diff --git a/kernel/memremap.c b/kernel/memremap.c
index 6ee03a8..bc3471c 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -72,6 +72,26 @@ static unsigned long pfn_next(unsigned long pfn)
 	return pfn + 1;
 }
 
+/*
+ * This returns true if the page is reserved by ZONE_DEVICE driver.
+ */
+bool pfn_zone_device_reserved(unsigned long pfn)
+{
+	struct dev_pagemap *pgmap;
+	struct vmem_altmap *altmap;
+	bool ret = false;
+
+	pgmap = get_dev_pagemap(pfn, NULL);
+	if (!pgmap)
+		return ret;
+	altmap = pgmap_altmap(pgmap);
+	if (altmap && pfn < (altmap->base_pfn + altmap->reserve))
+		ret = true;
+	put_dev_pagemap(pgmap);
+
+	return ret;
+}
+
 #define for_each_device_pfn(pfn, map) \
 	for (pfn = pfn_first(map); pfn < pfn_end(map); pfn = pfn_next(pfn))
 
-- 
1.8.3.1





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