From: Toshiki Fukasawa Subject: /proc/kpageflags: do not use uninitialized struct pages 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. Dan said: : The nvdimm implementation uses vmem_altmap to arrange for the 'struct : page' array to be allocated from a reservation of a pmem namespace. A : namespace in this mode contains an info-block that consumes the first : 8K of the namespace capacity, capacity designated for page mapping, : capacity for padding the start of data to optionally 4K, 2MB, or 1GB : (on x86), and then the namespace data itself. The implementation : specifies a section aligned (now sub-section aligned) address to : arch_add_memory() to establish the linear mapping to map the metadata, : and then vmem_altmap indicates to memmap_init_zone() which pfns : represent data. The implementation only specifies enough 'struct page' : capacity for pfn_to_page() to operate on the data space, not the : namespace metadata space. : : The proposal to validate ZONE_DEVICE pfns against the altmap seems the : right approach to me. Link: http://lkml.kernel.org/r/20190725023100.31141-3-t-fukasawa@vx.jp.nec.com Signed-off-by: Toshiki Fukasawa Cc: Alexey Dobriyan Cc: Christoph Hellwig Cc: Dan Williams Cc: Junichi Nomura Cc: Michal Hocko Cc: Naoya Horiguchi Cc: Signed-off-by: Andrew Morton --- fs/proc/page.c | 3 +++ include/linux/memremap.h | 6 ++++++ mm/memremap.c | 20 ++++++++++++++++++++ 3 files changed, 29 insertions(+) --- a/fs/proc/page.c~proc-kpageflags-do-not-use-uninitialized-struct-pages +++ a/fs/proc/page.c @@ -118,6 +118,9 @@ 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; --- a/include/linux/memremap.h~proc-kpageflags-do-not-use-uninitialized-struct-pages +++ a/include/linux/memremap.h @@ -151,6 +151,7 @@ static inline bool is_pci_p2pdma_page(co } #ifdef CONFIG_ZONE_DEVICE +bool pfn_zone_device_reserved(unsigned long pfn); void *memremap_pages(struct dev_pagemap *pgmap, int nid); void memunmap_pages(struct dev_pagemap *pgmap); void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap); @@ -163,6 +164,11 @@ unsigned long vmem_altmap_offset(struct void vmem_altmap_free(struct vmem_altmap *altmap, unsigned long nr_pfns); unsigned long memremap_compat_align(void); #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) { --- a/mm/memremap.c~proc-kpageflags-do-not-use-uninitialized-struct-pages +++ a/mm/memremap.c @@ -107,6 +107,26 @@ static unsigned long pfn_len(struct dev_ pfn_first(pgmap, range_id)) >> pgmap->vmemmap_shift; } +/* + * 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; +} + static void pageunmap_range(struct dev_pagemap *pgmap, int range_id) { struct range *range = &pgmap->ranges[range_id]; _