From: Baolin Wang baolin.wang@linux.alibaba.com
[ Upstream commit 8b3d838139bcd1e552f1899191f734264ce2a1a5 ]
We met a kernel crash issue when running stress-ng testing, and the system crashes when printing the dentry name in dump_mapping().
Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000 pc : dentry_name+0xd8/0x224 lr : pointer+0x22c/0x370 sp : ffff800025f134c0 ...... Call trace: dentry_name+0xd8/0x224 pointer+0x22c/0x370 vsnprintf+0x1ec/0x730 vscnprintf+0x2c/0x60 vprintk_store+0x70/0x234 vprintk_emit+0xe0/0x24c vprintk_default+0x3c/0x44 vprintk_func+0x84/0x2d0 printk+0x64/0x88 __dump_page+0x52c/0x530 dump_page+0x14/0x20 set_migratetype_isolate+0x110/0x224 start_isolate_page_range+0xc4/0x20c offline_pages+0x124/0x474 memory_block_offline+0x44/0xf4 memory_subsys_offline+0x3c/0x70 device_offline+0xf0/0x120 ......
The root cause is that, one thread is doing page migration, and we will use the target page's ->mapping field to save 'anon_vma' pointer between page unmap and page move, and now the target page is locked and refcount is 1.
Currently, there is another stress-ng thread performing memory hotplug, attempting to offline the target page that is being migrated. It discovers that the refcount of this target page is 1, preventing the offline operation, thus proceeding to dump the page. However, page_mapping() of the target page may return an incorrect file mapping to crash the system in dump_mapping(), since the target page->mapping only saves 'anon_vma' pointer without setting PAGE_MAPPING_ANON flag.
The page migration issue has been fixed by commit d1adb25df711 ("mm: migrate: fix getting incorrect page mapping during page migration"). In addition, Matthew suggested we should also improve dump_mapping()'s robustness to resilient against the kernel crash [1].
With checking the 'dentry.parent' and 'dentry.d_name.name' used by dentry_name(), I can see dump_mapping() will output the invalid dentry instead of crashing the system when this issue is reproduced again.
[12211.189128] page:fffff7de047741c0 refcount:1 mapcount:0 mapping:ffff989117f55ea0 index:0x1 pfn:0x211dd07 [12211.189144] aops:0x0 ino:1 invalid dentry:74786574206e6870 [12211.189148] flags: 0x57ffffc0000001(locked|node=1|zone=2|lastcpupid=0x1fffff) [12211.189150] page_type: 0xffffffff() [12211.189153] raw: 0057ffffc0000001 0000000000000000 dead000000000122 ffff989117f55ea0 [12211.189154] raw: 0000000000000001 0000000000000001 00000001ffffffff 0000000000000000 [12211.189155] page dumped because: unmovable page
[1] https://lore.kernel.org/all/ZXxn%2F0oixJxxAnpF@casper.infradead.org/
Suggested-by: Matthew Wilcox willy@infradead.org Signed-off-by: Baolin Wang baolin.wang@linux.alibaba.com Link: https://lore.kernel.org/r/937ab1f87328516821d39be672b6bc18861d9d3e.170539142... Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- fs/inode.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/fs/inode.c b/fs/inode.c index 91048c4c9c9e7..6d0d542303638 100644 --- a/fs/inode.c +++ b/fs/inode.c @@ -588,7 +588,8 @@ void dump_mapping(const struct address_space *mapping) }
dentry_ptr = container_of(dentry_first, struct dentry, d_u.d_alias); - if (get_kernel_nofault(dentry, dentry_ptr)) { + if (get_kernel_nofault(dentry, dentry_ptr) || + !dentry.d_parent || !dentry.d_name.name) { pr_warn("aops:%ps ino:%lx invalid dentry:%px\n", a_ops, ino, dentry_ptr); return;