CVE-2021-47635: In the Linux kernel, the following vulnerability has been resolved: ubifs: Fix to add refcount once page is set private MM defined the rule [1] very clearly t

In the Linux kernel, the following vulnerability has been resolved: ubifs: Fix to add refcount once page is set private MM defined the rule [1] very clearly t

In the Linux kernel, the following vulnerability has been resolved: ubifs: Fix to add refcount once page is set private MM defined the rule [1] very clearly that once page was set with PG_private flag, we should increment the refcount in that page, also main flows like pageout(), migrate_page() will assume there is one additional page reference count if page_has_private() returns true. Otherwise, we may get a BUG in page migration: page:0000000080d05b9d refcount:-1 mapcount:0 mapping:000000005f4d82a8 index:0xe2 pfn:0x14c12 aops:ubifs_file_address_operations [ubifs] ino:8f1 dentry name:"f30e" flags: 0x1fffff80002405(locked|uptodate|owner_priv_1|private|node=0| zone=1|lastcpupid=0x1fffff) page dumped because: VM_BUG_ON_PAGE(page_count(page) != 0) ------------[ cut here ]------------ kernel BUG at include/linux/page_ref.h:184! invalid opcode: 0000 [#1] SMP CPU: 3 PID: 38 Comm: kcompactd0 Not tainted 5.15.0-rc5 RIP: 0010:migrate_page_move_mapping+0xac3/0xe70 Call Trace: ubifs_migrate_page+0x22/0xc0 [ubifs] move_to_new_page+0xb4/0x600 migrate_pages+0x1523/0x1cc0 compact_zone+0x8c5/0x14b0 kcompactd+0x2bc/0x560 kthread+0x18c/0x1e0 ret

Description from the National Vulnerability Database, published by 416baaa9-dc9f-4396-8d5f-8c081fb06d67

What it means for a small business

Linux is on the computers, phones or browsers your team uses every day. Turn on automatic updates and restart: most of these fixes only take effect after a restart.

How likely is an attack? The Exploit Prediction Scoring System, run by FIRST (the global association of security response teams), puts the odds of this one being exploited in the next 30 days at 0.26%, higher than 17% of CVEs. That’s low, but low isn’t zero, and the odds jump the day someone publishes an exploit. Scored November 9, 2026, updated daily.

Check your own setup

Website check: reads what your site runs from the outside (WordPress version, plugins it can see, old server software) and matches it against this database.
Server check: what your web server says about itself, and whether that version has known flaws.

Sources

Part of the Huttenlocher Technologies vulnerability database, refreshed twice a day from CISA and the NVD. The title, category and plain-language guidance are ours; check the linked record before acting on a detail. This is general guidance, not a review of your systems.

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