CVE-2022-50534

In the Linux kernel, the following vulnerability has been resolved:

dm thin: Use last transaction's pmd->root when commit failed

Recently we found a softlock up problem in dm thin pool btree lookup code due to corrupted metadata:

Kernel panic - not syncing: softlockup: hung tasks CPU: 7 PID: 2669225 Comm: kworker/u16:3 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) Workqueue: dm-thin doworker [dmthinpool] Call Trace: <IRQ> dumpstack+0x9c/0xd3 panic+0x35d/0x6b9 watchdogtimerfn.cold+0x16/0x25 __run_hrtimer+0xa2/0x2d0 </IRQ> RIP: 0010:__relinklru+0x102/0x220 [dmbufio] __bufionew+0x11f/0x4f0 [dmbufio] newread+0xa3/0x1e0 [dmbufio] dmbmreadlock+0x33/0xd0 [dmpersistentdata] rostep+0x63/0x100 [dmpersistentdata] btreelookupraw.constprop.0+0x44/0x220 [dmpersistentdata] dmbtreelookup+0x16f/0x210 [dmpersistentdata] dmthinfindblock+0x12c/0x210 [dmthin_pool] __processbioreadonly+0xc5/0x400 [dmthinpool] processthindeferredbios+0x1a4/0x4a0 [dmthinpool] processonework+0x3c5/0x730

Following process may generate a broken btree mixed with fresh and stale btree nodes, which could get dm thin trapped in an infinite loop while looking up data block: Transaction 1: pmd->root = A, A->B->C // One path in btree pmd->root = X, X->Y->Z // Copy-up Transaction 2: X,Z is updated on disk, Y write failed. // Commit failed, dm thin becomes read-only. processbioreadonly dmthinfindblock _findblock dmbtreelookup(pmd->root) The pmd->root points to a broken btree, Y may contain stale node pointing to any block, for example X, which gets dm thin trapped into a dead loop while looking up Z.

Fix this by setting pmd->root in _openmetadata(), so that dm thin will use the last transaction's pmd->root if commit failed.

Fetch a reproducer in [Link].

Linke: https://bugzilla.kernel.org/show_bug.cgi?id=216790