CVE-2026-72193: ntfs3: cap RESTART_TABLE free-chain walker at rt->used

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

ntfs3: cap RESTARTTABLE free-chain walker at rt->used

A crafted NTFS3 disk image triggers an in-kernel infinite loop at mount time, hanging the mounting thread and firing the soft-lockup watchdog within ~22s on multi-CPU hosts (panic with kernel.softlockuppanic=1). The bug is reachable from desktop USB auto-mount on distributions where udisks2 routes the NTFS signature to the in-tree ntfs3 driver (Arch family and an increasing fraction of Fedora / openSUSE / RHEL deployments); CAPSYSADMIN-class manual mount elsewhere.

checkrstbl()'s second walker iterates the free-entry singly-linked list headed by rt->firstfree with no upper bound on iteration count:

for (off = ff; off;) { if (off == RESTARTENTRYALLOCATED) return false; off = le32tocpu((le32 )Add2Ptr(rt, off)); if (off > ts - sizeof(le32)) return false; }

The existing guards cover three exits: end-of-list (off == 0), the in-use marker (off == RESTARTENTRYALLOCATED), and out-of-bounds (off > ts - sizeof(le32)). None of the three prevents an in-bounds cycle.

A crafted on-disk RESTARTTABLE whose free chain contains a self-loop or A->B->A cycle whose offsets satisfy:

- in range [sizeof(struct RESTARTTABLE), ts - sizeof(le32)] - (off - sizeof(struct RESTARTTABLE)) % rsize == 0

passes all existing guards and spins the mount-time thread forever. Reproduced in UML by hand-forging a 2 MB NTFS3 image whose journal RESTARTTABLE firstfree = 0x18 and whose entry at offset 0x18 stores 0x18 as its next pointer; mount of the forged image with the in-tree ntfs3 driver never returns.

Bound the walker by rt->used. Each entry on a legitimate free chain is unique, and the total slot count is ne = le16tocpu (rt->used). A traversal that visits more than ne slots is by construction malformed; reject it as a corrupt RESTARTTABLE.

After this patch, mount of the forged image returns with -EINVAL and a logreplay failure message, and mkntfs-produced legitimate images mount cleanly (verified in the same UML harness).