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

Overview

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

ntfs3: cap RESTART_TABLE 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.softlockup_panic=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); CAP_SYS_ADMIN-class manual mount elsewhere.

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

for (off = ff; off;) { if (off == RESTART_ENTRY_ALLOCATED) return false; off = le32_to_cpu(*(__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 == RESTART_ENTRY_ALLOCATED), and out-of-bounds (off > ts - sizeof(__le32)). None of the three prevents an in-bounds cycle.

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

• in range [sizeof(struct RESTART_TABLE), ts - sizeof(__le32)]
• (off - sizeof(struct RESTART_TABLE)) % 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 RESTART_TABLE first_free = 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 = le16_to_cpu (rt->used). A traversal that visits more than ne slots is by construction malformed; reject it as a corrupt RESTART_TABLE.

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

Affected

• Linux >= b46acd6a6a627d876898e1c84d3f84902264b445 < 8128bec895075253c779d67afdc90ae513265fca
• Linux >= b46acd6a6a627d876898e1c84d3f84902264b445 < 7972df425687daa70d971fe6ed415e78683133dd
• Linux >= b46acd6a6a627d876898e1c84d3f84902264b445 < 7ac4c86915c24c208a0f0611b71d9676686fe756
• Linux >= b46acd6a6a627d876898e1c84d3f84902264b445 < 29b86dbe88cbbef53bb9aaec2e279359f8c450f8
• Linux >= b46acd6a6a627d876898e1c84d3f84902264b445 < 0fad25687d4d3fa1fdd313d31b9cb5817c425029
• Linux >= b46acd6a6a627d876898e1c84d3f84902264b445 < d313416280d41bea272f02a6034dfa88008692a0
• Linux >= b46acd6a6a627d876898e1c84d3f84902264b445 < 9611f644302c07d21bc8af97e3e06a3d30064253
• Linux 5.15

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.