mm/hugetlb: fix list corruption in allocate_file_region_entries()

Overview

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

mm/hugetlb: fix list corruption in allocate_file_region_entries()

allocate_file_region_entries() tops up resv->region_cache with freshly allocated file_region descriptors. The allocation uses GFP_KERNEL, so resv->lock is dropped around it: the new entries are gathered on a stack-local list head, allocated_regions, and spliced into resv->region_cache once the lock is re-acquired.

The splice used list_splice(), which moves the entries but does not re-initialize the source head, so allocated_regions is left pointing at an entry that now lives on resv->region_cache. The top-up runs in a while loop that re-checks the cache deficit after re-acquiring the lock. For a shared mapping the resv_map is shared by every mapper of the hugetlbfs inode, so a concurrent region_chg()/region_add()/region_del() on the same resv_map can consume cache entries during the unlocked window and force a second iteration. That iteration calls list_add() on the stale head and corrupts the list; with CONFIG_DEBUG_LIST the __list_add_valid() check trips:

list_add corruption. next->prev should be prev (ffffc900011ff7f8), but was ffff88814c281460. (next=ffff88814c545640). kernel BUG at lib/list_debug.c:31! allocate_file_region_entries+0x191/0x420 region_chg+0x267/0x300 hugetlb_reserve_pages+0x387/0xc80 hugetlbfs_file_mmap+0x2ce/0x3f0 mmap_region+0x1348/0x1a80 do_mmap+0x85e/0xb90 vm_mmap_pgoff+0x18c/0x330 ksys_mmap_pgoff+0x2a1/0x3e0 do_syscall_64+0xd7/0x420

Without CONFIG_DEBUG_LIST the bad list_add() silently links a kernel-stack address into resv->region_cache, leading to later use-after-free.

This was observed as a real host panic on a dense KVM host where a QEMU guest-RAM hugetlbfs file was mapped MAP_SHARED by both QEMU and a separate SPDK/DPDK vhost-user target, generating concurrent region_* traffic on one shared resv_map.

Use list_splice_init() so the source head is re-initialized empty after each splice, making the retry loop safe.

Affected

• Linux >= d3ec7b6e09e512ba902b86bcca2c512fb06d492f < 01b8569233e47693d6ff7efa96d9854c55f936fc
• Linux >= d3ec7b6e09e512ba902b86bcca2c512fb06d492f < 9c5fdffc5e1ce84403c58289ee72697051803bf7
• Linux >= d3ec7b6e09e512ba902b86bcca2c512fb06d492f < f3e54f6a5e1681f83d13e8716bc92ef5ecf121d3
• Linux >= d3ec7b6e09e512ba902b86bcca2c512fb06d492f < 62e1c2741a4d923d9854efd5927a6212aad7a187
• Linux >= d3ec7b6e09e512ba902b86bcca2c512fb06d492f < 587a0accc2b4fccc5cf7baf0fe34e50efde51f9c
• Linux >= d3ec7b6e09e512ba902b86bcca2c512fb06d492f < 126a70bf1a08ddc9d79c471ebdaa2b08cfbab8df
• Linux >= d3ec7b6e09e512ba902b86bcca2c512fb06d492f < ac1bb7fd45088d0db57a22ce7729f258ebd63cf5
• Linux >= d3ec7b6e09e512ba902b86bcca2c512fb06d492f < dd9623f58ec702a07b2d67179d6fcea79c52231a
• Linux 5.10

Remediation

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