[PATCH 6.12 137/337] mm/hugetlb: fix list corruption in allocate_file_region_entries()

6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Xiangfeng Cai <[email protected]>

commit dd9623f58ec702a07b2d67179d6fcea79c52231a upstream.

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.

Link: https://lore.kernel.org/[email protected]
Fixes: d3ec7b6e09e5 ("mm/hugetlb: use list_splice to merge two list at once")
Signed-off-by: Xiangfeng Cai <[email protected]>
Reviewed-by: Muchun Song <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 mm/hugetlb.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -676,7 +676,7 @@ static int allocate_file_region_entries(
 
 		spin_lock(&resv->lock);
 
-		list_splice(&allocated_regions, &resv->region_cache);
+		list_splice_init(&allocated_regions, &resv->region_cache);
 		resv->region_cache_count += to_allocate;
 	}