linux-kernel - Forwarded: [PATCH] Fix a lock imbalance bug in hugetlb_vmdelete

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Message-ID: <68d3c206.050a0220.139b6.002f.GAE@google.com>
Date: Wed, 24 Sep 2025 03:03:50 -0700
From: syzbot <syzbot+62edf7e27b2e8f754525@...kaller.appspotmail.com>
To: linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Forwarded: [PATCH] Fix a lock imbalance bug in hugetlb_vmdelete_list()
 that

For archival purposes, forwarding an incoming command email to
linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com.

***

Subject: [PATCH] Fix a lock imbalance bug in hugetlb_vmdelete_list() that
Author: kartikey406@...il.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master

Fix a lock imbalance bug in hugetlb_vmdelete_list() that causes:
WARNING: bad unlock balance detected!
  hugetlb_vmdelete_list+0x179/0x1c0 is trying to release lock 
  (&vma_lock->rw_sema) but there are no more locks to release!

The issue is a race condition between multiple threads operating on the
same VMA:

1. Thread 1 calls hugetlb_vma_trylock_write() when vma->vm_private_data=NULL
2. trylock returns success (no lock needed for this VMA type)
3. Thread 2 allocates a lock structure: vma->vm_private_data=&new_lock
4. Thread 1 calls hugetlb_vma_unlock_write(), sees non-NULL vm_private_data
5. Thread 1 tries to unlock a lock it never acquired → crash

The fix is to save the VMA lock state at the time we make the locking
decision, rather than checking it again at unlock time. This prevents
the time-of-check-time-of-use (TOCTOU) race condition.

Reported-by: syzbot+62edf7e27b2e8f754525@...kaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=62edf7e27b2e8f754525
Fixes: 8d9bfb2608cf ("hugetlb: add vma based lock for pmd sharing")
Signed-off-by: Deepanshu Kartikey <kartikey406@...il.com>
---
 fs/hugetlbfs/inode.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 9e0625167517..ae3e07eacd37 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -475,15 +475,16 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end,
 		      zap_flags_t zap_flags)
 {
 	struct vm_area_struct *vma;
-
 	/*
 	 * end == 0 indicates that the entire range after start should be
 	 * unmapped.  Note, end is exclusive, whereas the interval tree takes
 	 * an inclusive "last".
 	 */
 	vma_interval_tree_foreach(vma, root, start, end ? end - 1 : ULONG_MAX) {
+		struct hugetlb_vma_lock *vma_lock;
 		unsigned long v_start;
 		unsigned long v_end;
+		vma_lock = vma->vm_private_data;

 		if (!hugetlb_vma_trylock_write(vma))
 			continue;
@@ -498,7 +499,8 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end,
 		 * vmas.  Therefore, lock is not held when calling
 		 * unmap_hugepage_range for private vmas.
 		 */
-		hugetlb_vma_unlock_write(vma);
+		if (vma_lock)
+			hugetlb_vma_unlock_write(vma);
 	}
 }

-- 
2.43.0