Forwarded: [PATCH v2] hugetlbfs: skip VMAs without shareable locks in hugetlb_vmdelete_list

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Subject: [PATCH v2] hugetlbfs: skip VMAs without shareable locks in hugetlb_vmdelete_list
Author: kartikey406@gmail.com

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

hugetlb_vmdelete_list() uses trylock to acquire VMA locks during truncate
operations. As per the original design in commit 40549ba8f8e0 ("hugetlb:
use new vma_lock for pmd sharing synchronization"), if the trylock fails
or the VMA has no lock, it should skip that VMA. Any remaining mapped
pages are handled by remove_inode_hugepages() which is called after
hugetlb_vmdelete_list() and uses proper lock ordering to guarantee
unmapping success.

Currently, when hugetlb_vma_trylock_write() returns success (1) for VMAs
without shareable locks, the code proceeds to call unmap_hugepage_range().
This causes assertion failures in huge_pmd_unshare() → hugetlb_vma_assert_locked()
because no lock is actually held:

  WARNING: CPU: 1 PID: 6594 Comm: syz.0.28 Not tainted
  Call Trace:
   hugetlb_vma_assert_locked+0x1dd/0x250
   huge_pmd_unshare+0x2c8/0x540
   __unmap_hugepage_range+0x6e3/0x1aa0
   unmap_hugepage_range+0x32e/0x410
   hugetlb_vmdelete_list+0x189/0x1f0

Fix by checking for shareable lock before attempting trylock, avoiding
both the assertion failure and potential lock leaks from skipping VMAs
after locks are acquired.

Reported-by: syzbot+f26d7c75c26ec19790e7@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=f26d7c75c26ec19790e7
Fixes: 40549ba8f8e0 ("hugetlb: use new vma_lock for pmd sharing synchronization")
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>

---
Changes in v2:
- Check for shareable lock before trylock to avoid lock leaks (Andrew Morton)
- Add comment explaining why non-shareable VMAs are skipped (Andrew Morton)
---
 fs/hugetlbfs/inode.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 9e0625167517..44943e97adb0 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -484,6 +484,13 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end,
 	vma_interval_tree_foreach(vma, root, start, end ? end - 1 : ULONG_MAX) {
 		unsigned long v_start;
 		unsigned long v_end;
+		/*
+		 * Skip VMAs without shareable locks. Per the design in commit
+		 * 40549ba8f8e0, these will be handled by remove_inode_hugepages()
+		 * called after this function with proper locking.
+		 */
+		if (!__vma_shareable_lock(vma))
+			continue;
 
 		if (!hugetlb_vma_trylock_write(vma))
 			continue;
-- 
2.43.0