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 using goto to ensure locks acquired by trylock are always released, even
when skipping VMAs without shareable locks.

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")
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>

---
Changes in v2:
- Use goto to unlock after trylock, avoiding lock leaks (Andrew Morton)
- Add comment explaining why non-shareable VMAs are skipped (Andrew Morton)
---
 fs/hugetlbfs/inode.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 9e0625167517..9fa7c72ac1a6 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -488,6 +488,14 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end,
 		if (!hugetlb_vma_trylock_write(vma))
 			continue;
 
+		/*
+		 * 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))
+			goto skip;
+
 		v_start = vma_offset_start(vma, start);
 		v_end = vma_offset_end(vma, end);
 
@@ -498,7 +506,8 @@ 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);
+skip:
+		hugetlb_vma_unlock_write(vma);
 	}
 }
 
-- 
2.43.0