[PATCH stable 5.10] mm: hugetlb: fix UAF in hugetlb_handle_userfault

[Liu Shixin]

commit 958f32ce832ba781ac20e11bb2d12a9352ea28fc upstream.

The vma_lock and hugetlb_fault_mutex are dropped before handling
userfault and reacquire them again after handle_userfault(), but
reacquire the vma_lock could lead to UAF[1,2] due to the following
race,

hugetlb_fault
  hugetlb_no_page
    /*unlock vma_lock */
    hugetlb_handle_userfault
      handle_userfault
        /* unlock mm->mmap_lock*/
                                           vm_mmap_pgoff
                                             do_mmap
                                               mmap_region
                                                 munmap_vma_range
                                                   /* clean old vma */
        /* lock vma_lock again  <--- UAF */
    /* unlock vma_lock */

Since the vma_lock will unlock immediately after hugetlb_handle_userfault(),
let's drop the unneeded lock and unlock in hugetlb_handle_userfault() to fix
the issue.

[1] https://lore.kernel.org/linux-mm/000000000000d5e00a05e834962e@xxxxxxxxxx/
[2] https://lore.kernel.org/linux-mm/20220921014457.1668-1-liuzixian4@xxxxxxxxxx/
Reported-by: syzbot+193f9cee8638750b23cf@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: Liu Zixian <liuzixian4@xxxxxxxxxx>
Fixes: 1a1aad8a9b7b ("userfaultfd: hugetlbfs: add userfaultfd hugetlb hook")
CC: stable@xxxxxxxxxxxxxxx # 4.14+
Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/hugetlb.c | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c42c76447e10..c57c165bfbbc 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4337,6 +4337,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 	spinlock_t *ptl;
 	unsigned long haddr = address & huge_page_mask(h);
 	bool new_page = false;
+	u32 hash = hugetlb_fault_mutex_hash(mapping, idx);
 
 	/*
 	 * Currently, we are forced to kill the process in the event the
@@ -4346,7 +4347,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 	if (is_vma_resv_set(vma, HPAGE_RESV_UNMAPPED)) {
 		pr_warn_ratelimited("PID %d killed due to inadequate hugepage pool\n",
 			   current->pid);
-		return ret;
+		goto out;
 	}
 
 	/*
@@ -4365,7 +4366,6 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 		 * Check for page in userfault range
 		 */
 		if (userfaultfd_missing(vma)) {
-			u32 hash;
 			struct vm_fault vmf = {
 				.vma = vma,
 				.address = haddr,
@@ -4380,17 +4380,14 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 			};
 
 			/*
-			 * hugetlb_fault_mutex and i_mmap_rwsem must be
-			 * dropped before handling userfault.  Reacquire
-			 * after handling fault to make calling code simpler.
+			 * vma_lock and hugetlb_fault_mutex must be dropped
+			 * before handling userfault. Also mmap_lock will
+			 * be dropped during handling userfault, any vma
+			 * operation should be careful from here.
 			 */
-			hash = hugetlb_fault_mutex_hash(mapping, idx);
 			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
 			i_mmap_unlock_read(mapping);
-			ret = handle_userfault(&vmf, VM_UFFD_MISSING);
-			i_mmap_lock_read(mapping);
-			mutex_lock(&hugetlb_fault_mutex_table[hash]);
-			goto out;
+			return handle_userfault(&vmf, VM_UFFD_MISSING);
 		}
 
 		page = alloc_huge_page(vma, haddr, 0);
@@ -4497,6 +4494,8 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 
 	unlock_page(page);
 out:
+	mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+	i_mmap_unlock_read(mapping);
 	return ret;
 
 backout:
@@ -4592,10 +4591,12 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 	mutex_lock(&hugetlb_fault_mutex_table[hash]);
 
 	entry = huge_ptep_get(ptep);
-	if (huge_pte_none(entry)) {
-		ret = hugetlb_no_page(mm, vma, mapping, idx, address, ptep, flags);
-		goto out_mutex;
-	}
+	if (huge_pte_none(entry))
+		/*
+		 * hugetlb_no_page will drop vma lock and hugetlb fault
+		 * mutex internally, which make us return immediately.
+		 */
+		return hugetlb_no_page(mm, vma, mapping, idx, address, ptep, flags);
 
 	ret = 0;
 
-- 
2.25.1