[PATCH mm-unstable 2/2] mm/mmap: fix NULL pointer dereference in dup_mmap() error handling

[Hui Zhu <hui.zhu@linux.dev>]

From: Hui Zhu <zhuhui@kylinos.cn>

If dup_mmap() fails very early in its execution, it's possible that
no VMAs have been inserted into the new mm's maple tree. When
vma_next() is called in the cleanup block to retrieve the first
VMA ('tmp'), it may return NULL.

The UNMAP_STATE macro and the subsequent call to tear_down_vmas()
do not perform a NULL check on 'tmp' and directly attempt to
access its fields (such as tmp->vm_end). This results in a
NULL pointer dereference and a kernel panic.

This patch adds an explicit NULL check for 'tmp' before proceeding
with the unmap and tear down logic in the failure path of dup_mmap().

Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
---
 mm/mmap.c | 31 ++++++++++++++++++-------------
 1 file changed, 18 insertions(+), 13 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 498c88a54a36..ca5645a2e456 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1879,19 +1879,24 @@ __latent_entropy int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
 		if (end) {
 			vma_iter_set(&vmi, 0);
 			tmp = vma_next(&vmi);
-			UNMAP_STATE(unmap, &vmi, /* first = */ tmp,
-				    /* vma_start = */ 0, /* vma_end = */ end,
-				    /* prev = */ NULL, /* next = */ NULL);
-
-			/*
-			 * Don't iterate over vmas beyond the failure point for
-			 * both unmap_vma() and free_pgtables().
-			 */
-			unmap.tree_end = end;
-			flush_cache_mm(mm);
-			unmap_region(&unmap);
-			charge = tear_down_vmas(mm, &vmi, tmp, end);
-			vm_unacct_memory(charge);
+			if (tmp) {
+				UNMAP_STATE(unmap, &vmi,
+					    /* first = */ tmp,
+					    /* vma_start = */ 0,
+					    /* vma_end = */ end,
+					    /* prev = */ NULL,
+					    /* next = */ NULL);
+
+				/*
+				 * Don't iterate over vmas beyond the failure point for
+				 * both unmap_vma() and free_pgtables().
+				 */
+				unmap.tree_end = end;
+				flush_cache_mm(mm);
+				unmap_region(&unmap);
+				charge = tear_down_vmas(mm, &vmi, tmp, end);
+				vm_unacct_memory(charge);
+			}
 		}
 		vma_iter_free(&vmi);
 		__mt_destroy(&mm->mm_mt);
-- 
2.43.0