[PATCH] mm/rmap: fix unlink_anon_vmas() handling of error case from anon_vma_fork

[Deepanshu Kartikey <kartikey406@gmail.com>]

When anon_vma_fork() encounters a memory allocation failure after
anon_vma_clone() has succeeded, unlink_anon_vmas() is called with
vma->anon_vma being NULL but the anon_vma_chain populated with entries
that are present in the anon_vma interval trees.

This happens in the following sequence:
1. anon_vma_clone() succeeds, populating vma->anon_vma_chain and
   inserting entries into interval trees
2. maybe_reuse_anon_vma() does not set vma->anon_vma because reuse
   conditions are not met (common case for active processes)
3. anon_vma_alloc() or anon_vma_chain_alloc() fails due to memory
   pressure
4. Error path invokes unlink_anon_vmas() with vma->anon_vma == NULL

The existing code triggered VM_WARN_ON_ONCE and returned without
performing cleanup, leaving entries in interval trees and causing
memory leaks.

Fix this by detecting the condition and properly cleaning up:
- Iterate through the populated chain
- Lock each anon_vma
- Remove entries from interval trees
- Unlock and free chain entries

This prevents both the warning and the resource leaks.

Reported-by: syzbot+c27fa543e10a45d4e149@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c27fa543e10a45d4e149
Tested-by: syzbot+c27fa543e10a45d4e149@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 mm/rmap.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index f13480cb9f2e..acc8df6ad4a7 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -477,7 +477,31 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 
 	/* Unfaulted is a no-op. */
 	if (!active_anon_vma) {
-		VM_WARN_ON_ONCE(!list_empty(&vma->anon_vma_chain));
+		/*
+		 * Handle anon_vma_fork() error path where anon_vma_clone()
+		 * succeeded and populated the chain (with entries in interval
+		 * trees), but maybe_reuse_anon_vma() didn't set vma->anon_vma
+		 * because reuse conditions weren't met, and a later allocation
+		 * failed before we could allocate and assign a new anon_vma.
+		 *
+		 * We must properly remove entries from interval trees before
+		 * freeing to avoid leaving dangling pointers.
+		 */
+		if (!list_empty(&vma->anon_vma_chain)) {
+			struct anon_vma_chain *avc, *next;
+
+			list_for_each_entry_safe(avc, next, &vma->anon_vma_chain,
+						same_vma) {
+				struct anon_vma *anon_vma = avc->anon_vma;
+
+				anon_vma_lock_write(anon_vma);
+				anon_vma_interval_tree_remove(avc, &anon_vma->rb_root);
+				anon_vma_unlock_write(anon_vma);
+				list_del(&avc->same_vma);
+				anon_vma_chain_free(avc);
+			}
+		}
+
 		return;
 	}
 
-- 
2.43.0