[PATCH bpf-next v5] bpf: Fix use-after-free on mm_struct in bpf_find_vma()

[Sanghyun Park]

bpf_find_vma() reads task->mm and calls mmap_read_trylock(mm) without
holding a reference on the mm. On a foreign task, a concurrent exit_mm()
can free the mm_struct between the lockless read and the trylock,
resulting in a use-after-free. mm_struct is not SLAB_TYPESAFE_BY_RCU.

For the current task, task->mm is stable. For a foreign task, pin the mm
under task->alloc_lock and release it with mmput_async(), mirroring commit
d8e27d2d22b6 ("bpf: fix mm lifecycle in open-coded task_vma iterator").
Use spin_trylock() instead of get_task_mm() so BPF context does not block
on alloc_lock. Reject irqs-disabled contexts and !CONFIG_MMU on the
foreign-task path because dropping the mm reference is not safe there.

Race:

  CPU0 (BPF program)                  CPU1 (exiting task)
  ============================        ==========================
  bpf_find_vma(foreign_task):
    mm = task->mm
                                      exit_mm():
                                        task->mm = NULL
                                        mmput(mm) -> frees mm_struct
    mmap_read_trylock(mm)
        // UAF on mm

Fixes: 7c7e3d31e785 ("bpf: Introduce helper bpf_find_vma")
Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
Signed-off-by: Sanghyun Park <sanghyun.park.cnu@xxxxxxxxx>
---
v5:
 - Replace bpf_iter_mmput_async() with mmput_async() directly as suggested
   by Yonghong.
 - Add Yonghong's Acked-by.
v4: https://lore.kernel.org/bpf/20260610024637.343364-1-sanghyun.park.cnu@xxxxxxxxx/
 - Use [PATCH bpf-next] subject as requested by Alexei.
 - Add the missing BPF maintainers/reviewers to Cc.
v3: https://lore.kernel.org/bpf/20260609105216.3536839-1-sanghyun.park.cnu@xxxxxxxxx/
 - Drop get_task_mm()+mmput(); mirror d8e27d2d22b6 with alloc_lock
   trylock + mmput_async(). (Yonghong Song)
 - Reject irqs-disabled contexts on the foreign-task path.
 - Reject foreign-task path when !CONFIG_MMU: bpf_iter_mmput_async()
   falls back to mmput() which may sleep, and bpf_find_vma() can run
   in non-sleepable context.
 - Shorten the foreign-task rationale comment and trim the changelog body.
 - Fix the v2's whitespace damage.
v2: https://lore.kernel.org/bpf/CAOrxSK5_7e4114VyfEU9htGi+UneuNt88fGVKOAa3_ZenPOFkA@xxxxxxxxxxxxxx/

 kernel/bpf/task_iter.c | 43 ++++++++++++++++++++++++++++++------------
 1 file changed, 31 insertions(+), 12 deletions(-)

diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index e791ae065c39b..27ceead874651 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -756,6 +756,7 @@ BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,
 	struct mmap_unlock_irq_work *work = NULL;
 	struct vm_area_struct *vma;
 	bool irq_work_busy = false;
+	bool mmput_needed = false;
 	struct mm_struct *mm;
 	int ret = -ENOENT;
 
@@ -765,14 +766,38 @@ BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,
 	if (!task)
 		return -ENOENT;
 
-	mm = task->mm;
+	if (task == current) {
+		mm = task->mm;
+	} else {
+		/*
+		 * Foreign task: pin task->mm against a concurrent exit_mm().
+		 * Use trylock on alloc_lock instead of get_task_mm()'s
+		 * blocking task_lock() to avoid deadlocking the target task.
+		 */
+		if (!IS_ENABLED(CONFIG_MMU))
+			return -EOPNOTSUPP;
+		if (irqs_disabled())
+			return -EBUSY;
+		if (!spin_trylock(&task->alloc_lock))
+			return -EBUSY;
+		mm = task->mm;
+		if (mm && !(task->flags & PF_KTHREAD)) {
+			mmget(mm);
+			mmput_needed = true;
+		} else {
+			mm = NULL;
+		}
+		spin_unlock(&task->alloc_lock);
+	}
 	if (!mm)
 		return -ENOENT;
 
 	irq_work_busy = bpf_mmap_unlock_get_irq_work(&work);
 
-	if (irq_work_busy || !mmap_read_trylock(mm))
-		return -EBUSY;
+	if (irq_work_busy || !mmap_read_trylock(mm)) {
+		ret = -EBUSY;
+		goto out;
+	}
 
 	vma = find_vma(mm, start);
 
@@ -782,6 +807,9 @@ BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,
 		ret = 0;
 	}
 	bpf_mmap_unlock_mm(work, mm);
+out:
+	if (mmput_needed)
+		mmput_async(mm);
 	return ret;
 }
 
@@ -796,15 +824,6 @@ const struct bpf_func_proto bpf_find_vma_proto = {
 	.arg5_type	= ARG_ANYTHING,
 };
 
-static inline void bpf_iter_mmput_async(struct mm_struct *mm)
-{
-#ifdef CONFIG_MMU
-	mmput_async(mm);
-#else
-	mmput(mm);
-#endif
-}
-
 struct bpf_iter_task_vma_kern_data {
 	struct task_struct *task;
 	struct mm_struct *mm;
-- 
2.48.1