From 47b079e2117a2ee52e21f8b72935900c702fc0b5 Mon Sep 17 00:00:00 2001 From: Sanghyun Park Date: Wed, 8 Jul 2026 16:21:04 +0900 Subject: [PATCH] bpf: Fix use-after-free on mm_struct in bpf_find_vma() 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") Signed-off-by: Sanghyun Park Reviewed-by: Puranjay Mohan Acked-by: Yonghong Song Link: https://lore.kernel.org/bpf/20260708072106.199637-2-sanghyun.park.cnu@gmail.com Signed-off-by: Kumar Kartikeya Dwivedi --- kernel/bpf/task_iter.c | 36 +++++++++++++++++++++++++++++++++--- 1 file changed, 33 insertions(+), 3 deletions(-) diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c index e791ae065c39..b256fb9c1214 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 __maybe_unused 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 +807,11 @@ BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start, ret = 0; } bpf_mmap_unlock_mm(work, mm); +out: +#ifdef CONFIG_MMU + if (mmput_needed) + mmput_async(mm); +#endif return ret; }