From 3356f2f7d2fd451b1412fe15d3910c64fc7d5220 Mon Sep 17 00:00:00 2001 From: Suren Baghdasaryan Date: Mon, 4 Aug 2025 16:33:49 -0700 Subject: [PATCH] mm: change vma_start_read() to drop RCU lock on failure vma_start_read() can drop and reacquire RCU lock in certain failure cases. It's not apparent that the RCU session started by the caller of this function might be interrupted when vma_start_read() fails to lock the vma. This might become a source of subtle bugs and to prevent that we change the locking rules for vma_start_read() to drop RCU read lock upon failure. This way it's more obvious that RCU-protected objects are unsafe after vma locking fails. Link: https://lkml.kernel.org/r/20250804233349.1278678-2-surenb@google.com Suggested-by: Vlastimil Babka Signed-off-by: Suren Baghdasaryan Tested-by: Lorenzo Stoakes Reviewed-by: Vlastimil Babka Reviewed-by: Lorenzo Stoakes Cc: Jann Horn Cc: Liam Howlett Cc: Suren Baghdasaryan Signed-off-by: Andrew Morton --- mm/mmap_lock.c | 84 +++++++++++++++++++++++++++----------------------- 1 file changed, 45 insertions(+), 39 deletions(-) diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c index 10826f347a9f7..0a0db5849b8e7 100644 --- a/mm/mmap_lock.c +++ b/mm/mmap_lock.c @@ -136,15 +136,16 @@ void vma_mark_detached(struct vm_area_struct *vma) * Returns the vma on success, NULL on failure to lock and EAGAIN if vma got * detached. * - * WARNING! The vma passed to this function cannot be used if the function - * fails to lock it because in certain cases RCU lock is dropped and then - * reacquired. Once RCU lock is dropped the vma can be concurently freed. + * IMPORTANT: RCU lock must be held upon entering the function, but upon error + * IT IS RELEASED. The caller must handle this correctly. */ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm, struct vm_area_struct *vma) { + struct mm_struct *other_mm; int oldcnt; + RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "no rcu lock held"); /* * Check before locking. A race might cause false locked result. * We can use READ_ONCE() for the mm_lock_seq here, and don't need @@ -152,8 +153,10 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm, * we don't rely on for anything - the mm_lock_seq read against which we * need ordering is below. */ - if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(mm->mm_lock_seq.sequence)) - return NULL; + if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(mm->mm_lock_seq.sequence)) { + vma = NULL; + goto err; + } /* * If VMA_LOCK_OFFSET is set, __refcount_inc_not_zero_limited_acquire() @@ -164,34 +167,14 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm, if (unlikely(!__refcount_inc_not_zero_limited_acquire(&vma->vm_refcnt, &oldcnt, VMA_REF_LIMIT))) { /* return EAGAIN if vma got detached from under us */ - return oldcnt ? NULL : ERR_PTR(-EAGAIN); + vma = oldcnt ? NULL : ERR_PTR(-EAGAIN); + goto err; } rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_); - /* - * If vma got attached to another mm from under us, that mm is not - * stable and can be freed in the narrow window after vma->vm_refcnt - * is dropped and before rcuwait_wake_up(mm) is called. Grab it before - * releasing vma->vm_refcnt. - */ - if (unlikely(vma->vm_mm != mm)) { - /* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */ - struct mm_struct *other_mm = vma->vm_mm; - - /* - * __mmdrop() is a heavy operation and we don't need RCU - * protection here. Release RCU lock during these operations. - * We reinstate the RCU read lock as the caller expects it to - * be held when this function returns even on error. - */ - rcu_read_unlock(); - mmgrab(other_mm); - vma_refcount_put(vma); - mmdrop(other_mm); - rcu_read_lock(); - return NULL; - } + if (unlikely(vma->vm_mm != mm)) + goto err_unstable; /* * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result. @@ -206,10 +189,31 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm, */ if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&mm->mm_lock_seq))) { vma_refcount_put(vma); - return NULL; + vma = NULL; + goto err; } return vma; +err: + rcu_read_unlock(); + + return vma; +err_unstable: + /* + * If vma got attached to another mm from under us, that mm is not + * stable and can be freed in the narrow window after vma->vm_refcnt + * is dropped and before rcuwait_wake_up(mm) is called. Grab it before + * releasing vma->vm_refcnt. + */ + other_mm = vma->vm_mm; /* use a copy as vma can be freed after we drop vm_refcnt */ + + /* __mmdrop() is a heavy operation, do it after dropping RCU lock. */ + rcu_read_unlock(); + mmgrab(other_mm); + vma_refcount_put(vma); + mmdrop(other_mm); + + return NULL; } /* @@ -223,11 +227,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, MA_STATE(mas, &mm->mm_mt, address, address); struct vm_area_struct *vma; - rcu_read_lock(); retry: + rcu_read_lock(); vma = mas_walk(&mas); - if (!vma) + if (!vma) { + rcu_read_unlock(); goto inval; + } vma = vma_start_read(mm, vma); if (IS_ERR_OR_NULL(vma)) { @@ -247,18 +253,17 @@ retry: * From here on, we can access the VMA without worrying about which * fields are accessible for RCU readers. */ + rcu_read_unlock(); /* Check if the vma we locked is the right one. */ - if (unlikely(address < vma->vm_start || address >= vma->vm_end)) - goto inval_end_read; + if (unlikely(address < vma->vm_start || address >= vma->vm_end)) { + vma_end_read(vma); + goto inval; + } - rcu_read_unlock(); return vma; -inval_end_read: - vma_end_read(vma); inval: - rcu_read_unlock(); count_vm_vma_lock_event(VMA_LOCK_ABORT); return NULL; } @@ -313,6 +318,7 @@ retry: */ if (PTR_ERR(vma) == -EAGAIN) { /* reset to search from the last address */ + rcu_read_lock(); vma_iter_set(vmi, from_addr); goto retry; } @@ -342,9 +348,9 @@ retry: return vma; fallback_unlock: + rcu_read_unlock(); vma_end_read(vma); fallback: - rcu_read_unlock(); vma = lock_next_vma_under_mmap_lock(mm, vmi, from_addr); rcu_read_lock(); /* Reinitialize the iterator after re-entering rcu read section */ -- 2.51.0