mmap locking API: don't check locking if the mm isn't live yet

In preparation for adding a mmap_assert_locked() check in
__get_user_pages(), teach the mmap_assert_*locked() helpers that it's fine
to operate on an mm without locking in the middle of execve() as long as
it hasn't been installed on a process yet.

Existing code paths that do this are (reverse callgraph):

  get_user_pages_remote
    get_arg_page
      copy_strings
      copy_string_kernel
      remove_arg_zero
    tomoyo_dump_page
      tomoyo_print_bprm
      tomoyo_scan_bprm
      tomoyo_environ

Link: https://lkml.kernel.org/r/CAG48ez03YJG9JU_6tGiMcaVjuTyRE_o4LEQ7901b5ZoCnNAjcg@mail.gmail.com
Signed-off-by: Jann Horn <[email protected]>
Cc: "Eric W . Biederman" <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Sakari Ailus <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Stephen Rothwell <[email protected]>

fs/exec.c

@@ -1000,6 +1000,14 @@ static int exec_mmap(struct mm_struct *mm)
 		}
 	}
 
+#if defined(CONFIG_LOCKDEP) || defined(CONFIG_DEBUG_VM)
+	/*
+	 * From here on, the mm may be accessed concurrently, and proper locking
+	 * is required for things like get_user_pages_remote().
+	 */
+	mm->mmap_lock_required = 1;
+#endif
+
 	task_lock(tsk);
 	membarrier_exec_mmap(mm);
 

include/linux/mm_types.h

@@ -556,6 +556,16 @@ struct mm_struct {
 #ifdef CONFIG_IOMMU_SUPPORT
 		u32 pasid;
 #endif
+
+#if defined(CONFIG_LOCKDEP) || defined(CONFIG_DEBUG_VM)
+		/*
+		 * Notes whether this mm has been installed on a process yet.
+		 * If not, only the task going through execve() can access this
+		 * mm, and no locking is needed around get_user_pages_remote().
+		 * This flag is only used for debug checks.
+		 */
+		bool mmap_lock_required;
+#endif
 	} __randomize_layout;
 
 	/*

include/linux/mmap_lock.h

@@ -77,14 +77,22 @@ static inline void mmap_read_unlock_non_owner(struct mm_struct *mm)
 
 static inline void mmap_assert_locked(struct mm_struct *mm)
 {
-	lockdep_assert_held(&mm->mmap_lock);
-	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
+#if defined(CONFIG_LOCKDEP) || defined(CONFIG_DEBUG_VM)
+	if (mm->mmap_lock_required) {
+		lockdep_assert_held(&mm->mmap_lock);
+		VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
+	}
+#endif
 }
 
 static inline void mmap_assert_write_locked(struct mm_struct *mm)
 {
-	lockdep_assert_held_write(&mm->mmap_lock);
-	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
+#if defined(CONFIG_LOCKDEP) || defined(CONFIG_DEBUG_VM)
+	if (mm->mmap_lock_required) {
+		lockdep_assert_held_write(&mm->mmap_lock);
+		VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
+	}
+#endif
 }
 
 static inline int mmap_lock_is_contended(struct mm_struct *mm)