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 <jannh@google.com>
Cc: "Eric W . Biederman" <ebiederm@xmission.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Michel Lespinasse <walken@google.com>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

diff --git a/fs/exec.c b/fs/exec.c
index 547a2390baf54f4e285fb41d3488025f1954aed9..f7009df270aa7fb29f78d1b6aad33de805ef147f 100644 (file)
--- a/fs/exec.c
+++ b/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);
 

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 80f5d755c037430883dcbc2d0af701cbb92fc16e..6a6b078b9d6aba69f0b38f7d85bbb417f815244e 100644 (file)
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -553,6 +553,16 @@ 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;
 
        /*

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 18e7eae9b5ba1740bbba91811bede4fff04e291b..e1afb420fc94f7b159aa83b37e4fe8d62036f00f 100644 (file)
--- a/include/linux/mmap_lock.h
+++ b/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)