mm: don't drop VMA locks in mm_drop_all_locks()

Despite its name, mm_drop_all_locks() does not drop _all_ locks; the mmap
lock is held write-locked by the caller, and the caller is responsible for
dropping the mmap lock at a later point (which will also release the VMA
locks).

Calling vma_end_write_all() here is dangerous because the caller might
have write-locked a VMA with the expectation that it will stay
write-locked until the mmap_lock is released, as usual.

This _almost_ becomes a problem in the following scenario:

An anonymous VMA A and an SGX VMA B are mapped adjacent to each other.
Userspace calls munmap() on a range starting at the start address of A and
ending in the middle of B.

Hypothetical call graph with additional notes in brackets:

do_vmi_align_munmap
  [begin first for_each_vma_range loop]
  vma_start_write [on VMA A]
  vma_mark_detached [on VMA A]
  __split_vma [on VMA B]
    sgx_vma_open [== new->vm_ops->open]
      sgx_encl_mm_add
        __mmu_notifier_register [luckily THIS CAN'T ACTUALLY HAPPEN]
          mm_take_all_locks
          mm_drop_all_locks
            vma_end_write_all [drops VMA lock taken on VMA A before]
  vma_start_write [on VMA B]
  vma_mark_detached [on VMA B]
  [end first for_each_vma_range loop]
  vma_iter_clear_gfp [removes VMAs from maple tree]
  mmap_write_downgrade
  unmap_region
  mmap_read_unlock

In this hypothetical scenario, while do_vmi_align_munmap() thinks it still
holds a VMA write lock on VMA A, the VMA write lock has actually been
invalidated inside __split_vma().

The call from sgx_encl_mm_add() to __mmu_notifier_register() can't
actually happen here, as far as I understand, because we are duplicating
an existing SGX VMA, but sgx_encl_mm_add() only calls
__mmu_notifier_register() for the first SGX VMA created in a given
process.  So this could only happen in fork(), not on munmap().  But in my
view it is just pure luck that this can't happen.

Also, we wouldn't actually have any bad consequences from this in
do_vmi_align_munmap(), because by the time the bug drops the lock on VMA
A, we've already marked VMA A as detached, which makes it completely
ineligible for any VMA-locked page faults.  But again, that's just pure
luck.

So remove the vma_end_write_all(), so that VMA write locks are only ever
released on mmap_write_unlock() or mmap_write_downgrade().

Also add comments to document the locking rules established by this patch.

Link: https://lkml.kernel.org/r/20230720193436.454247-1-jannh@google.com
Fixes: eeff9a5d47f8 ("mm/mmap: prevent pagefault handler from racing with mmu_notifier registration")
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 84988d4ff8fb2b18e135a317bd7768c856747d0c..93eb291181f799bfdb0da7f0567936b2a96c1011 100644 (file)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -691,6 +691,11 @@ static bool __is_vma_write_locked(struct vm_area_struct *vma, int *mm_lock_seq)
        return (vma->vm_lock_seq == *mm_lock_seq);
 }
 
+/*
+ * Begin writing to a VMA.
+ * Exclude concurrent readers under the per-VMA lock until the currently
+ * write-locked mmap_lock is dropped or downgraded.
+ */
 static inline void vma_start_write(struct vm_area_struct *vma)
 {
        int mm_lock_seq;

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index a5c63b6d7d46c1b64f770607373657d9a7b07ee5..8d38dcb6d044cb73ac9338515f73e69817622fd9 100644 (file)
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -73,6 +73,14 @@ static inline void mmap_assert_write_locked(struct mm_struct *mm)
 }
 
 #ifdef CONFIG_PER_VMA_LOCK
+/*
+ * Drop all currently-held per-VMA locks.
+ * This is called from the mmap_lock implementation directly before releasing
+ * a write-locked mmap_lock (or downgrading it to read-locked).
+ * This should normally NOT be called manually from other places.
+ * If you want to call this manually anyway, keep in mind that this will release
+ * *all* VMA write locks, including ones from further up the stack.
+ */
 static inline void vma_end_write_all(struct mm_struct *mm)
 {
        mmap_assert_write_locked(mm);

diff --git a/mm/mmap.c b/mm/mmap.c
index 7bd1caa09dddf072ccfe16ec30f38d9fe46a4225..4a9466b76648375699aec359090a7a287535ac38 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3642,6 +3642,12 @@ int mm_take_all_locks(struct mm_struct *mm)
 
        mutex_lock(&mm_all_locks_mutex);
 
+       /*
+        * vma_start_write() does not have a complement in mm_drop_all_locks()
+        * because vma_start_write() is always asymmetrical; it marks a VMA as
+        * being written to until mmap_write_unlock() or mmap_write_downgrade()
+        * is reached.
+        */
        mas_for_each(&mas, vma, ULONG_MAX) {
                if (signal_pending(current))
                        goto out_unlock;
@@ -3738,7 +3744,6 @@ void mm_drop_all_locks(struct mm_struct *mm)
                if (vma->vm_file && vma->vm_file->f_mapping)
                        vm_unlock_mapping(vma->vm_file->f_mapping);
        }
-       vma_end_write_all(mm);
 
        mutex_unlock(&mm_all_locks_mutex);
 }