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authorIra Weiny <ira.weiny@intel.com>2022-10-23 21:34:52 -0700
committerAndrew Morton <akpm@linux-foundation.org>2022-10-28 13:37:23 -0700
commit5521de7dddd211e3a9403d7bde0b614fd0936ac6 (patch)
tree7a81377409f7c736b4b7875bf0b2cb55b0ac81d9 /mm
parent78a498c3a227f2ac773a8234b2ce092a4403f2c3 (diff)
mm/userfaultfd: replace kmap/kmap_atomic() with kmap_local_page()
kmap() and kmap_atomic() are being deprecated in favor of kmap_local_page() which is appropriate for any thread local context.[1] A recent locking bug report with userfaultfd showed that the conversion of the kmap_atomic()'s in those code flows requires care with regard to the prevention of deadlock.[2] git archaeology implied that the recursion may not be an actual bug.[3] However, depending on the implementation of the mmap_lock and the condition of the call there may still be a deadlock.[4] So this is not purely a lockdep issue. Considering a single threaded call stack there are 3 options. 1) Different mm's are in play (no issue) 2) Readlock implementation is recursive and same mm is in play (no issue) 3) Readlock implementation is _not_ recursive (issue) The mmap_lock is recursive so with a single thread there is no issue. However, Matthew pointed out a deadlock scenario when you consider additional process' and threads thusly. "The readlock implementation is only recursive if nobody else has taken a write lock. If you have a multithreaded process, one of the other threads can call mmap() and that will prevent recursion (due to fairness). Even if it's a different process that you're trying to acquire the mmap read lock on, you can still get into a deadly embrace. eg: process A thread 1 takes read lock on own mmap_lock process A thread 2 calls mmap, blocks taking write lock process B thread 1 takes page fault, read lock on own mmap lock process B thread 2 calls mmap, blocks taking write lock process A thread 1 blocks taking read lock on process B process B thread 1 blocks taking read lock on process A Now all four threads are blocked waiting for each other." Regardless using pagefault_disable() ensures that no matter what locking implementation is used a deadlock will not occur. Complete kmap conversion in userfaultfd by replacing the kmap() and kmap_atomic() calls with kmap_local_page(). When replacing the kmap_atomic() call ensure page faults continue to be disabled to support the correct fall back behavior and add a comment to inform future souls of the requirement. [1] https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com/ [2] https://lore.kernel.org/all/Y1Mh2S7fUGQ%2FiKFR@iweiny-desk3/ [3] https://lore.kernel.org/all/Y1MymJ%2FINb45AdaY@iweiny-desk3/ [4] https://lore.kernel.org/lkml/Y1bXBtGTCym77%2FoD@casper.infradead.org/ [ira.weiny@intel.com: v2] Link: https://lkml.kernel.org/r/20221025220136.2366143-1-ira.weiny@intel.com Link: https://lkml.kernel.org/r/20221024043452.1491677-1-ira.weiny@intel.com Signed-off-by: Ira Weiny <ira.weiny@intel.com> Cc: Matthew Wilcox <willy@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Peter Xu <peterx@redhat.com> Cc: Axel Rasmussen <axelrasmussen@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'mm')
-rw-r--r--mm/userfaultfd.c25
1 files changed, 21 insertions, 4 deletions
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index e24e8a47ce8a..3d0fef3980b3 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -157,11 +157,28 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
if (!page)
goto out;
- page_kaddr = kmap_atomic(page);
+ page_kaddr = kmap_local_page(page);
+ /*
+ * The read mmap_lock is held here. Despite the
+ * mmap_lock being read recursive a deadlock is still
+ * possible if a writer has taken a lock. For example:
+ *
+ * process A thread 1 takes read lock on own mmap_lock
+ * process A thread 2 calls mmap, blocks taking write lock
+ * process B thread 1 takes page fault, read lock on own mmap lock
+ * process B thread 2 calls mmap, blocks taking write lock
+ * process A thread 1 blocks taking read lock on process B
+ * process B thread 1 blocks taking read lock on process A
+ *
+ * Disable page faults to prevent potential deadlock
+ * and retry the copy outside the mmap_lock.
+ */
+ pagefault_disable();
ret = copy_from_user(page_kaddr,
(const void __user *) src_addr,
PAGE_SIZE);
- kunmap_atomic(page_kaddr);
+ pagefault_enable();
+ kunmap_local(page_kaddr);
/* fallback to copy_from_user outside mmap_lock */
if (unlikely(ret)) {
@@ -646,11 +663,11 @@ retry:
mmap_read_unlock(dst_mm);
BUG_ON(!page);
- page_kaddr = kmap(page);
+ page_kaddr = kmap_local_page(page);
err = copy_from_user(page_kaddr,
(const void __user *) src_addr,
PAGE_SIZE);
- kunmap(page);
+ kunmap_local(page_kaddr);
if (unlikely(err)) {
err = -EFAULT;
goto out;