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-rw-r--r--mm/shmem.c17
1 files changed, 17 insertions, 0 deletions
diff --git a/mm/shmem.c b/mm/shmem.c
index 8280a5cb48df..c1d8b8a1aa3b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2424,9 +2424,26 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
if (!zeropage) { /* COPY */
page_kaddr = kmap_local_folio(folio, 0);
+ /*
+ * 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);
+ pagefault_enable();
kunmap_local(page_kaddr);
/* fallback to copy_from_user outside mmap_lock */