1 files changed, 7 insertions, 1 deletions

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 25b8de0f21c0..a092a0cce0b7 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -894,7 +894,13 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
 
 #else
 /*
- * User space process size. 47bits minus one guard page.
+ * User space process size. 47bits minus one guard page.  The guard
+ * page is necessary on Intel CPUs: if a SYSCALL instruction is at
+ * the highest possible canonical userspace address, then that
+ * syscall will enter the kernel with a non-canonical return
+ * address, and SYSRET will explode dangerously.  We avoid this
+ * particular problem by preventing anything from being mapped
+ * at the maximum canonical address.
  */
 #define TASK_SIZE_MAX   ((1UL << 47) - PAGE_SIZE)