1 files changed, 28 insertions, 0 deletions

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 59f4524984a..00a8fe4c58b 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -58,6 +58,8 @@
 #include <asm/idle.h>
 #include <asm/syscalls.h>
 #include <asm/ds.h>
+#include <asm/debugreg.h>
+#include <asm/hw_breakpoint.h>
 
 asmlinkage void ret_from_fork(void) __asm__("ret_from_fork");
 
@@ -262,7 +264,13 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
 
         task_user_gs(p) = get_user_gs(regs);
 
+        p->thread.io_bitmap_ptr = NULL;
         tsk = current;
+        err = -ENOMEM;
+        if (unlikely(test_tsk_thread_flag(tsk, TIF_DEBUG)))
+                if (copy_thread_hw_breakpoint(tsk, p, clone_flags))
+                        goto out;
+
         if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {
                 p->thread.io_bitmap_ptr = kmemdup(tsk->thread.io_bitmap_ptr,
                                                 IO_BITMAP_BYTES, GFP_KERNEL);
@@ -282,10 +290,13 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
                 err = do_set_thread_area(p, -1,
                         (struct user_desc __user *)childregs->si, 0);
 
+out:
         if (err && p->thread.io_bitmap_ptr) {
                 kfree(p->thread.io_bitmap_ptr);
                 p->thread.io_bitmap_max = 0;
         }
+        if (err)
+                flush_thread_hw_breakpoint(p);
 
         clear_tsk_thread_flag(p, TIF_DS_AREA_MSR);
         p->thread.ds_ctx = NULL;
@@ -424,6 +435,23 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
                 lazy_load_gs(next->gs);
 
         percpu_write(current_task, next_p);
+        /*
+         * There's a problem with moving the arch_install_thread_hw_breakpoint()
+         * call before current is updated.  Suppose a kernel breakpoint is
+         * triggered in between the two, the hw-breakpoint handler will see that
+         * the 'current' task does not have TIF_DEBUG flag set and will think it
+         * is leftover from an old task (lazy switching) and will erase it. Then
+         * until the next context switch, no user-breakpoints will be installed.
+         *
+         * The real problem is that it's impossible to update both current and
+         * physical debug registers at the same instant, so there will always be
+         * a window in which they disagree and a breakpoint might get triggered.
+         * Since we use lazy switching, we are forced to assume that a
+         * disagreement means that current is correct and the exception is due
+         * to lazy debug register switching.
+         */
+        if (unlikely(test_tsk_thread_flag(next_p, TIF_DEBUG)))
+                arch_install_thread_hw_breakpoint(next_p);
 
         return prev_p;
 }