x86, ptrace, mm: fix double-free on race
Ptrace_detach() races with __ptrace_unlink() if the traced task is
reaped while detaching. This might cause a double-free of the BTS
buffer.
Change the ptrace_detach() path to only do the memory accounting in
ptrace_bts_detach() and leave the buffer free to ptrace_bts_untrace()
which will be called from __ptrace_unlink().
The fix follows a proposal from Oleg Nesterov.
Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 0a5df5f..5a4c23d 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -810,12 +810,16 @@
static void ptrace_bts_detach(struct task_struct *child)
{
- if (unlikely(child->bts)) {
- ds_release_bts(child->bts);
- child->bts = NULL;
-
- ptrace_bts_free_buffer(child);
- }
+ /*
+ * Ptrace_detach() races with ptrace_untrace() in case
+ * the child dies and is reaped by another thread.
+ *
+ * We only do the memory accounting at this point and
+ * leave the buffer deallocation and the bts tracer
+ * release to ptrace_bts_untrace() which will be called
+ * later on with tasklist_lock held.
+ */
+ release_locked_buffer(child->bts_buffer, child->bts_size);
}
#else
static inline void ptrace_bts_fork(struct task_struct *tsk) {}