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>

3 files changed, 17 insertions, 7 deletions

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 0a5df5f82fb9..5a4c23d89892 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -810,12 +810,16 @@ static void ptrace_bts_untrace(struct task_struct *child)
 
 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) {}
diff --git a/include/linux/mm.h b/include/linux/mm.h
index e8ddc98b8405..3d7fb44d7d7e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1305,5 +1305,6 @@ void vmemmap_populate_print_last(void);
 
 extern void *alloc_locked_buffer(size_t size);
 extern void free_locked_buffer(void *buffer, size_t size);
+extern void release_locked_buffer(void *buffer, size_t size);
 #endif /* __KERNEL__ */
 #endif /* _LINUX_MM_H */
diff --git a/mm/mlock.c b/mm/mlock.c
index 028ec482fdd4..2b57f7e60390 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -657,7 +657,7 @@ void *alloc_locked_buffer(size_t size)
         return buffer;
 }
 
-void free_locked_buffer(void *buffer, size_t size)
+void release_locked_buffer(void *buffer, size_t size)
 {
         unsigned long pgsz = PAGE_ALIGN(size) >> PAGE_SHIFT;
 
@@ -667,6 +667,11 @@ void free_locked_buffer(void *buffer, size_t size)
         current->mm->locked_vm -= pgsz;
 
         up_write(&current->mm->mmap_sem);
+}
+
+void free_locked_buffer(void *buffer, size_t size)
+{
+        release_locked_buffer(buffer, size);
 
         kfree(buffer);
 }