[patch] x86, ptrace: fix double-free on race

[Markus Metzger]

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@xxxxxxxxxx>
Signed-off-by: Markus Metzger <markus.t.metzger@xxxxxxxxx>
---

Index: gits/arch/x86/kernel/ptrace.c
===================================================================
--- gits.orig/arch/x86/kernel/ptrace.c	2009-01-22 13:09:37.000000000 +0100
+++ gits/arch/x86/kernel/ptrace.c	2009-02-11 08:55:49.000000000 +0100
@@ -810,12 +810,14 @@
 
 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) {}
Index: gits/include/linux/mm.h
===================================================================
--- gits.orig/include/linux/mm.h	2009-02-11 08:40:37.000000000 +0100
+++ gits/include/linux/mm.h	2009-02-11 08:51:28.000000000 +0100
@@ -1304,5 +1304,6 @@
 
 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 */
Index: gits/mm/mlock.c
===================================================================
--- gits.orig/mm/mlock.c	2009-02-11 08:40:37.000000000 +0100
+++ gits/mm/mlock.c	2009-02-11 08:49:17.000000000 +0100
@@ -660,7 +660,7 @@
 	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;
 
@@ -670,6 +670,11 @@
 	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);
 }
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