[PATCH] kprobes: Fix return probes on sys_execve

Fix a bug in kprobes that can cause an Oops or even a crash when a return
probe is installed on one of the following functions: sys_execve,
do_execve, load_*_binary, flush_old_exec, or flush_thread. The fix is to
remove the call to kprobe_flush_task() in flush_thread(). This fix has
been tested on all architectures for which the return-probes feature has
been implemented (i386, x86_64, ppc64, ia64). Please apply.

BACKGROUND

Up to now, we have called kprobe_flush_task() under two situations: when a
task exits, and when it execs. Flushing kretprobe_instances on exit is
correct because (a) do_exit() doesn't return, and (b) one or more
return-probed functions may be active when a task calls do_exit(). Neither
is the case for sys_execve() and its callees.

Initially, the mistaken call to kprobe_flush_task() on exec was harmless
because we put the "real" return address of each active probed function
back in the stack, just to be safe, when we recycled its
kretprobe_instance. When support for ppc64 and ia64 was added, this safety
measure couldn't be employed, and was eventually dropped even for i386 and
x86_64. sys_execve() and its callees were informally blacklisted for
return probes until this fix was developed.

Acked-by: Prasanna S Panchamukhi <[e-mail]>
Signed-off-by: Jim Keniston <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/i386/kernel/process.c

···
 {
 	struct task_struct *tsk = current;
 
-	/*
-	 * Remove function-return probe instances associated with this task
-	 * and put them back on the free list. Do not insert an exit probe for
-	 * this function, it will be disabled by kprobe_flush_task if you do.
-	 */
-	kprobe_flush_task(tsk);
-
 	memset(tsk->thread.debugreg, 0, sizeof(unsigned long)*8);
 	memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));	
 	/*

arch/ia64/kernel/process.c

···
 void
 flush_thread (void)
 {
-	/*
-	 * Remove function-return probe instances associated with this task
-	 * and put them back on the free list. Do not insert an exit probe for
-	 * this function, it will be disabled by kprobe_flush_task if you do.
-	 */
-	kprobe_flush_task(current);
-
 	/* drop floating-point and debug-register state if it exists: */
 	current->thread.flags &= ~(IA64_THREAD_FPH_VALID | IA64_THREAD_DBG_VALID);
 	ia64_drop_fpu(current);

arch/powerpc/kernel/process.c

···
 	if (t->flags & _TIF_ABI_PENDING)
 		t->flags ^= (_TIF_ABI_PENDING | _TIF_32BIT);
 #endif
-	kprobe_flush_task(current);
 
 #ifndef CONFIG_SMP
 	if (last_task_used_math == current)

arch/x86_64/kernel/process.c

···
 	struct task_struct *tsk = current;
 	struct thread_info *t = current_thread_info();
 
-	/*
-	 * Remove function-return probe instances associated with this task
-	 * and put them back on the free list. Do not insert an exit probe for
-	 * this function, it will be disabled by kprobe_flush_task if you do.
-	 */
-	kprobe_flush_task(tsk);
-
 	if (t->flags & _TIF_ABI_PENDING)
 		t->flags ^= (_TIF_ABI_PENDING | _TIF_IA32);