[PATCH 4/7] uretprobes: return probe entry, prepare_uretprobe()

From: Anton Arapov
Date: Fri Mar 22 2013 - 09:09:53 EST

When a uprobe with return probe consumer is hit, prepare_uretprobe()
function is invoked. It creates return_instance, hijacks return address
and replaces it with the trampoline.

- Return instances are kept as stack per uprobed task.
- Return instance is dirty, when the original return address is
trampoline's page vaddr (e.g. recursive call of the probed function).

N.B. it might be a good idea to introduce get_uprobe() to reflect
put_uprobe() later, but it is not a subject of this patchset.

RFCv6 changes:
- rework prepare_uretprobe() logic in order to make further unwinding
in handler_uretprobe() simplier.
- introduce the 'dirty' field.

RFCv5 changes:
- switch from hlist to simply linked list for tracking ->*return_uprobes.
- revert (*) from v4.
- preallocate first slot xol_area for return probes, see xol_get_area()
changes.
- add get_trampoline_vaddr() helper, to emphasize area->vaddr overload.

RFCv4 changes:
- get rid of area->rp_trampoline_vaddr as it always the same as ->vaddr.
- cleanup ->return_uprobes list in uprobe_free_utask(), because the
task can exit from inside the ret-probe'd function(s).
- in find_active_uprobe(): Once we inserted "int3" we must ensure that
handle_swbp() will be called even if this uprobe goes away. We have
the reference but it only protects uprobe itself, it can't protect
agains delete_uprobe().
IOW, we must ensure that uprobe_pre_sstep_notifier() can't return 0.

RFCv3 changes:
- protected uprobe with refcounter. See atomic_inc in prepare_uretprobe()
and put_uprobe() in a following patch in handle_uretprobe().

RFCv2 changes:
- get rid of ->return_consumers member from struct uprobe, introduce
rp_handler() in consumer.

Signed-off-by: Anton Arapov <anton@xxxxxxxxxx>
---
include/linux/uprobes.h | 1 +
kernel/events/uprobes.c | 91 ++++++++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 91 insertions(+), 1 deletion(-)

diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index a28bdee..145d466 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -69,6 +69,7 @@ struct uprobe_task {
enum uprobe_task_state state;
struct arch_uprobe_task autask;

+ struct return_instance *return_instances;
struct uprobe *active_uprobe;

unsigned long xol_vaddr;
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 86706d1..4ea3e91 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -75,6 +75,14 @@ struct uprobe {
struct arch_uprobe arch;
};

+struct return_instance {
+ struct uprobe *uprobe;
+ unsigned long orig_ret_vaddr; /* original return address */
+ bool dirty; /* true, if instance is nested */
+
+ struct return_instance *next; /* keep as stack */
+};
+
/*
* valid_vma: Verify if the specified vma is an executable vma
* Relax restrictions while unregistering: vm_flags might have
@@ -1318,6 +1326,7 @@ unsigned long __weak uprobe_get_swbp_addr(struct pt_regs *regs)
void uprobe_free_utask(struct task_struct *t)
{
struct uprobe_task *utask = t->utask;
+ struct return_instance *ri, *tmp;

if (!utask)
return;
@@ -1325,6 +1334,15 @@ void uprobe_free_utask(struct task_struct *t)
if (utask->active_uprobe)
put_uprobe(utask->active_uprobe);

+ ri = utask->return_instances;
+ while (ri) {
+ put_uprobe(ri->uprobe);
+
+ tmp = ri;
+ ri = ri->next;
+ kfree(tmp);
+ }
+
xol_free_insn_slot(t);
kfree(utask);
t->utask = NULL;
@@ -1358,6 +1376,71 @@ static unsigned long get_trampoline_vaddr(struct xol_area *area)
return area->vaddr;
}

+static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
+{
+ struct return_instance *ri;
+ struct uprobe_task *utask;
+ struct xol_area *area;
+ unsigned long trampoline_vaddr;
+ unsigned long prev_ret_vaddr, ret_vaddr;
+
+ area = get_xol_area();
+ if (!area)
+ return;
+
+ utask = get_utask();
+ if (!utask)
+ return;
+
+ prev_ret_vaddr = -1;
+ if (utask->return_instances)
+ prev_ret_vaddr = utask->return_instances->orig_ret_vaddr;
+
+ ri = kzalloc(sizeof(struct return_instance), GFP_KERNEL);
+ if (!ri)
+ return;
+
+ ri->dirty = false;
+ trampoline_vaddr = get_trampoline_vaddr(area);
+ ret_vaddr = arch_uretprobe_hijack_return_addr(trampoline_vaddr, regs);
+
+ /*
+ * We don't want to keep trampoline address in stack, rather keep the
+ * original return address of first caller thru all the consequent
+ * instances. This also makes breakpoint unwrapping easier.
+ */
+ if (ret_vaddr == trampoline_vaddr) {
+ if (likely(prev_ret_vaddr != -1)) {
+ ri->dirty = true;
+ ret_vaddr = prev_ret_vaddr;
+ } else {
+ /*
+ * This situation is not possible. Likely we have an
+ * attack from user-space. Die.
+ */
+ printk(KERN_ERR "uprobe: something went wrong "
+ "pid/tgid=%d/%d", current->pid, current->tgid);
+ send_sig(SIGSEGV, current, 0);
+ kfree(ri);
+ return;
+ }
+ }
+
+ if (likely(ret_vaddr != -1)) {
+ atomic_inc(&uprobe->ref);
+ ri->uprobe = uprobe;
+ ri->orig_ret_vaddr = ret_vaddr;
+
+ /* add instance to the stack */
+ ri->next = utask->return_instances;
+ utask->return_instances = ri;
+
+ return;
+ }
+
+ kfree(ri);
+}
+
/* Prepare to single-step probed instruction out of line. */
static int
pre_ssout(struct uprobe *uprobe, struct pt_regs *regs, unsigned long bp_vaddr)
@@ -1513,20 +1596,26 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
{
struct uprobe_consumer *uc;
int remove = UPROBE_HANDLER_REMOVE;
+ bool prep = false; /* prepare return uprobe, when needed */
int rc = 0;

down_read(&uprobe->register_rwsem);
for (uc = uprobe->consumers; uc; uc = uc->next) {
if (uc->handler)
rc = uc->handler(uc, regs);
- else
+ else {
+ prep |= true;
remove = 0;
+ }

WARN(rc & ~UPROBE_HANDLER_MASK,
"bad rc=0x%x from %pf()\n", rc, uc->handler);
remove &= rc;
}

+ if (prep)
+ prepare_uretprobe(uprobe, regs); /* put bp at return */
+
if (remove && uprobe->consumers) {
WARN_ON(!uprobe_is_active(uprobe));
unapply_uprobe(uprobe, current->mm);
--
1.8.1.4

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/