[PATCH v3 2/4] fprobe: make fprobe_kprobe_handler recursion free

Ze Gao
2023-05-17T03:45:50Z

Current implementation calls kprobe related functions before doing
ftrace recursion check in fprobe_kprobe_handler, which opens door
to kernel crash due to stack recursion if preempt_count_{add, sub}
is traceable in kprobe_busy_{begin, end}.

Things goes like this without this patch quoted from Steven:
"
fprobe_kprobe_handler() {
kprobe_busy_begin() {
preempt_disable() {
preempt_count_add() { <-- trace
fprobe_kprobe_handler() {
[ wash, rinse, repeat, CRASH!!! ]
"

By refactoring the common part out of fprobe_kprobe_handler and
fprobe_handler and call ftrace recursion detection at the very beginning,
the whole fprobe_kprobe_handler is free from recursion.

Signed-off-by: Ze Gao <zegao@tencent.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/linux-trace-kernel/20230516071830.8190-3-zegao@tencent.com
---
kernel/trace/fprobe.c | 59 ++++++++++++++++++++++++++++++++-----------
1 file changed, 44 insertions(+), 15 deletions(-)

diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index 9abb3905bc8e..097c740799ba 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c
@@ -20,30 +20,22 @@ struct fprobe_rethook_node {
char data[];
};

-static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
- struct ftrace_ops *ops, struct ftrace_regs *fregs)
+static inline void __fprobe_handler(unsigned long ip, unsigned long
+ parent_ip, struct ftrace_ops *ops, struct ftrace_regs *fregs)
{
struct fprobe_rethook_node *fpr;
struct rethook_node *rh = NULL;
struct fprobe *fp;
void *entry_data = NULL;
- int bit, ret;
+ int ret;

fp = container_of(ops, struct fprobe, ops);
- if (fprobe_disabled(fp))
- return;
-
- bit = ftrace_test_recursion_trylock(ip, parent_ip);
- if (bit < 0) {
- fp->nmissed++;
- return;
- }

if (fp->exit_handler) {
rh = rethook_try_get(fp->rethook);
if (!rh) {
fp->nmissed++;
- goto out;
+ return;
}
fpr = container_of(rh, struct fprobe_rethook_node, node);
fpr->entry_ip = ip;
@@ -61,23 +53,60 @@ static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
else
rethook_hook(rh, ftrace_get_regs(fregs), true);
}
-out:
+}
+
+static void fprobe_handler(unsigned long ip, unsigned long parent_ip,
+ struct ftrace_ops *ops, struct ftrace_regs *fregs)
+{
+ struct fprobe *fp;
+ int bit;
+
+ fp = container_of(ops, struct fprobe, ops);
+ if (fprobe_disabled(fp))
+ return;
+
+ /* recursion detection has to go before any traceable function and
+ * all functions before this point should be marked as notrace
+ */
+ bit = ftrace_test_recursion_trylock(ip, parent_ip);
+ if (bit < 0) {
+ fp->nmissed++;
+ return;
+ }
+ __fprobe_handler(ip, parent_ip, ops, fregs);
ftrace_test_recursion_unlock(bit);
+
}
NOKPROBE_SYMBOL(fprobe_handler);

static void fprobe_kprobe_handler(unsigned long ip, unsigned long parent_ip,
struct ftrace_ops *ops, struct ftrace_regs *fregs)
{
- struct fprobe *fp = container_of(ops, struct fprobe, ops);
+ struct fprobe *fp;
+ int bit;
+
+ fp = container_of(ops, struct fprobe, ops);
+ if (fprobe_disabled(fp))
+ return;
+
+ /* recursion detection has to go before any traceable function and
+ * all functions called before this point should be marked as notrace
+ */
+ bit = ftrace_test_recursion_trylock(ip, parent_ip);
+ if (bit < 0) {
+ fp->nmissed++;
+ return;
+ }

if (unlikely(kprobe_running())) {
fp->nmissed++;
return;
}
+
kprobe_busy_begin();
- fprobe_handler(ip, parent_ip, ops, fregs);
+ __fprobe_handler(ip, parent_ip, ops, fregs);
kprobe_busy_end();
+ ftrace_test_recursion_unlock(bit);
}

static void fprobe_exit_handler(struct rethook_node *rh, void *data,
--
2.40.1