Hi Ingo,
Here is v3 series of fixing kretprobe incorrect stacking order patches.
I found that this series did not merged yet, so I just ported it on the tip tree.
In this version, I just added Steve's Ack and sort tags. Please push it as an urgent fix.
(1) kprobe incorrct stacking order problem
On recent talk with Andrea, I started more precise investigation on the kernel panic with kretprobes on notrace functions, which Francis had been reported last year ( https://lkml.org/lkml/2017/7/14/466 ).
See the investigation details in https://lkml.kernel.org/r/154686789378.15479.2886543882215785247.stgit@devbo...
When we put a kretprobe on ftrace_ops_assist_func() and put another kretprobe on probed-function, below happens
<caller> -><probed-function> ->fentry ->ftrace_ops_assist_func() ->int3 ->kprobe_int3_handler() ...->pre_handler_kretprobe() push the return address (*fentry*) of ftrace_ops_assist_func() to top of the kretprobe list and replace it with kretprobe_trampoline. <-kprobe_int3_handler() <-(int3) ->kprobe_ftrace_handler() ...->pre_handler_kretprobe() push the return address (caller) of probed-function to top of the kretprobe list and replace it with kretprobe_trampoline. <-(kprobe_ftrace_handler()) <-(ftrace_ops_assist_func()) [kretprobe_trampoline] ->tampoline_handler() pop the return address (caller) from top of the kretprobe list <-(trampoline_handler()) <caller> [run caller with incorrect stack information] <-(<caller>) !!KERNEL PANIC!!
Therefore, this kernel panic happens only when we put 2 k*ret*probes on ftrace_ops_assist_func() and other functions. If we put kprobes, it doesn't cause any issue, since it doesn't change the return address.
To fix (or just avoid) this issue, we can introduce a frame pointer verification to skip wrong order entries. And I also would like to blacklist those functions because those are part of ftrace-based kprobe handling routine.
(2) kretprobe trampoline recursion problem
This was found by Andrea in the previous thread https://lkml.kernel.org/r/20190107183444.GA5966@xps-13
---- echo "r:event_1 __fdget" >> kprobe_events echo "r:event_2 _raw_spin_lock_irqsave" >> kprobe_events echo 1 > events/kprobes/enable [DEADLOCK] ----
Because kretprobe trampoline_handler uses spinlock for protecting hash table, if we probe the spinlock itself, it causes deadlock. Thank you Andrea and Steve for discovering this root cause!!
This bug has been introduced with the asm-coded trampoline code, since previously it used another kprobe for hooking the function return placeholder (which only has a nop) and trampoline handler was called from that kprobe.
To fix this bug, I introduced a dummy kprobe and set it in current_kprobe as we did in old days.
Thank you,
---
Masami Hiramatsu (3): x86/kprobes: Verify stack frame on kretprobe kprobes: Mark ftrace mcount handler functions nokprobe x86/kprobes: Fix to avoid kretprobe recursion
arch/x86/kernel/kprobes/core.c | 48 ++++++++++++++++++++++++++++++++++++++-- include/linux/kprobes.h | 1 + kernel/trace/ftrace.c | 6 ++++- 3 files changed, 52 insertions(+), 3 deletions(-)
-- Masami Hiramatsu (Linaro) mhiramat@kernel.org