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Message-ID: <a8cf7252-4d67-fb8c-6c3e-77f7a1c66dee@huawei.com>
Date: Tue, 9 Apr 2024 14:20:45 +0800
From: Zheng Yejian <zhengyejian1@...wei.com>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
CC: <naveen.n.rao@...ux.ibm.com>, <anil.s.keshavamurthy@...el.com>,
<davem@...emloft.net>, <linux-kernel@...r.kernel.org>,
<linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] kprobes: Avoid possible warn in __arm_kprobe_ftrace()
On 2024/4/8 20:41, Masami Hiramatsu (Google) wrote:
> Hi Zheng,
>
> On Mon, 8 Apr 2024 16:34:03 +0800
> Zheng Yejian <zhengyejian1@...wei.com> wrote:
>
>> There is once warn in __arm_kprobe_ftrace() on:
>>
>> ret = ftrace_set_filter_ip(ops, (unsigned long)p->addr, 0, 0);
>> if (WARN_ONCE(..., "Failed to arm kprobe-ftrace at %pS (error %d)\n", ...)
>> return ret;
>>
>> This warning is generated because 'p->addr' is detected to be not a valid
>> ftrace location in ftrace_set_filter_ip(). The ftrace address check is done
>> by check_ftrace_location() at the beginning of check_kprobe_address_safe().
>> At that point, ftrace_location(addr) == addr should return true if the
>> module is loaded. Then the module is searched twice:
>> 1. in is_module_text_address(), we find that 'p->addr' is in a module;
>> 2. in __module_text_address(), we find the module;
>>
>> If the module has just been unloaded before the second search, then
>> '*probed_mod' is NULL and we would not go to get the module refcount,
>> then the return value of check_kprobe_address_safe() would be 0, but
>> actually we need to return -EINVAL.
>
> OK, so you found a race window in check_kprobe_address_safe().
>
> It does something like below.
>
> check_kprobe_address_safe() {
> ...
>
> /* Timing [A] */
>
> if (!(core_kernel_text(p->addr) ||
> is_module_text_address(p->addr)) ||
> ...(other reserved address check)) {
> return -EINVAL;
> }
>
> /* Timing [B] */
>
> *probed_mod = __module_text_address(p->addr):
> if (*probe_mod) {
> if (!try_module_get(*probed_mod)) {
> return -ENOENT;
> }
> ...
> }
> }
>
> So, if p->addr is in a module which is alive at the timing [A], but
> unloaded at timing [B], 'p->addr' is passed the
> 'is_module_text_address(p->addr)' check, but *probed_mod becomes NULL.
> Thus the corresponding module is not referenced and kprobe_arm(p) will
> access a wrong address (use after free).
> This happens either kprobe on ftrace is enabled or not.
Yes, This is the problem. And for this case, check_kprobe_address_safe()
still return 0, and then going on to arm kprobe may cause problems. So
we should make check_kprobe_address_safe() return -EINVAL when refcount
of the module is not got.
>
> To fix this problem, we should move the mutex_lock(kprobe_mutex) before
> check_kprobe_address_safe() because kprobe_module_callback() also lock it
> so it can stop module unloading.
>
> Can you ensure this will fix your problem?
It seems not, the warning in __arm_kprobe_ftrace() still occurs. I
contrived following simple test:
#!/bin/bash
sysctl -w kernel.panic_on_warn=1
while [ True ]; do
insmod mod.ko # contain function 'foo'
rmmod mod.ko
done &
while [ True ]; do
insmod kprobe.ko # register kprobe on function 'foo'
rmmod kprobe.ko
done &
I think holding kprobe_mutex cannot make sure we get the refcount of the
module.
> I think your patch is just optimizing but not fixing the fundamental
> problem, which is we don't have an atomic search symbol and get module
Sorry, this patch is a little confusing, but it is not just optimizing :)
As shown below, after my patch, if p->addr is in a module which is alive
at the timing [A'] but unloaded at timing [B'], then *probed_mod must
not be NULL. Then after timing [B'], it will go to try_module_get() and
expected to fail and return -ENOENT. So this is the different.
check_kprobe_address_safe() {
...
*probed_mod = NULL;
if (!core_kernel_text((unsigned long) p->addr)) {
/* Timing [A'] */
*probed_mod = __module_text_address((unsigned long) p->addr);
if (!(*probed_mod)) {
return -EINVAL;
}
}
...
/* Timing [B'] */
if (*probed_mod) {
if (!try_module_get(*probed_mod)) {
return -ENOENT;
}
...
}
> API. In that case, we should stop a whole module unloading system until
> registering a new kprobe on a module. (After registering the kprobe,
> the callback can mark it gone and disarm_kprobe does not work anymore.)
>
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 9d9095e81792..94eaefd1bc51 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1633,11 +1633,11 @@ int register_kprobe(struct kprobe *p)
> p->nmissed = 0;
> INIT_LIST_HEAD(&p->list);
>
> + mutex_lock(&kprobe_mutex);
> +
> ret = check_kprobe_address_safe(p, &probed_mod);
> if (ret)
> - return ret;
> -
> - mutex_lock(&kprobe_mutex);
> + goto out;
>
> if (on_func_entry)
> p->flags |= KPROBE_FLAG_ON_FUNC_ENTRY;
>
> ----
>
> Thank you,
>
>>
>> To fix it, originally we can simply check 'p->addr' is out of text again,
>> like below. But that would check twice respectively in kernel text and
>> module text, so finally I reduce them to be once.
>>
>> if (!(core_kernel_text((unsigned long) p->addr) ||
>> is_module_text_address((unsigned long) p->addr)) || ...) {
>> ret = -EINVAL;
>> goto out;
>> }
>> ...
>> *probed_mod = __module_text_address((unsigned long) p->addr);
>> if (*probed_mod) {
>> ...
>> } else if (!core_kernel_text((unsigned long) p->addr)) { // check again!
>> ret = -EINVAL;
>> goto out;
>> }
>>
>> Signed-off-by: Zheng Yejian <zhengyejian1@...wei.com>
>> ---
>> kernel/kprobes.c | 18 ++++++++++++------
>> 1 file changed, 12 insertions(+), 6 deletions(-)
>>
>> v2:
>> - Update commit messages and comments as suggested by Masami.
>> Link: https://lore.kernel.org/all/20240408115038.b0c85767bf1f249eccc32fff@kernel.org/
>>
>> v1:
>> - Link: https://lore.kernel.org/all/20240407035904.2556645-1-zhengyejian1@huawei.com/
>>
>> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
>> index 9d9095e81792..65adc815fc6e 100644
>> --- a/kernel/kprobes.c
>> +++ b/kernel/kprobes.c
>> @@ -1567,10 +1567,17 @@ static int check_kprobe_address_safe(struct kprobe *p,
>> jump_label_lock();
>> preempt_disable();
>>
>> - /* Ensure it is not in reserved area nor out of text */
>> - if (!(core_kernel_text((unsigned long) p->addr) ||
>> - is_module_text_address((unsigned long) p->addr)) ||
>> - in_gate_area_no_mm((unsigned long) p->addr) ||
>> + /* Ensure the address is in a text area, and find a module if exists. */
>> + *probed_mod = NULL;
>> + if (!core_kernel_text((unsigned long) p->addr)) {
>> + *probed_mod = __module_text_address((unsigned long) p->addr);
>> + if (!(*probed_mod)) {
>> + ret = -EINVAL;
>> + goto out;
>> + }
>> + }
>> + /* Ensure it is not in reserved area. */
>> + if (in_gate_area_no_mm((unsigned long) p->addr) ||
>> within_kprobe_blacklist((unsigned long) p->addr) ||
>> jump_label_text_reserved(p->addr, p->addr) ||
>> static_call_text_reserved(p->addr, p->addr) ||
>> @@ -1580,8 +1587,7 @@ static int check_kprobe_address_safe(struct kprobe *p,
>> goto out;
>> }
>>
>> - /* Check if 'p' is probing a module. */
>> - *probed_mod = __module_text_address((unsigned long) p->addr);
>> + /* Get module refcount and reject __init functions for loaded modules. */
>> if (*probed_mod) {
>> /*
>> * We must hold a refcount of the probed module while updating
>> --
>> 2.25.1
>>
>
--
Thanks
Zheng Yejian
>
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