[PATCH v2] kprobes: Avoid possible warn in __arm_kprobe_ftrace()

[Zheng Yejian]

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.

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@xxxxxxxxxx>
---
 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@xxxxxxxxxx/

v1:
 - Link: https://lore.kernel.org/all/20240407035904.2556645-1-zhengyejian1@xxxxxxxxxx/

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