linux-kernel - Re: [PATCH 00/14] Fix wrong %pF and %pS printk format specifier usages

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Message-ID: <20170907083207.GC533@jagdpanzerIV.localdomain>
Date:   Thu, 7 Sep 2017 17:32:07 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Helge Deller <deller@....de>, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 00/14] Fix wrong %pF and %pS printk format specifier
 usages

On (09/07/17 16:56), Sergey Senozhatsky wrote:
[..]
> BTW, are we sure we can crash? when attempt to deference IP from
> the given descriptor? shall we handle page fault in this case and
> do something sane? just asking.

I don't know... does the below code make any sense?

quick and dirty. NOT TESTED at all (not even compile tested).
we can avoid extra probe_kernel_address() on anything that is
not ia64, ppc64, etc.

basically it checks that it's safe to access ptr (we can access it
without page fault in __dereference_function_descriptor()). then
we do ptr->ip, and also check if it's safe, but in
dereference_function_descriptor().

I suppose somethign like

	pr_err("%pF\n", 1);

can crash ia64, etc. correct?


well. not tested.

---

 lib/vsprintf.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 86c3385b9eb3..0dc39b95e1d9 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1593,6 +1593,16 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
 
 int kptr_restrict __read_mostly;
 
+static void *__dereference_function_descriptor(void *ptr)
+{
+	void *p;
+
+	if (!probe_kernel_address(ptr, p))
+		return dereference_function_descriptor(ptr);
+
+	return ptr;
+}
+
 /*
  * Show a '%p' thing.  A kernel extension is that the '%p' is followed
  * by an extra set of alphanumeric characters that are extended format
@@ -1723,7 +1733,7 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 	switch (*fmt) {
 	case 'F':
 	case 'f':
-		ptr = dereference_function_descriptor(ptr);
+		ptr = __dereference_function_descriptor(ptr);
 		/* Fallthrough */
 	case 'S':
 	case 's':