linux-kernel - [PATCH] vsprintf: Make "null" pointer dereference more robust

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Date:   Fri, 2 Mar 2018 13:53:59 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     "Tobin C . Harding" <me@...in.cc>, linux@...musvillemoes.dk,
        Joe Perches <joe@...ches.com>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.cz>
Subject: [PATCH] vsprintf: Make "null" pointer dereference more robust

%p has many modifiers where the pointer is dereferenced. An invalid
pointer might cause kernel to crash silently.

Note that printk() formats the string under logbuf_lock. Any recursive
printks are redirected to the printk_safe implementation and the messages
are stored into per-CPU buffers. These buffers might be eventually flushed
in printk_safe_flush_on_panic() but it is not guaranteed.

In general, we should do our best to get useful message from printk().
All pointers to the first memory page must be invalid. Let's prevent
the dereference and print "(null)" in this case. This is already done
in many other situations, including "%s" format handling and many
page fault handlers.

Signed-off-by: Petr Mladek <pmladek@...e.com>
---
 lib/vsprintf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index d7a708f82559..5c2d1f44218a 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1849,7 +1849,7 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 {
 	const int default_width = 2 * sizeof(void *);

-	if (!ptr && *fmt != 'K' && *fmt != 'x') {
+	if ((unsigned long)ptr < PAGE_SIZE && *fmt != 'K' && *fmt != 'x') {
 		/*
 		 * Print (null) with the same width as a pointer so it makes
 		 * tabular output look nice.
-- 
2.13.6