vsprintf: Prevent crash when dereferencing invalid pointers

We already prevent crash when dereferencing some obviously broken
pointers. But the handling is not consistent. Sometimes we print "(null)"
only for pure NULL pointer, sometimes for pointers in the first
page and sometimes also for pointers in the last page (error codes).

Note that printk() call this code 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.

This patch adds a check using probe_kernel_read(). It is not a full-proof
test. But it should help to see the error message in 99% situations where
the kernel would silently crash otherwise.

Also it makes the error handling unified for "%s" and the many %p*
specifiers that need to read the data from a given address. We print:

   + (null)   when accessing data on pure pure NULL address
   + (efault) when accessing data on an invalid address

It does not affect the %p* specifiers that just print the given address
in some form, namely %pF, %pf, %pS, %ps, %pB, %pK, %px, and plain %p.

Note that we print (efault) from security reasons. In fact, the real
address can be seen only by %px or eventually %pK.

Link: http://lkml.kernel.org/r/20190417115350.20479-9-pmladek@suse.com
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Tobin C . Harding" <me@tobin.cc>
Cc: Joe Perches <joe@perches.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

1 files changed, 21 insertions, 1 deletions

diff --git a/lib/test_printf.c b/lib/test_printf.c
index 250ee864b8b8..359ae4fb1ece 100644
--- a/lib/test_printf.c
+++ b/lib/test_printf.c
@@ -239,6 +239,7 @@ plain_format(void)
 #define PTR ((void *)0x456789ab)
 #define PTR_STR "456789ab"
 #define PTR_VAL_NO_CRNG "(ptrval)"
+#define ZEROS ""
 
 static int __init
 plain_format(void)
@@ -268,7 +269,6 @@ plain_hash_to_buffer(const void *p, char *buf, size_t len)
 	return 0;
 }
 
-
 static int __init
 plain_hash(void)
 {
@@ -326,6 +326,24 @@ test_hashed(const char *fmt, const void *p)
 }
 
 static void __init
+null_pointer(void)
+{
+	test_hashed("%p", NULL);
+	test(ZEROS "00000000", "%px", NULL);
+	test("(null)", "%pE", NULL);
+}
+
+#define PTR_INVALID ((void *)0x000000ab)
+
+static void __init
+invalid_pointer(void)
+{
+	test_hashed("%p", PTR_INVALID);
+	test(ZEROS "000000ab", "%px", PTR_INVALID);
+	test("(efault)", "%pE", PTR_INVALID);
+}
+
+static void __init
 symbol_ptr(void)
 {
 }
@@ -571,6 +589,8 @@ static void __init
 test_pointer(void)
 {
 	plain();
+	null_pointer();
+	invalid_pointer();
 	symbol_ptr();
 	kernel_ptr();
 	struct_resource();