[PATCH v2] vsprintf: Prevent crash when dereferencing invalid pointers

[Petr Mladek]

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.

In general, we want to get a message from printk() than a silent crash.
This patch adds a check using probe_kernel_read().

The check is used _only_ in situations where we would really crash. This is
why this patch adds a white list of %p* specifiers that do the dereference.
Note that "%p" might be followed by any character. Only few of them are
valid specifiers and only few specifiers need to access the data.

Also it makes the handling unified. We print:

   + (null) when pure NULL pointer is dereferenced
   + (efault) when an invalid address is dereferenced
   + pointer address otherwise

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

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 lib/vsprintf.c | 41 ++++++++++++++++++++++++++++++++---------
 1 file changed, 32 insertions(+), 9 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index d7a708f82559..36fa8a15c169 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -520,6 +520,19 @@ char *number(char *buf, char *end, unsigned long long num,
 	return buf;
 }
 
+static const char *check_pointer_access(const void *ptr)
+{
+	unsigned char byte;
+
+	if (!ptr)
+		return "(null)";
+
+	if (probe_kernel_read(&byte, ptr, 1))
+		return "(efault)";
+
+	return 0;
+}
+
 static noinline_for_stack
 char *special_hex_number(char *buf, char *end, unsigned long long num, int size)
 {
@@ -586,9 +599,11 @@ char *string(char *buf, char *end, const char *s, struct printf_spec spec)
 {
 	int len = 0;
 	size_t lim = spec.precision;
+	const char *err_msg;
 
-	if ((unsigned long)s < PAGE_SIZE)
-		s = "(null)";
+	err_msg = check_pointer_access(s);
+	if (err_msg)
+		s = err_msg;
 
 	while (lim--) {
 		char c = *s++;
@@ -1847,16 +1862,22 @@ static noinline_for_stack
 char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 	      struct printf_spec spec)
 {
+	static const char data_access_fmt[] = "RrhbMmIiEUVNadCDgGO";
 	const int default_width = 2 * sizeof(void *);
+	const char *err_msg = NULL;
+
+	/* Prevent silent crash when this is called under logbuf_lock. */
+	if (strchr(data_access_fmt, *fmt) != NULL)
+		err_msg = check_pointer_access(ptr);
 
-	if (!ptr && *fmt != 'K' && *fmt != 'x') {
+	if (err_msg) {
 		/*
-		 * Print (null) with the same width as a pointer so it makes
-		 * tabular output look nice.
+		 * Print the error message with the same width as a pointer
+		 * so it makes tabular output look nice.
 		 */
 		if (spec.field_width == -1)
 			spec.field_width = default_width;
-		return string(buf, end, "(null)", spec);
+		return string(buf, end, err_msg, spec);
 	}
 
 	switch (*fmt) {
@@ -2571,11 +2592,13 @@ int vbin_printf(u32 *bin_buf, size_t size, const char *fmt, va_list args)
 
 		case FORMAT_TYPE_STR: {
 			const char *save_str = va_arg(args, char *);
+			const char *err_msg;
 			size_t len;
 
-			if ((unsigned long)save_str > (unsigned long)-PAGE_SIZE
-					|| (unsigned long)save_str < PAGE_SIZE)
-				save_str = "(null)";
+			err_msg = check_pointer_access(save_str);
+			if (err_msg)
+				save_str = err_msg;
+
 			len = strlen(save_str) + 1;
 			if (str + len < end)
 				memcpy(str, save_str, len);
-- 
2.13.6