printk: avoid and/or handle record truncation

If a reader provides a buffer that is smaller than the message text,
the @text_len field of @info will have a value larger than the buffer
size. If readers blindly read @text_len bytes of data without
checking the size, they will read beyond their buffer.

Add this check to record_print_text() to properly recognize when such
truncation has occurred.

Add a maximum size argument to the ringbuffer function to extend
records so that records can not be created that are larger than the
buffer size of readers.

When extending records (LOG_CONT), do not extend records beyond
LOG_LINE_MAX since that is the maximum size available in the buffers
used by consoles and syslog.

Fixes: f5f022e53b ("printk: reimplement log_cont using record extension")
Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20200930090134.8723-2-john.ogness@linutronix.de

kernel/printk/printk.c

@@ -1349,6 +1349,13 @@ static size_t record_print_text(struct printk_record *r, bool syslog,
 	size_t len = 0;
 	char *next;
 
+	/*
+	 * If the message was truncated because the buffer was not large
+	 * enough, treat the available text as if it were the full text.
+	 */
+	if (text_len > buf_size)
+		text_len = buf_size;
+
 	prefix_len = info_print_prefix(r->info, syslog, time, prefix);
 
 	/*
@@ -1903,7 +1910,7 @@ static size_t log_output(int facility, int level, enum log_flags lflags,
 		struct printk_record r;
 
 		prb_rec_init_wr(&r, text_len);
-		if (prb_reserve_in_last(&e, prb, &r, caller_id)) {
+		if (prb_reserve_in_last(&e, prb, &r, caller_id, LOG_LINE_MAX)) {
 			memcpy(&r.text_buf[r.info->text_len], text, text_len);
 			r.info->text_len += text_len;
 			if (lflags & LOG_NEWLINE) {

kernel/printk/printk_ringbuffer.c

@@ -202,7 +202,8 @@
  *	// specify additional 5 bytes text space to extend
  *	prb_rec_init_wr(&r, 5);
  *
- *	if (prb_reserve_in_last(&e, &test_rb, &r, printk_caller_id())) {
+ *	// try to extend, but only if it does not exceed 32 bytes
+ *	if (prb_reserve_in_last(&e, &test_rb, &r, printk_caller_id()), 32) {
  *		snprintf(&r.text_buf[r.info->text_len],
  *			 r.text_buf_size - r.info->text_len, "hello");
  *
@@ -1309,6 +1310,7 @@ static struct prb_desc *desc_reopen_last(struct prb_desc_ring *desc_ring,
  * @rb:        The ringbuffer to re-reserve and extend data in.
  * @r:         The record structure to allocate buffers for.
  * @caller_id: The caller ID of the caller (reserving writer).
+ * @max_size:  Fail if the extended size would be greater than this.
  *
  * This is the public function available to writers to re-reserve and extend
  * data.
@@ -1343,7 +1345,7 @@ static struct prb_desc *desc_reopen_last(struct prb_desc_ring *desc_ring,
  *            @r->info->text_len after concatenating.
  */
 bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
-			 struct printk_record *r, u32 caller_id)
+			 struct printk_record *r, u32 caller_id, unsigned int max_size)
 {
 	struct prb_desc_ring *desc_ring = &rb->desc_ring;
 	struct printk_info *info;
@@ -1389,6 +1391,9 @@ bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer
 		if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
 			goto fail;
 
+		if (r->text_buf_size > max_size)
+			goto fail;
+
 		r->text_buf = data_alloc(rb, &rb->text_data_ring, r->text_buf_size,
 					 &d->text_blk_lpos, id);
 	} else {
@@ -1410,6 +1415,9 @@ bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer
 		if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
 			goto fail;
 
+		if (r->text_buf_size > max_size)
+			goto fail;
+
 		r->text_buf = data_realloc(rb, &rb->text_data_ring, r->text_buf_size,
 					   &d->text_blk_lpos, id);
 	}

kernel/printk/printk_ringbuffer.h

@@ -303,7 +303,7 @@ static inline void prb_rec_init_wr(struct printk_record *r,
 bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
 		 struct printk_record *r);
 bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
-			 struct printk_record *r, u32 caller_id);
+			 struct printk_record *r, u32 caller_id, unsigned int max_size);
 void prb_commit(struct prb_reserved_entry *e);
 void prb_final_commit(struct prb_reserved_entry *e);