printk: fix syslog() overflowing user buffer

This is not a buffer overflow in the traditional sense: we don't overflow any *kernel* buffers, but we do mis-count the amount of data we copy back to user space for the SYSLOG_ACTION_READ_ALL case. In particular, if the user buffer is too small to hold everything, and *if* there is a continuation line at just the right place, we can end up giving the user more data than he asked for. The reason is that we first count up the number of bytes all the log records contains, then we walk the records again until we've skipped the records at the beginning that won't fit, and then we walk the rest of the records and copy them to the user space buffer. And in between that "skip the initial records that won't fit" and the "copy the records that *will* fit to user space", we reset the 'prev' variable that contained the record information for the last record not copied. That meant that when we started copying to user space, we now had a different character count than what we had originally calculated in the first record walk-through. The fix is to simply not clear the 'prev' flags value (in both cases where we had the same logic: syslog_print_all and kmsg_dump_get_buffer: the latter is used for pstore-like dumping) Reported-and-tested-by: Debabrata Banerjee <[email protected]> Acked-by: Kay Sievers <[email protected]> Cc: Greg Kroah-Hartman <[email protected]> Cc: Jeff Mahoney <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
AshishNamdev · AshishNamdev · Feb 22, 2014 · Feb 13, 2014 · Feb 13, 2014 · Feb 13, 2014
commit e4178d809fdaee32a56833fff1f5056c99e90a1a
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
@@ -1076,7 +1076,6 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
 		next_seq = log_next_seq;
 
 		len = 0;
-		prev = 0;
 		while (len >= 0 && seq < next_seq) {
 			struct printk_log *msg = log_from_idx(idx);
 			int textlen;
@@ -2788,7 +2787,6 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
 	next_idx = idx;
 
 	l = 0;
-	prev = 0;
 	while (seq < dumper->next_seq) {
 		struct printk_log *msg = log_from_idx(idx);