From: John Ogness john.ogness@linutronix.de
[ Upstream commit bb07b16c44b2c6ddbafa44bb06454719002e828e ]
The second loop of syslog_print_all() subtracts lengths that were added in the first loop. With commit b031a684bfd0 ("printk: remove logbuf_lock writer-protection of ringbuffer") it is possible that records are (over)written during syslog_print_all(). This allows the possibility of the second loop subtracting lengths that were never added in the first loop.
This situation can result in syslog_print_all() filling the buffer starting from a later record, even though there may have been room to fit the earlier record(s) as well.
Fixes: b031a684bfd0 ("printk: remove logbuf_lock writer-protection of ringbuffer") 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/20210303101528.29901-4-john.ogness@linutronix.de Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/printk/printk.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 575a34b88936..77ae2704e979 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1494,6 +1494,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear) struct printk_info info; unsigned int line_count; struct printk_record r; + u64 max_seq; char *text; int len = 0; u64 seq; @@ -1512,9 +1513,15 @@ static int syslog_print_all(char __user *buf, int size, bool clear) prb_for_each_info(clear_seq, prb, seq, &info, &line_count) len += get_record_print_text_size(&info, line_count, true, time);
+ /* + * Set an upper bound for the next loop to avoid subtracting lengths + * that were never added. + */ + max_seq = seq; + /* move first record forward until length fits into the buffer */ prb_for_each_info(clear_seq, prb, seq, &info, &line_count) { - if (len <= size) + if (len <= size || info.seq >= max_seq) break; len -= get_record_print_text_size(&info, line_count, true, time); }