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printk: limit second loop of syslog_print_all

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The second loop of syslog_print_all() subtracts lengths that were
added in the first loop. With commit b031a684bf ("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: b031a684bf ("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
This commit is contained in:
John Ogness 2021-03-03 11:15:16 +01:00 • committed by Petr Mladek
parent 40ddbbac7f
commit bb07b16c44
1 changed files with 8 additions and 1 deletions
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9
kernel/printk/printk.c
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@ -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);
}