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printk: syslog: close window between wait and read

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Syslog's SYSLOG_ACTION_READ is supposed to block until the next
syslog record can be read, and then it should read that record.
However, because @syslog_lock is not held between waking up and
reading the record, another reader could read the record first,
thus causing SYSLOG_ACTION_READ to return with a value of 0, never
having read _anything_.

By holding @syslog_lock between waking up and reading, it can be
guaranteed that SYSLOG_ACTION_READ blocks until it successfully
reads a syslog record (or a real error occurs).

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/20210715193359.25946-7-john.ogness@linutronix.de
This commit is contained in:
John Ogness 2021-07-15 21:39:59 +02:06 committed by Petr Mladek
parent b371cbb584
commit 8d909b2333
1 changed files with 36 additions and 19 deletions
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55
kernel/printk/printk.c
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@ -1480,12 +1480,14 @@ static u64 find_first_fitting_seq(u64 start_seq, u64 max_seq, size_t size,
return seq;
}
/* The caller is responsible for making sure @size is greater than 0. */
static int syslog_print(char __user *buf, int size)
{
struct printk_info info;
struct printk_record r;
char *text;
int len = 0;
u64 seq;
text = kmalloc(CONSOLE_LOG_MAX, GFP_KERNEL);
if (!text)
@ -1493,15 +1495,35 @@ static int syslog_print(char __user *buf, int size)
prb_rec_init_rd(&r, &info, text, CONSOLE_LOG_MAX);
while (size > 0) {
mutex_lock(&syslog_lock);
/*
* Wait for the @syslog_seq record to be available. @syslog_seq may
* change while waiting.
*/
do {
seq = syslog_seq;
mutex_unlock(&syslog_lock);
len = wait_event_interruptible(log_wait, prb_read_valid(prb, seq, NULL));
mutex_lock(&syslog_lock);
if (len)
goto out;
} while (syslog_seq != seq);
/*
* Copy records that fit into the buffer. The above cycle makes sure
* that the first record is always available.
*/
do {
size_t n;
size_t skip;
int err;
mutex_lock(&syslog_lock);
if (!prb_read_valid(prb, syslog_seq, &r)) {
mutex_unlock(&syslog_lock);
if (!prb_read_valid(prb, syslog_seq, &r))
break;
}
if (r.info->seq != syslog_seq) {
/* message is gone, move to next valid one */
syslog_seq = r.info->seq;
@ -1528,12 +1550,15 @@ static int syslog_print(char __user *buf, int size)
syslog_partial += n;
} else
n = 0;
mutex_unlock(&syslog_lock);
if (!n)
break;
if (copy_to_user(buf, text + skip, n)) {
mutex_unlock(&syslog_lock);
err = copy_to_user(buf, text + skip, n);
mutex_lock(&syslog_lock);
if (err) {
if (!len)
len = -EFAULT;
break;
@ -1542,8 +1567,9 @@ static int syslog_print(char __user *buf, int size)
len += n;
size -= n;
buf += n;
}
} while (size);
out:
mutex_unlock(&syslog_lock);
kfree(text);
return len;
}
@ -1614,7 +1640,6 @@ int do_syslog(int type, char __user *buf, int len, int source)
bool clear = false;
static int saved_console_loglevel = LOGLEVEL_DEFAULT;
int error;
u64 seq;
error = check_syslog_permissions(type, source);
if (error)
@ -1632,15 +1657,6 @@ int do_syslog(int type, char __user *buf, int len, int source)
return 0;
if (!access_ok(buf, len))
return -EFAULT;
/* Get a consistent copy of @syslog_seq. */
mutex_lock(&syslog_lock);
seq = syslog_seq;
mutex_unlock(&syslog_lock);
error = wait_event_interruptible(log_wait, prb_read_valid(prb, seq, NULL));
if (error)
return error;
error = syslog_print(buf, len);
break;
/* Read/clear last kernel messages */
@ -1707,6 +1723,7 @@ int do_syslog(int type, char __user *buf, int len, int source)
} else {
bool time = syslog_partial ? syslog_time : printk_time;
unsigned int line_count;
u64 seq;
prb_for_each_info(syslog_seq, prb, seq, &info,
&line_count) {