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printk: fix printk_time race.
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Since printk_time can be toggled via /sys/module/printk/parameters/time ,
it is not safe to assume that output length does not change across
multiple msg_print_text() calls. If we hit this race, we can observe
failures such as SYSLOG_ACTION_READ_ALL writes more bytes than userspace
has supplied, SYSLOG_ACTION_SIZE_UNREAD returns -EFAULT when succeeded,
SYSLOG_ACTION_READ reads garbage memory or even triggers an kernel oops
at _copy_to_user() due to integer overflow.

To close this race, get a snapshot value of printk_time and pass it to
SYSLOG_ACTION_READ, SYSLOG_ACTION_READ_ALL, SYSLOG_ACTION_SIZE_UNREAD and
kmsg_dump_get_buffer().

Link: http://lkml.kernel.org/r/[email protected]
To: Sergey Senozhatsky <[email protected]>
Cc: [email protected]
Signed-off-by: Tetsuo Handa <[email protected]>
Reviewed-by: Sergey Senozhatsky <[email protected]>
Signed-off-by: Petr Mladek <[email protected]>
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@pmladek
Tetsuo Handa authored and pmladek committed Dec 10, 2018
1 parent 9adcfaf commit e80c1a9
Showing 1 changed file with 39 additions and 31 deletions.
70 changes: 39 additions & 31 deletions kernel/printk/printk.c
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Original file line number Diff line number Diff line change
Expand Up @@ -404,6 +404,7 @@ DECLARE_WAIT_QUEUE_HEAD(log_wait);
static u64 syslog_seq;
static u32 syslog_idx;
static size_t syslog_partial;
static bool syslog_time;

/* index and sequence number of the first record stored in the buffer */
static u64 log_first_seq;
Expand Down Expand Up @@ -1229,12 +1230,7 @@ module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);

static size_t print_time(u64 ts, char *buf)
{
unsigned long rem_nsec;

if (!printk_time)
return 0;

rem_nsec = do_div(ts, 1000000000);
unsigned long rem_nsec = do_div(ts, 1000000000);

if (!buf)
return snprintf(NULL, 0, "[%5lu.000000] ", (unsigned long)ts);
Expand All @@ -1243,7 +1239,8 @@ static size_t print_time(u64 ts, char *buf)
(unsigned long)ts, rem_nsec / 1000);
}

static size_t print_prefix(const struct printk_log *msg, bool syslog, char *buf)
static size_t print_prefix(const struct printk_log *msg, bool syslog,
bool time, char *buf)
{
size_t len = 0;
unsigned int prefix = (msg->facility << 3) | msg->level;
Expand All @@ -1262,11 +1259,13 @@ static size_t print_prefix(const struct printk_log *msg, bool syslog, char *buf)
}
}

len += print_time(msg->ts_nsec, buf ? buf + len : NULL);
if (time)
len += print_time(msg->ts_nsec, buf ? buf + len : NULL);
return len;
}

static size_t msg_print_text(const struct printk_log *msg, bool syslog, char *buf, size_t size)
static size_t msg_print_text(const struct printk_log *msg, bool syslog,
bool time, char *buf, size_t size)
{
const char *text = log_text(msg);
size_t text_size = msg->text_len;
Expand All @@ -1285,17 +1284,17 @@ static size_t msg_print_text(const struct printk_log *msg, bool syslog, char *bu
}

if (buf) {
if (print_prefix(msg, syslog, NULL) +
if (print_prefix(msg, syslog, time, NULL) +
text_len + 1 >= size - len)
break;

len += print_prefix(msg, syslog, buf + len);
len += print_prefix(msg, syslog, time, buf + len);
memcpy(buf + len, text, text_len);
len += text_len;
buf[len++] = '\n';
} else {
/* SYSLOG_ACTION_* buffer size only calculation */
len += print_prefix(msg, syslog, NULL);
len += print_prefix(msg, syslog, time, NULL);
len += text_len;
len++;
}
Expand Down Expand Up @@ -1332,9 +1331,17 @@ static int syslog_print(char __user *buf, int size)
break;
}

/*
* To keep reading/counting partial line consistent,
* use printk_time value as of the beginning of a line.
*/
if (!syslog_partial)
syslog_time = printk_time;

skip = syslog_partial;
msg = log_from_idx(syslog_idx);
n = msg_print_text(msg, true, text, LOG_LINE_MAX + PREFIX_MAX);
n = msg_print_text(msg, true, syslog_time, text,
LOG_LINE_MAX + PREFIX_MAX);
if (n - syslog_partial <= size) {
/* message fits into buffer, move forward */
syslog_idx = log_next(syslog_idx);
Expand Down Expand Up @@ -1374,11 +1381,13 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
u64 next_seq;
u64 seq;
u32 idx;
bool time;

text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL);
if (!text)
return -ENOMEM;

time = printk_time;
logbuf_lock_irq();
/*
* Find first record that fits, including all following records,
Expand All @@ -1389,7 +1398,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
while (seq < log_next_seq) {
struct printk_log *msg = log_from_idx(idx);

len += msg_print_text(msg, true, NULL, 0);
len += msg_print_text(msg, true, time, NULL, 0);
idx = log_next(idx);
seq++;
}
Expand All @@ -1400,7 +1409,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
while (len > size && seq < log_next_seq) {
struct printk_log *msg = log_from_idx(idx);

len -= msg_print_text(msg, true, NULL, 0);
len -= msg_print_text(msg, true, time, NULL, 0);
idx = log_next(idx);
seq++;
}
Expand All @@ -1411,14 +1420,9 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
len = 0;
while (len >= 0 && seq < next_seq) {
struct printk_log *msg = log_from_idx(idx);
int textlen;
int textlen = msg_print_text(msg, true, time, text,
LOG_LINE_MAX + PREFIX_MAX);

textlen = msg_print_text(msg, true, text,
LOG_LINE_MAX + PREFIX_MAX);
if (textlen < 0) {
len = textlen;
break;
}
idx = log_next(idx);
seq++;

Expand Down Expand Up @@ -1542,11 +1546,14 @@ int do_syslog(int type, char __user *buf, int len, int source)
} else {
u64 seq = syslog_seq;
u32 idx = syslog_idx;
bool time = syslog_partial ? syslog_time : printk_time;

while (seq < log_next_seq) {
struct printk_log *msg = log_from_idx(idx);

error += msg_print_text(msg, true, NULL, 0);
error += msg_print_text(msg, true, time, NULL,
0);
time = printk_time;
idx = log_next(idx);
seq++;
}
Expand Down Expand Up @@ -2004,6 +2011,7 @@ EXPORT_SYMBOL(printk);

#define LOG_LINE_MAX 0
#define PREFIX_MAX 0
#define printk_time false

static u64 syslog_seq;
static u32 syslog_idx;
Expand All @@ -2027,8 +2035,8 @@ static void console_lock_spinning_enable(void) { }
static int console_lock_spinning_disable_and_check(void) { return 0; }
static void call_console_drivers(const char *ext_text, size_t ext_len,
const char *text, size_t len) {}
static size_t msg_print_text(const struct printk_log *msg,
bool syslog, char *buf, size_t size) { return 0; }
static size_t msg_print_text(const struct printk_log *msg, bool syslog,
bool time, char *buf, size_t size) { return 0; }
static bool suppress_message_printing(int level) { return false; }

#endif /* CONFIG_PRINTK */
Expand Down Expand Up @@ -2386,8 +2394,7 @@ void console_unlock(void)

len += msg_print_text(msg,
console_msg_format & MSG_FORMAT_SYSLOG,
text + len,
sizeof(text) - len);
printk_time, text + len, sizeof(text) - len);
if (nr_ext_console_drivers) {
ext_len = msg_print_ext_header(ext_text,
sizeof(ext_text),
Expand Down Expand Up @@ -3111,7 +3118,7 @@ bool kmsg_dump_get_line_nolock(struct kmsg_dumper *dumper, bool syslog,
goto out;

msg = log_from_idx(dumper->cur_idx);
l = msg_print_text(msg, syslog, line, size);
l = msg_print_text(msg, syslog, printk_time, line, size);

dumper->cur_idx = log_next(dumper->cur_idx);
dumper->cur_seq++;
Expand Down Expand Up @@ -3182,6 +3189,7 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
u32 next_idx;
size_t l = 0;
bool ret = false;
bool time = printk_time;

if (!dumper->active)
goto out;
Expand All @@ -3205,7 +3213,7 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
while (seq < dumper->next_seq) {
struct printk_log *msg = log_from_idx(idx);

l += msg_print_text(msg, true, NULL, 0);
l += msg_print_text(msg, true, time, NULL, 0);
idx = log_next(idx);
seq++;
}
Expand All @@ -3216,7 +3224,7 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
while (l > size && seq < dumper->next_seq) {
struct printk_log *msg = log_from_idx(idx);

l -= msg_print_text(msg, true, NULL, 0);
l -= msg_print_text(msg, true, time, NULL, 0);
idx = log_next(idx);
seq++;
}
Expand All @@ -3229,7 +3237,7 @@ bool kmsg_dump_get_buffer(struct kmsg_dumper *dumper, bool syslog,
while (seq < dumper->next_seq) {
struct printk_log *msg = log_from_idx(idx);

l += msg_print_text(msg, syslog, buf + l, size - l);
l += msg_print_text(msg, syslog, time, buf + l, size - l);
idx = log_next(idx);
seq++;
}
Expand Down

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