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SubjectRe: [PATCH] printk: don't unconditionally shortcut print time()
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Date
Petr Mladek wrote:
> See syslog_print_all() and kmsg_dump_get_buffer(). They
> start with calling msg_print_text() many times to calculate
> how many messages fit into the given buffer. Then they
> blindly copy the messages into the buffer.
>
> Now, the buffer might overflow if the size is calculated
> with printk_time disabled and the messages are copied with
> printk_time enabled.
>

Indeed, SYSLOG_ACTION_READ_ALL case can write more bytes than userspace
has supplied, resulting in possible memory corruption in userpsace. And
obtaining a snapshot of printk_time under logbuf_lock won't help, for we
need to drop logbuf_lock before calling copy_to_user().

----------
@@ -1365,6 +1365,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
if (!text)
return -ENOMEM;

+ printk_time = false;
logbuf_lock_irq();
/*
* Find first record that fits, including all following records,
@@ -1409,6 +1410,7 @@ static int syslog_print_all(char __user *buf, int size, bool clear)
seq++;

logbuf_unlock_irq();
+ printk_time = true;
if (copy_to_user(buf + len, text, textlen))
len = -EFAULT;
else
----------
----------
#include <sys/klog.h>
#define SYSLOG_ACTION_READ_ALL 3
int main(int argc, char *argv[])
{
char buffer[4096];
int i;
for (i = 0; i < sizeof(buffer); i++)
if (klogctl(SYSLOG_ACTION_READ_ALL, buffer, i) > i)
return 1;
return 0;
}
----------
----------
syslog(SYSLOG_ACTION_READ_ALL, "", 0) = 0
(...snipped...)
syslog(SYSLOG_ACTION_READ_ALL, "", 59) = 0
syslog(SYSLOG_ACTION_READ_ALL, "<6>IPv6: ADDRCONF(NETDEV_CHANGE): ens32: link becomes ready\n", 60) = 60
(...snipped...)
syslog(SYSLOG_ACTION_READ_ALL, "<6>IPv6: ADDRCONF(NETDEV_CHANGE): ens32: link becomes ready\n", 114) = 60
syslog(SYSLOG_ACTION_READ_ALL, "<6>IPv6: ADDRCONF(NETDEV_UP): ens32: link is not ready\n<6>[ 10.517132] IPv6: ADDRCONF(NETDEV_CHANGE): ens32: link becomes ready\n", 115) = 130
----------
> IMHO, passing it as parameter to msg_print_text(), print_prefix(),
> and print_time() looks acceptable.

Below is a patch for passing a snapshot of printk_time to msg_print_text(),
but we are still failing to close a race explained at bottom.

kernel/printk/printk.c | 59 ++++++++++++++++++++++++--------------------------
1 file changed, 28 insertions(+), 31 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1b2a029..3838a6b 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1215,12 +1215,7 @@ static inline void boot_delay_msec(int level)

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);
@@ -1229,7 +1224,8 @@ 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;
@@ -1248,11 +1244,13 @@ 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;
@@ -1271,17 +1269,17 @@ 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++;
}
@@ -1320,7 +1318,8 @@ static int syslog_print(char __user *buf, int size)

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, printk_time, text,
+ LOG_LINE_MAX + PREFIX_MAX);
if (n - syslog_partial <= size) {
/* message fits into buffer, move forward */
syslog_idx = log_next(syslog_idx);
@@ -1360,11 +1359,13 @@ 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,
@@ -1375,7 +1376,7 @@ 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++;
}
@@ -1386,7 +1387,7 @@ 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++;
}
@@ -1397,14 +1398,9 @@ 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++;

@@ -1532,7 +1528,8 @@ int do_syslog(int type, char __user *buf, int len, int source)
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, printk_time,
+ NULL, 0);
idx = log_next(idx);
seq++;
}
@@ -2028,8 +2025,8 @@ 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 */
@@ -2387,8 +2384,7 @@ 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),
@@ -3112,7 +3108,7 @@ 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++;
@@ -3183,6 +3179,7 @@ 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;
@@ -3206,7 +3203,7 @@ 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++;
}
@@ -3217,7 +3214,7 @@ 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++;
}
@@ -3230,7 +3227,7 @@ 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++;
}
--
1.8.3.1
The remaining race is that klogctl(SYSLOG_ACTION_SIZE_UNREAD) after klogctl(SYSLOG_ACTION_READ)
can return negative value upon success, for SYSLOG_ACTION_SIZE_UNREAD depends that printk_time
did not change (from true to false) after SYSLOG_ACTION_READ updated syslog_partial. To close
this race, we need to make sure that printk_time is evaluated only once for each record.
That is, evaluate printk_time upon e.g. log_store() rather than upon msg_print_text().

kernel/printk/printk.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1b2a029..c692fbc 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -421,6 +421,8 @@ __packed __aligned(4)
static u64 clear_seq;
static u32 clear_idx;

+static bool printk_time = IS_ENABLED(CONFIG_PRINTK_TIME);
+
#define PREFIX_MAX 32
#define LOG_LINE_MAX (1024 - PREFIX_MAX)

@@ -620,7 +622,9 @@ static int log_store(int facility, int level,
msg->facility = facility;
msg->level = level & 7;
msg->flags = flags & 0x1f;
- if (ts_nsec > 0)
+ if (!printk_time)
+ msg->ts_nsec = (u64) -1ULL;
+ else if (ts_nsec > 0)
msg->ts_nsec = ts_nsec;
else
msg->ts_nsec = local_clock();
@@ -1210,14 +1214,13 @@ static inline void boot_delay_msec(int level)
}
#endif

-static bool printk_time = IS_ENABLED(CONFIG_PRINTK_TIME);
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)
+ if (ts == (u64) -1ULL)
return 0;

rem_nsec = do_div(ts, 1000000000);
--
1.8.3.1
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