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SubjectRe: [PATCH] printk: don't unconditionally shortcut print_time()
From
Date
On 2018/12/04 11:56, Sergey Senozhatsky wrote:
> Can we please have something better than 'f'?

OK. To pass 80 column limit, one line increased. ;-)

< + bool f = syslog_partial ? syslog_time : printk_time;
> + bool time = syslog_partial ? syslog_time : printk_time;

< + error += msg_print_text(msg, true, f, NULL, 0);
< + f = printk_time;
> + error += msg_print_text(msg, true, time, NULL,
> + 0);
> + time = printk_time;

From d233699cbd1b43178ef6ce1f801e8a3b8e4326b9 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Tue, 4 Dec 2018 19:00:01 +0900
Subject: [PATCH v3] printk: fix printk_time race.

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().

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Petr Mladek <pmladek@suse.com>
---
kernel/printk/printk.c | 70 ++++++++++++++++++++++++++++----------------------
1 file changed, 39 insertions(+), 31 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5c2079d..4f18472 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -403,6 +403,7 @@ __packed __aligned(4)
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;
@@ -1228,12 +1229,7 @@ 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);
@@ -1242,7 +1238,8 @@ 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;
@@ -1261,11 +1258,13 @@ 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;
@@ -1284,17 +1283,17 @@ 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++;
}
@@ -1331,9 +1330,17 @@ static int syslog_print(char __user *buf, int size)
break;
}

+ /*
+ * To keep reading/counting partial line consistent,
+ * use printk_time value as of the biginning 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);
@@ -1373,11 +1380,13 @@ 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,
@@ -1388,7 +1397,7 @@ 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++;
}
@@ -1399,7 +1408,7 @@ 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++;
}
@@ -1410,14 +1419,9 @@ 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++;

@@ -1541,11 +1545,14 @@ 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++;
}
@@ -2003,6 +2010,7 @@ asmlinkage __visible int printk(const char *fmt, ...)

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

static u64 syslog_seq;
static u32 syslog_idx;
@@ -2026,8 +2034,8 @@ 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 */
@@ -2385,8 +2393,7 @@ 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),
@@ -3110,7 +3117,7 @@ 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++;
@@ -3181,6 +3188,7 @@ 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;
@@ -3204,7 +3212,7 @@ 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++;
}
@@ -3215,7 +3223,7 @@ 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++;
}
@@ -3228,7 +3236,7 @@ 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
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