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printk: fix buffer overflow potential for print_text()

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commit f0e386ee0c0b71ea6f7238506a4d0965a2dbef11 upstream.

Before the commit 896fbe20b4 ("printk: use the lockless
ringbuffer"), msg_print_text() would only write up to size-1 bytes
into the provided buffer. Some callers expect this behavior and
append a terminator to returned string. In particular:

arch/powerpc/xmon/xmon.c:dump_log_buf()
arch/um/kernel/kmsg_dump.c:kmsg_dumper_stdout()

msg_print_text() has been replaced by record_print_text(), which
currently fills the full size of the buffer. This causes a
buffer overflow for the above callers.

Change record_print_text() so that it will only use size-1 bytes
for text data. Also, for paranoia sakes, add a terminator after
the text data.

And finally, document this behavior so that it is clear that only
size-1 bytes are used and a terminator is added.

Fixes: 896fbe20b4 ("printk: use the lockless ringbuffer")
Cc: stable@vger.kernel.org # 5.10+
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210114170412.4819-1-john.ogness@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
John Ogness 2021-01-14 18:10:12 +01:06 committed by Greg Kroah-Hartman
parent cb14bbbb7b
commit 861c2e349a
1 changed files with 27 additions and 9 deletions
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36
kernel/printk/printk.c
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@ -1338,11 +1338,16 @@ static size_t info_print_prefix(const struct printk_info *info, bool syslog,
* done: * done:
* *
* - Add prefix for each line. * - Add prefix for each line.
* - Drop truncated lines that no longer fit into the buffer.
* - Add the trailing newline that has been removed in vprintk_store(). * - Add the trailing newline that has been removed in vprintk_store().
* - Drop truncated lines that do not longer fit into the buffer. * - Add a string terminator.
*
* Since the produced string is always terminated, the maximum possible
* return value is @r->text_buf_size - 1;
* *
* Return: The length of the updated/prepared text, including the added * Return: The length of the updated/prepared text, including the added
* prefixes and the newline. The dropped line(s) are not counted. * prefixes and the newline. The terminator is not counted. The dropped
* line(s) are not counted.
*/ */
static size_t record_print_text(struct printk_record *r, bool syslog, static size_t record_print_text(struct printk_record *r, bool syslog,
bool time) bool time)
@ -1385,26 +1390,31 @@ static size_t record_print_text(struct printk_record *r, bool syslog,
/* /*
* Truncate the text if there is not enough space to add the * Truncate the text if there is not enough space to add the
* prefix and a trailing newline. * prefix and a trailing newline and a terminator.
*/ */
if (len + prefix_len + text_len + 1 > buf_size) { if (len + prefix_len + text_len + 1 + 1 > buf_size) {
/* Drop even the current line if no space. */ /* Drop even the current line if no space. */
if (len + prefix_len + line_len + 1 > buf_size) if (len + prefix_len + line_len + 1 + 1 > buf_size)
break; break;
text_len = buf_size - len - prefix_len - 1; text_len = buf_size - len - prefix_len - 1 - 1;
truncated = true; truncated = true;
} }
memmove(text + prefix_len, text, text_len); memmove(text + prefix_len, text, text_len);
memcpy(text, prefix, prefix_len); memcpy(text, prefix, prefix_len);
/*
* Increment the prepared length to include the text and
* prefix that were just moved+copied. Also increment for the
* newline at the end of this line. If this is the last line,
* there is no newline, but it will be added immediately below.
*/
len += prefix_len + line_len + 1; len += prefix_len + line_len + 1;
if (text_len == line_len) { if (text_len == line_len) {
/* /*
* Add the trailing newline removed in * This is the last line. Add the trailing newline
* vprintk_store(). * removed in vprintk_store().
*/ */
text[prefix_len + line_len] = '\n'; text[prefix_len + line_len] = '\n';
break; break;
@ -1429,6 +1439,14 @@ static size_t record_print_text(struct printk_record *r, bool syslog,
text_len -= line_len + 1; text_len -= line_len + 1;
} }
/*
* If a buffer was provided, it will be terminated. Space for the
* string terminator is guaranteed to be available. The terminator is
* not counted in the return value.
*/
if (buf_size > 0)
text[len] = 0;
return len; return len;
} }