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author | Alexandre SIMON <Alexandre.Simon@univ-lorraine.fr> | 2013-02-01 15:31:54 +0100 |
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committer | Ben Hutchings <ben@decadent.org.uk> | 2013-02-06 04:33:57 +0000 |
commit | fed286bde47fde5bf24916f48dd43811e916c139 (patch) | |
tree | 369d2b34997e3828f0c041e6c5d4758feeb3a62a | |
parent | cccd7d46b541d365a3cb1a87946a7fc30cf8e858 (diff) | |
download | lwn-fed286bde47fde5bf24916f48dd43811e916c139.tar.gz lwn-fed286bde47fde5bf24916f48dd43811e916c139.zip |
printk: fix buffer overflow when calling log_prefix function from call_console_drivers
This patch corrects a buffer overflow in kernels from 3.0 to 3.4 when calling
log_prefix() function from call_console_drivers().
This bug existed in previous releases but has been revealed with commit
162a7e7500f9664636e649ba59defe541b7c2c60 (2.6.39 => 3.0) that made changes
about how to allocate memory for early printk buffer (use of memblock_alloc).
It disappears with commit 7ff9554bb578ba02166071d2d487b7fc7d860d62 (3.4 => 3.5)
that does a refactoring of printk buffer management.
In log_prefix(), the access to "p[0]", "p[1]", "p[2]" or
"simple_strtoul(&p[1], &endp, 10)" may cause a buffer overflow as this
function is called from call_console_drivers by passing "&LOG_BUF(cur_index)"
where the index must be masked to do not exceed the buffer's boundary.
The trick is to prepare in call_console_drivers() a buffer with the necessary
data (PRI field of syslog message) to be safely evaluated in log_prefix().
This patch can be applied to stable kernel branches 3.0.y, 3.2.y and 3.4.y.
Without this patch, one can freeze a server running this loop from shell :
$ export DUMMY=`cat /dev/urandom | tr -dc '12345AZERTYUIOPQSDFGHJKLMWXCVBNazertyuiopqsdfghjklmwxcvbn' | head -c255`
$ while true do ; echo $DUMMY > /dev/kmsg ; done
The "server freeze" depends on where memblock_alloc does allocate printk buffer :
if the buffer overflow is inside another kernel allocation the problem may not
be revealed, else the server may hangs up.
Signed-off-by: Alexandre SIMON <Alexandre.Simon@univ-lorraine.fr>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
-rw-r--r-- | include/linux/syslog.h | 6 | ||||
-rw-r--r-- | kernel/printk.c | 13 |
2 files changed, 18 insertions, 1 deletions
diff --git a/include/linux/syslog.h b/include/linux/syslog.h index 38911391a139..ce4c66556059 100644 --- a/include/linux/syslog.h +++ b/include/linux/syslog.h @@ -47,6 +47,12 @@ #define SYSLOG_FROM_CALL 0 #define SYSLOG_FROM_FILE 1 +/* + * Syslog priority (PRI) maximum length in char : '<[0-9]{1,3}>' + * See RFC5424 for details +*/ +#define SYSLOG_PRI_MAX_LENGTH 5 + int do_syslog(int type, char __user *buf, int count, bool from_file); #endif /* _LINUX_SYSLOG_H */ diff --git a/kernel/printk.c b/kernel/printk.c index 7982a0a841ea..c0d12ea31c86 100644 --- a/kernel/printk.c +++ b/kernel/printk.c @@ -633,8 +633,19 @@ static void call_console_drivers(unsigned start, unsigned end) start_print = start; while (cur_index != end) { if (msg_level < 0 && ((end - cur_index) > 2)) { + /* + * prepare buf_prefix, as a contiguous array, + * to be processed by log_prefix function + */ + char buf_prefix[SYSLOG_PRI_MAX_LENGTH+1]; + unsigned i; + for (i = 0; i < ((end - cur_index)) && (i < SYSLOG_PRI_MAX_LENGTH); i++) { + buf_prefix[i] = LOG_BUF(cur_index + i); + } + buf_prefix[i] = '\0'; /* force '\0' as last string character */ + /* strip log prefix */ - cur_index += log_prefix(&LOG_BUF(cur_index), &msg_level, NULL); + cur_index += log_prefix((const char *)&buf_prefix, &msg_level, NULL); start_print = cur_index; } while (cur_index != end) { |