| From | Luis Henriques <> | Subject | [PATCH 3.16.y-ckt 125/126] printk: prevent userland from spoofing kernel messages | Date | Wed, 9 Dec 2015 09:38:15 +0000 |
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3.16.7-ckt21 -stable review patch. If anyone has any objections, please let me know.
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From: Mathias Krause <minipli@googlemail.com>
commit 3824657c522f19f85a76bd932821174a5557a382 upstream.
The following statement of ABI/testing/dev-kmsg is not quite right:
It is not possible to inject messages from userspace with the facility number LOG_KERN (0), to make sure that the origin of the messages can always be reliably determined.
Userland actually can inject messages with a facility of 0 by abusing the fact that the facility is stored in a u8 data type. By using a facility which is a multiple of 256 the assignment of msg->facility in log_store() implicitly truncates it to 0, i.e. LOG_KERN, allowing users of /dev/kmsg to spoof kernel messages as shown below:
The following call... # printf '<%d>Kernel panic - not syncing: beer empty\n' 0 >/dev/kmsg ...leads to the following log entry (dmesg -x | tail -n 1): user :emerg : [ 66.137758] Kernel panic - not syncing: beer empty
However, this call... # printf '<%d>Kernel panic - not syncing: beer empty\n' 0x800 >/dev/kmsg ...leads to the slightly different log entry (note the kernel facility): kern :emerg : [ 74.177343] Kernel panic - not syncing: beer empty
Fix that by limiting the user provided facility to 8 bit right from the beginning and catch the truncation early.
Fixes: 7ff9554bb578 ("printk: convert byte-buffer to variable-length...") Signed-off-by: Mathias Krause <minipli@googlemail.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Petr Mladek <pmladek@suse.cz> Cc: Alex Elder <elder@linaro.org> Cc: Joe Perches <joe@perches.com> Cc: Kay Sievers <kay@vrfy.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> [ kamal: backport to 3.13-stable: retain local 'int i' ] Signed-off-by: Kamal Mostafa <kamal@canonical.com> Signed-off-by: Luis Henriques <luis.henriques@canonical.com> --- kernel/printk/printk.c | 12 ++++++++---- 1 file changed, 8 insertions(+), 4 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index f7f6f7e5ff8a..5fa3f3b3b338 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -259,6 +259,9 @@ static u32 clear_idx; #define PREFIX_MAX 32 #define LOG_LINE_MAX 1024 - PREFIX_MAX +#define LOG_LEVEL(v) ((v) & 0x07) +#define LOG_FACILITY(v) ((v) >> 3 & 0xff) + /* record buffer */ #if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) #define LOG_ALIGN 4 @@ -547,12 +550,13 @@ static ssize_t devkmsg_writev(struct kiocb *iocb, const struct iovec *iv, line = buf; if (line[0] == '<') { char *endp = NULL; + unsigned int u; - i = simple_strtoul(line+1, &endp, 10); + u = simple_strtoul(line + 1, &endp, 10); if (endp && endp[0] == '>') { - level = i & 7; - if (i >> 3) - facility = i >> 3; + level = LOG_LEVEL(u); + if (LOG_FACILITY(u) != 0) + facility = LOG_FACILITY(u); endp++; len -= endp - line; line = endp;
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