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Message-Id: <1445700024-19895-1-git-send-email-minipli@googlemail.com> Date: Sat, 24 Oct 2015 17:20:24 +0200 From: Mathias Krause <minipli@...glemail.com> To: Andrew Morton <akpm@...ux-foundation.org> Cc: linux-kernel@...r.kernel.org, Mathias Krause <minipli@...glemail.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Petr Mladek <pmladek@...e.cz>, Alex Elder <elder@...aro.org>, Joe Perches <joe@...ches.com>, Kay Sievers <kay@...y.org> Subject: [PATCH] printk: prevent userland from spoofing kernel messages 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@...glemail.com> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org> Cc: Petr Mladek <pmladek@...e.cz> Cc: Alex Elder <elder@...aro.org> Cc: Joe Perches <joe@...ches.com> Cc: Kay Sievers <kay@...y.org> --- Might be worth to apply to stable, too. Don't know. Prior to commit 7ff9554bb578 there was no facility so user injected messages had an implicit facility for LOG_KERN. But commit 7ff9554bb578 explicitly mentions this feature, so, dunno :/ kernel/printk/printk.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 8f0324ef72ab..9982616ce712 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -269,6 +269,9 @@ 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 @@ -611,7 +614,6 @@ struct devkmsg_user { static ssize_t devkmsg_write(struct kiocb *iocb, struct iov_iter *from) { char *buf, *line; - int i; int level = default_message_loglevel; int facility = 1; /* LOG_USER */ size_t len = iov_iter_count(from); @@ -641,12 +643,13 @@ static ssize_t devkmsg_write(struct kiocb *iocb, struct iov_iter *from) 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; -- 1.7.10.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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