um: mconsole: Fix out-of-bounds read in mconsole_log()

mconsole_parse() matches the 3-byte prefix "log", but mconsole_log()
skips strlen("log ") = 4 bytes, advancing ptr past the NUL terminator
when a client sends "log" without a trailing space.

The length then comes from req->len, the raw recvfrom() count, instead
of req->request.len, so printk() reads up to req->len - 4 bytes of
stale data from a previous request left in the static mc_request
buffer, leaking it to the kernel log.

Use req->cmd->command for the actual command length, add skip_spaces(),
use req->request.len, and guard against non-positive length.

Signed-off-by: Shengzhuo Wei <me@cherr.cc>
Link: https://patch.msgid.link/20260425-mconsole-oob-read-leak-v1-1-7d46e5892c5c@cherr.cc
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c
index 18d0ec15557c..f01b7d8a07bc 100644
--- a/arch/um/drivers/mconsole_kern.c
+++ b/arch/um/drivers/mconsole_kern.c
@@ -117,10 +117,12 @@ void mconsole_log(struct mc_request *req)
 	int len;
 	char *ptr = req->request.data;
 
-	ptr += strlen("log ");
+	ptr += strlen(req->cmd->command);
+	ptr = skip_spaces(ptr);
 
-	len = req->len - (ptr - req->request.data);
-	printk(KERN_WARNING "%.*s", len, ptr);
+	len = req->request.len - (ptr - req->request.data);
+	if (len > 0)
+		printk(KERN_WARNING "%.*s", len, ptr);
 	mconsole_reply(req, "", 0, 0);
 }