[PATCH 4/6] gpu: host1x: Avoid stack over-read in debug output helpers

[Mikko Perttunen]

host1x_debug_output() and host1x_debug_cont() used vsnprintf(), which
returns the length the formatted string would have reached with an
unbounded buffer. That return value was passed straight to o->fn as
the number of bytes to emit.

This could cause a read past end of the output buffer if a call to
host1x_debug_* produced a string longer than 256 bytes. This only
affected the debugfs files as the printk debug sink ignores the
number of bytes. In practice, this is very unlikely to occur.

Fix by switching to vscnprintf(), which returns the number of bytes
actually written.

Fixes: 6236451d83a7 ("gpu: host1x: Add debug support")
Signed-off-by: Mikko Perttunen <mperttunen@xxxxxxxxxx>
---
 drivers/gpu/host1x/debug.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/host1x/debug.c b/drivers/gpu/host1x/debug.c
index 6433c00d5d7e..b828f773fc06 100644
--- a/drivers/gpu/host1x/debug.c
+++ b/drivers/gpu/host1x/debug.c
@@ -31,7 +31,7 @@ void host1x_debug_output(struct output *o, const char *fmt, ...)
 	int len;
 
 	va_start(args, fmt);
-	len = vsnprintf(o->buf, sizeof(o->buf), fmt, args);
+	len = vscnprintf(o->buf, sizeof(o->buf), fmt, args);
 	va_end(args);
 
 	o->fn(o->ctx, o->buf, len, false);
@@ -43,7 +43,7 @@ void host1x_debug_cont(struct output *o, const char *fmt, ...)
 	int len;
 
 	va_start(args, fmt);
-	len = vsnprintf(o->buf, sizeof(o->buf), fmt, args);
+	len = vscnprintf(o->buf, sizeof(o->buf), fmt, args);
 	va_end(args);
 
 	o->fn(o->ctx, o->buf, len, true);

-- 
2.53.0