FAILED: patch "[PATCH] drm/amd/display: guard against overflow in HDCP message dump" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 93c8fe6d56037f284be7116d0c8155847c6d7fbe
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072944-flagman-thimble-4ccb@gregkh' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 93c8fe6d56037f284be7116d0c8155847c6d7fbe Mon Sep 17 00:00:00 2001
From: Harry Wentland <[email protected]>
Date: Tue, 16 Jun 2026 12:17:45 -0400
Subject: [PATCH] drm/amd/display: guard against overflow in HDCP message dump

[Why]
mod_hdcp_dump_binary_message() computed target_size (a uint32_t) as roughly byte_size * msg_size and gated the whole write on buf_size >= target_size. A large msg_size can overflow target_size, wrapping it to a small value that passes the check while the loop still writes byte_size * msg_size bytes into buf. All current callers pass small constants so this is not reachable today, but the unchecked arithmetic should be hardened.

[How]
Drop the overflow-prone target_size precomputation and instead bounds-check the output position on every iteration, stopping once the next entry would not leave room for the trailing terminator. This cannot overflow and, for oversized messages, dumps as much as fits rather than printing nothing.

index 1164fd96b714..f0f8e280ed30 100644
@@ -33,22 +33,28 @@ void mod_hdcp_dump_binary_message(uint8_t *msg, uint32_t msg_size,
 			byte_size = 3,
 			newline_size = 1,
 			terminator_size = 1;
-	uint32_t line_count = msg_size / bytes_per_line,
-			trailing_bytes = msg_size % bytes_per_line;
-	uint32_t target_size = (byte_size * bytes_per_line + newline_size) * line_count +
-			byte_size * trailing_bytes + newline_size + terminator_size;
 	uint32_t buf_pos = 0;
 	uint32_t i = 0;
 
-	if (buf_size >= target_size) {
-		for (i = 0; i < msg_size; i++) {
-			if (i % bytes_per_line == 0)
-				buf[buf_pos++] = '\n';
-			sprintf((char *)&buf[buf_pos], "%02X ", msg[i]);
-			buf_pos += byte_size;
-		}
-		buf[buf_pos++] = '\0';
+	/* Need room for at least the terminator. */
+	if (buf_size < terminator_size)
+		return;
+
+	for (i = 0; i < msg_size; i++) {
+		uint32_t needed = byte_size + terminator_size;
+
+		if (i % bytes_per_line == 0)
+			needed += newline_size;
+
+		if (buf_pos + needed > buf_size)
+			break;
+
+		if (i % bytes_per_line == 0)
+			buf[buf_pos++] = '\n';
+		sprintf((char *)&buf[buf_pos], "%02X ", msg[i]);
+		buf_pos += byte_size;
 	}
+	buf[buf_pos++] = '\0';
 }
 
 void mod_hdcp_log_ddc_trace(struct mod_hdcp *hdcp)