From 876d291b9b854094537aed557a6ebbb7dc5c2f5e Mon Sep 17 00:00:00 2001
From: Michael Kelley <mikelley@microsoft.com>
Date: Sun, 16 Jan 2022 11:18:31 -0800
Subject: video: hyperv_fb: Fix validation of screen resolution
Git-commit: 9ff5549b1d1d3c3a9d71220d44bd246586160f1d
Patch-mainline: v5.17-rc2
References: jsc#PED-1166 jsc#PED-1168 jsc#PED-1170 jsc#PED-1218 jsc#PED-1220 jsc#PED-1222 jsc#PED-1223 jsc#PED-1225

In the WIN10 version of the Synthetic Video protocol with Hyper-V,
Hyper-V reports a list of supported resolutions as part of the protocol
negotiation. The driver calculates the maximum width and height from
the list of resolutions, and uses those maximums to validate any screen
resolution specified in the video= option on the kernel boot line.

This method of validation is incorrect. For example, the list of
supported resolutions could contain 1600x1200 and 1920x1080, both of
which fit in an 8 Mbyte frame buffer.  But calculating the max width
and height yields 1920 and 1200, and 1920x1200 resolution does not fit
in an 8 Mbyte frame buffer.  Unfortunately, this resolution is accepted,
causing a kernel fault when the driver accesses memory outside the
frame buffer.

Instead, validate the specified screen resolution by calculating
its size, and comparing against the frame buffer size.  Delete the
code for calculating the max width and height from the list of
resolutions, since these max values have no use.  Also add the
frame buffer size to the info message to aid in understanding why
a resolution might be rejected.

Fixes: 67e7cdb4829d ("video: hyperv: hyperv_fb: Obtain screen resolution from Hyper-V host")
Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://lore.kernel.org/r/1642360711-2335-1-git-send-email-mikelley@microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Acked-by: Patrik Jakobsson <pjakobsson@suse.de>
---
 drivers/video/fbdev/hyperv_fb.c | 16 +++-------------
 1 file changed, 3 insertions(+), 13 deletions(-)

diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c
index 23999df52739..c8e0ea27caf1 100644
--- a/drivers/video/fbdev/hyperv_fb.c
+++ b/drivers/video/fbdev/hyperv_fb.c
@@ -287,8 +287,6 @@ struct hvfb_par {
 
 static uint screen_width = HVFB_WIDTH;
 static uint screen_height = HVFB_HEIGHT;
-static uint screen_width_max = HVFB_WIDTH;
-static uint screen_height_max = HVFB_HEIGHT;
 static uint screen_depth;
 static uint screen_fb_size;
 static uint dio_fb_size; /* FB size for deferred IO */
@@ -582,7 +580,6 @@ static int synthvid_get_supported_resolution(struct hv_device *hdev)
 	int ret = 0;
 	unsigned long t;
 	u8 index;
-	int i;
 
 	memset(msg, 0, sizeof(struct synthvid_msg));
 	msg->vid_hdr.type = SYNTHVID_RESOLUTION_REQUEST;
@@ -613,13 +610,6 @@ static int synthvid_get_supported_resolution(struct hv_device *hdev)
 		goto out;
 	}
 
-	for (i = 0; i < msg->resolution_resp.resolution_count; i++) {
-		screen_width_max = max_t(unsigned int, screen_width_max,
-		    msg->resolution_resp.supported_resolution[i].width);
-		screen_height_max = max_t(unsigned int, screen_height_max,
-		    msg->resolution_resp.supported_resolution[i].height);
-	}
-
 	screen_width =
 		msg->resolution_resp.supported_resolution[index].width;
 	screen_height =
@@ -941,7 +931,7 @@ static void hvfb_get_option(struct fb_info *info)
 
 	if (x < HVFB_WIDTH_MIN || y < HVFB_HEIGHT_MIN ||
 	    (synthvid_ver_ge(par->synthvid_version, SYNTHVID_VERSION_WIN10) &&
-	    (x > screen_width_max || y > screen_height_max)) ||
+	    (x * y * screen_depth / 8 > screen_fb_size)) ||
 	    (par->synthvid_version == SYNTHVID_VERSION_WIN8 &&
 	     x * y * screen_depth / 8 > SYNTHVID_FB_SIZE_WIN8) ||
 	    (par->synthvid_version == SYNTHVID_VERSION_WIN7 &&
@@ -1194,8 +1184,8 @@ static int hvfb_probe(struct hv_device *hdev,
 	}
 
 	hvfb_get_option(info);
-	pr_info("Screen resolution: %dx%d, Color depth: %d\n",
-		screen_width, screen_height, screen_depth);
+	pr_info("Screen resolution: %dx%d, Color depth: %d, Frame buffer size: %d\n",
+		screen_width, screen_height, screen_depth, screen_fb_size);
 
 	ret = hvfb_getmem(hdev, info);
 	if (ret) {
-- 
2.38.1

