From d72cf01f410aa09868d98b672f3f92328c96b32d Mon Sep 17 00:00:00 2001
From: Dan Carpenter <dan.carpenter@oracle.com>
Date: Wed, 21 Aug 2019 10:24:56 +0300
Subject: drm/mipi-dbi: fix a loop in debugfs code
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Git-commit: d72cf01f410aa09868d98b672f3f92328c96b32d
Patch-mainline: v5.5-rc1
References: bsc#1152489

This code will likely crash if we try to do a zero byte write.  The code
looks like this:

        /* strip trailing whitespace */
        for (i = count - 1; i > 0; i--)
                if (isspace(buf[i]))
			...

We're writing zero bytes so count = 0.  You would think that "count - 1"
would be negative one, but because "i" is unsigned it is a large
positive numer instead.  The "i > 0" condition is true and the "buf[i]"
access will be out of bounds.

The fix is to make "i" signed and now everything works as expected.  The
upper bound of "count" is capped in __kernel_write() at MAX_RW_COUNT so
we don't have to worry about it being higher than INT_MAX.

Fixes: 02dd95fe3169 ("drm/tinydrm: Add MIPI DBI support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[noralf: Adjust title]
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190821072456.GJ26957@mwanda
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/tinydrm/mipi-dbi.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/drivers/gpu/drm/tinydrm/mipi-dbi.c
+++ b/drivers/gpu/drm/tinydrm/mipi-dbi.c
@@ -1033,8 +1033,7 @@ static ssize_t mipi_dbi_debugfs_command_
 	struct mipi_dbi *mipi = m->private;
 	u8 val, cmd = 0, parameters[64];
 	char *buf, *pos, *token;
-	unsigned int i;
-	int ret, idx;
+	int i, ret, idx;
 
 	if (!drm_dev_enter(&mipi->drm, &idx))
 		return -ENODEV;