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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Dan Carpenter <dan.carpenter@oracle.com>,
        =?UTF-8?q?Noralf=20Tr=C3=B8nnes?= <noralf@tronnes.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.4 046/434] drm/mipi-dbi: fix a loop in debugfs code
Date:   Sun, 29 Dec 2019 18:21:39 +0100
Message-Id: <20191229172705.060721032@linuxfoundation.org>
In-Reply-To: <20191229172702.393141737@linuxfoundation.org>
References: <20191229172702.393141737@linuxfoundation.org>
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From: Dan Carpenter <dan.carpenter@oracle.com>

[ Upstream commit d72cf01f410aa09868d98b672f3f92328c96b32d ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/gpu/drm/drm_mipi_dbi.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_mipi_dbi.c b/drivers/gpu/drm/drm_mipi_dbi.c
index 1961f713aaab..c4ee2709a6f3 100644
--- a/drivers/gpu/drm/drm_mipi_dbi.c
+++ b/drivers/gpu/drm/drm_mipi_dbi.c
@@ -1187,8 +1187,7 @@ static ssize_t mipi_dbi_debugfs_command_write(struct file *file,
 	struct mipi_dbi_dev *dbidev = 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(&dbidev->drm, &idx))
 		return -ENODEV;
-- 
2.20.1