From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Geert Uytterhoeven <geert@linux-m68k.org>,
        Miguel Ojeda <ojeda@kernel.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.19 209/323] auxdisplay: img-ascii-lcd: Fix lock-up when displaying empty string
Date:   Wed, 24 Nov 2021 12:56:39 +0100
Message-Id: <20211124115725.986627946@linuxfoundation.org>
In-Reply-To: <20211124115718.822024889@linuxfoundation.org>
References: <20211124115718.822024889@linuxfoundation.org>
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From: Geert Uytterhoeven <geert@linux-m68k.org>

[ Upstream commit afcb5a811ff3ab3969f09666535eb6018a160358 ]

While writing an empty string to a device attribute is a no-op, and thus
does not need explicit safeguards, the user can still write a single
newline to an attribute file:

    echo > .../message

If that happens, img_ascii_lcd_display() trims the newline, yielding an
empty string, and causing an infinite loop in img_ascii_lcd_scroll().

Fix this by adding a check for empty strings.  Clear the display in case
one is encountered.

Fixes: 0cad855fbd083ee5 ("auxdisplay: img-ascii-lcd: driver for simple ASCII LCD displays")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/auxdisplay/img-ascii-lcd.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/auxdisplay/img-ascii-lcd.c b/drivers/auxdisplay/img-ascii-lcd.c
index 834509506ef64..c4bc6723acfa5 100644
--- a/drivers/auxdisplay/img-ascii-lcd.c
+++ b/drivers/auxdisplay/img-ascii-lcd.c
@@ -284,6 +284,16 @@ static int img_ascii_lcd_display(struct img_ascii_lcd_ctx *ctx,
 	if (msg[count - 1] == '\n')
 		count--;
 
+	if (!count) {
+		/* clear the LCD */
+		devm_kfree(&ctx->pdev->dev, ctx->message);
+		ctx->message = NULL;
+		ctx->message_len = 0;
+		memset(ctx->curr, ' ', ctx->cfg->num_chars);
+		ctx->cfg->update(ctx);
+		return 0;
+	}
+
 	new_msg = devm_kmalloc(&ctx->pdev->dev, count + 1, GFP_KERNEL);
 	if (!new_msg)
 		return -ENOMEM;
-- 
2.33.0