[PATCH 4.9 137/207] auxdisplay: img-ascii-lcd: Fix lock-up when displaying empty string

Greg Kroah-Hartman
2021-11-24T12:22:08Z

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 6e8eaa7fe7a6f..b5f849d2f7623 100644
--- a/drivers/auxdisplay/img-ascii-lcd.c
+++ b/drivers/auxdisplay/img-ascii-lcd.c
@@ -283,6 +283,16 @@ 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