[PATCH 4.14 018/323] ACPI: custom_method: fix potential use-after-free issue

[Greg Kroah-Hartman]

From: Mark Langsdorf <mlangsdo@xxxxxxxxxx>

commit e483bb9a991bdae29a0caa4b3a6d002c968f94aa upstream.

In cm_write(), buf is always freed when reaching the end of the
function.  If the requested count is less than table.length, the
allocated buffer will be freed but subsequent calls to cm_write() will
still try to access it.

Remove the unconditional kfree(buf) at the end of the function and
set the buf to NULL in the -EINVAL error path to match the rest of
function.

Fixes: 03d1571d9513 ("ACPI: custom_method: fix memory leaks")
Signed-off-by: Mark Langsdorf <mlangsdo@xxxxxxxxxx>
Cc: 5.4+ <stable@xxxxxxxxxxxxxxx> # 5.4+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/acpi/custom_method.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/acpi/custom_method.c
+++ b/drivers/acpi/custom_method.c
@@ -50,6 +50,7 @@ static ssize_t cm_write(struct file *fil
 	    (*ppos + count < count) ||
 	    (count > uncopied_bytes)) {
 		kfree(buf);
+		buf = NULL;
 		return -EINVAL;
 	}
 
@@ -71,7 +72,6 @@ static ssize_t cm_write(struct file *fil
 		add_taint(TAINT_OVERRIDDEN_ACPI_TABLE, LOCKDEP_NOW_UNRELIABLE);
 	}
 
-	kfree(buf);
 	return count;
 }