[PATCH] ACPI: custom_method: fix potential use-after-free issue

[PATCH] ACPI: custom_method: fix potential use-after-free issue

[Mark Langsdorf]

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@redhat.com>
---
 drivers/acpi/custom_method.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
index 443fdf62dd22..8844f895f9be 100644
--- a/drivers/acpi/custom_method.c
+++ b/drivers/acpi/custom_method.c
@@ -55,6 +55,7 @@ static ssize_t cm_write(struct file *file, const char __user *user_buf,
 	    (*ppos + count < count) ||
 	    (count > uncopied_bytes)) {
 		kfree(buf);
+		buf = NULL;
 		return -EINVAL;
 	}
 
@@ -76,7 +77,6 @@ static ssize_t cm_write(struct file *file, const char __user *user_buf,
 		add_taint(TAINT_OVERRIDDEN_ACPI_TABLE, LOCKDEP_NOW_UNRELIABLE);
 	}
 
-	kfree(buf);
 	return count;
 }
 
-- 
2.26.3

Re: [PATCH] ACPI: custom_method: fix potential use-after-free issue

[Rafael J. Wysocki]

On Fri, Apr 23, 2021 at 5:28 PM Mark Langsdorf <mlangsdo@redhat.com> wrote:
>
> 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@redhat.com>
> ---
>  drivers/acpi/custom_method.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
> index 443fdf62dd22..8844f895f9be 100644
> --- a/drivers/acpi/custom_method.c
> +++ b/drivers/acpi/custom_method.c
> @@ -55,6 +55,7 @@ static ssize_t cm_write(struct file *file, const char __user *user_buf,
>             (*ppos + count < count) ||
>             (count > uncopied_bytes)) {
>                 kfree(buf);
> +               buf = NULL;
>                 return -EINVAL;
>         }
>
> @@ -76,7 +77,6 @@ static ssize_t cm_write(struct file *file, const char __user *user_buf,
>                 add_taint(TAINT_OVERRIDDEN_ACPI_TABLE, LOCKDEP_NOW_UNRELIABLE);
>         }
>
> -       kfree(buf);
>         return count;
>  }
>
> --

Applied as 5.13-rc material, thanks!