On Sun, May 02, 2021 at 10:23:26AM -0700, Kees Cook wrote:
> This reverts commit 03d1571d9513369c17e6848476763ebbd10ec2cb.
>
> While /sys/kernel/debug/acpi/custom_method is already a privileged-only
> API providing proxied arbitrary write access to kernel memory[1][2],
> with existing race conditions[3] in buffer allocation and use that could
> lead to memory leaks and use-after-free conditions, the above commit
> appears to accidentally make the use-after-free conditions even easier
> to accomplish. ("buf" is a global variable and prior kfree()s would set
> buf back to NULL.)
>
> This entire interface needs to be reworked (if not entirely removed).
>
> [1] https://lore.kernel.org/lkml/20110222193250.GA23913@xxxxxxxxxxx/
> [2] https://lore.kernel.org/lkml/201906221659.B618D83@keescook/
> [3] https://lore.kernel.org/lkml/20170109231323.GA89642@beast/
>
> Cc: Wenwen Wang <wenwen@xxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> drivers/acpi/custom_method.c | 5 +----
> 1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
> index 7b54dc95d36b..36d95a02cd30 100644
> --- a/drivers/acpi/custom_method.c
> +++ b/drivers/acpi/custom_method.c
> @@ -53,10 +53,8 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
> if ((*ppos > max_size) ||
> (*ppos + count > max_size) ||
> (*ppos + count < count) ||
> - (count > uncopied_bytes)) {
> - kfree(buf);
> + (count > uncopied_bytes))
> return -EINVAL;
> - }
>
> if (copy_from_user(buf + (*ppos), user_buf, count)) {
> kfree(buf);
> @@ -76,7 +74,6 @@ 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;
> }
>
> --
> 2.25.1
>
Thanks for the revert, I'll queue it up on my larger "umn.edu reverts"
branch that I'll be sending out for review in a day or so.
greg k-h
