[PATCH] KEYS: Don't write out to userspace while holding key semaphore

[Eric Biggers]

On Thu, Mar 05, 2020 at 04:06:40PM -0500, Waiman Long wrote:
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index 9b898c969558..564a4d187329 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -846,14 +846,36 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
>  can_read_key:
>  	ret = -EOPNOTSUPP;
>  	if (key->type->read) {
> -		/* Read the data with the semaphore held (since we might sleep)
> +		/*
> +		 * Read the data with the semaphore held (since we might sleep)
>  		 * to protect against the key being updated or revoked.
> +		 *
> +		 * Allocating a temporary buffer to hold the keys before
> +		 * transferring them to user buffer to avoid potential
> +		 * deadlock involving page fault and mmap_sem.
>  		 */
> +		char *tmpbuf = kmalloc(buflen, GFP_KERNEL);

This is passing an arbitrarily large size from userspace into kmalloc().

- Eric