Re: [PATCH v3 4/5] livepatch: Load built-in key during boot

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.06.2025 15:36, Ross Lagerwall wrote:
> Parse the raw data of the embedded RSA key into a form that can be later
> used for verifying live patch signatures.
> 
> Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> ---
> 
> * Fix endianness of builtin key lengths
> * Set builtin key to __ro_after_init
> 
>  xen/common/livepatch.c | 36 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
> index 9a0df5363b59..92d1d342d872 100644
> --- a/xen/common/livepatch.c
> +++ b/xen/common/livepatch.c
> @@ -11,6 +11,8 @@
>  #include <xen/lib.h>
>  #include <xen/list.h>
>  #include <xen/mm.h>
> +#include <xen/mpi.h>
> +#include <xen/rsa.h>
>  #include <xen/sched.h>
>  #include <xen/smp.h>
>  #include <xen/softirq.h>
> @@ -73,6 +75,10 @@ static struct livepatch_work livepatch_work;
>  static DEFINE_PER_CPU(bool, work_to_do);
>  static DEFINE_PER_CPU(struct tasklet, livepatch_tasklet);
>  
> +#ifdef CONFIG_PAYLOAD_VERIFY
> +static struct rsa_public_key __ro_after_init builtin_payload_key;
> +#endif
> +
>  static int get_name(const struct xen_livepatch_name *name, char *n)
>  {
>      if ( !name->size || name->size > XEN_LIVEPATCH_NAME_SIZE )
> @@ -2300,6 +2306,31 @@ static void cf_check livepatch_printall(unsigned char 
> key)
>      spin_unlock(&payload_lock);
>  }
>  
> +static int load_builtin_payload_key(void)

__init

> +{
> +#ifdef CONFIG_PAYLOAD_VERIFY
> +    const uint8_t *ptr;
> +    uint32_t len;
> +
> +    rsa_public_key_init(&builtin_payload_key);
> +
> +    ptr = xen_livepatch_key_data;
> +
> +    memcpy(&len, ptr, sizeof(len));

How do you know enough data is available?

> +    ptr += sizeof(len);
> +    builtin_payload_key.n = mpi_read_raw_data(ptr, le32_to_cpu(len));

Even more so here. And then again below.

> +    ptr += len;
> +
> +    memcpy(&len, ptr, sizeof(len));
> +    ptr += sizeof(len);
> +    builtin_payload_key.e = mpi_read_raw_data(ptr, le32_to_cpu(len));
> +
> +    return rsa_public_key_prepare(&builtin_payload_key);
> +#else
> +    return 0;
> +#endif
> +}
> +
>  static int cf_check cpu_callback(
>      struct notifier_block *nfb, unsigned long action, void *hcpu)
>  {
> @@ -2318,6 +2349,11 @@ static struct notifier_block cpu_nfb = {
>  static int __init cf_check livepatch_init(void)
>  {
>      unsigned int cpu;
> +    int err;
> +
> +    err = load_builtin_payload_key();
> +    if ( err )
> +        return err;
>  
>      for_each_online_cpu ( cpu )
>      {

So far this function can't fail. Therefore it makes sense that I can't find any
provisions for this case elsewhere in the file. Such provisions will need
adding as a prereq, I expect. Or alternatively it would want clarifying
explicitly that (why) none are required.

Jan