Re: [PATCH v2 4/6] xen/livepatch: do Xen build-id check earlier

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/09/2024 9:42 am, Roger Pau Monne wrote:
> The check against the expected Xen build ID should be done ahead of attempting
> to apply the alternatives contained in the livepatch.
>
> If the CPUID in the alternatives patching data is out of the scope of the
> running Xen featureset the BUG() in _apply_alternatives() will trigger thus
> bringing the system down.  Note the layout of struct alt_instr could also
> change between versions.  It's also possible for struct exception_table_entry
> to have changed format, hence leading to other kind of errors if parsing of 
> the
> payload is done ahead of checking if the Xen build-id matches.
>
> Move the Xen build ID check as early as possible.  To do so introduce a new
> check_xen_buildid() function that parses and checks the Xen build-id before
> moving the payload.  Since the expected Xen build-id is used early to
> detect whether the livepatch payload could be loaded, there's no reason to
> store it in the payload struct, as a non-matching Xen build-id won't get the
> payload populated in the first place.
>
> Note parse_buildid() has to be slightly adjusted to allow fetching the section
> data from the 'data' field instead of the 'load_addr' one: with the Xen build
> ID moved ahead of move_payload() 'load_addr' is not yet set when the Xen build
> ID check is performed.  Also printing the expected Xen build ID has part of
> dumping the payload is no longer done, as all loaded payloads would have Xen
> build IDs matching the running Xen, otherwise they would have failed to load.
>
> Fixes: 879615f5db1d ('livepatch: Always check hypervisor build ID upon 
> livepatch upload')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Much nicer.  A couple of suggestions.

> ---
> Changes since v1:
>  - Do the Xen build-id check even earlier.
> ---
>  xen/common/livepatch.c              | 66 +++++++++++++++++++----------
>  xen/include/xen/livepatch_payload.h |  1 -
>  2 files changed, 44 insertions(+), 23 deletions(-)
>
> diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
> index 8e61083f23a7..895c425cd5ea 100644
> --- a/xen/common/livepatch.c
> +++ b/xen/common/livepatch.c
> @@ -448,24 +448,21 @@ static bool section_ok(const struct livepatch_elf *elf,
>      return true;
>  }
>  
> -static int xen_build_id_dep(const struct payload *payload)
> +static int xen_build_id_dep(const struct livepatch_build_id *expected)
>  {
>      const void *id = NULL;
>      unsigned int len = 0;
>      int rc;
>  
> -    ASSERT(payload->xen_dep.len);
> -    ASSERT(payload->xen_dep.p);
> +    ASSERT(expected->len);
> +    ASSERT(expected->p);
>  
>      rc = xen_build_id(&id, &len);
>      if ( rc )
>          return rc;
>  
> -    if ( payload->xen_dep.len != len || memcmp(id, payload->xen_dep.p, len) 
> ) {
> -        printk(XENLOG_ERR LIVEPATCH "%s: check against hypervisor build-id 
> failed\n",
> -               payload->name);
> +    if ( expected->len != len || memcmp(id, expected->p, len) )
>          return -EINVAL;
> -    }

I'd suggest merging this into check_xen_buildid(), which is the single
caller and serves the same singular purpose.

It removes the ASSERT() (expected is now a local variable), and it helps
with some diagnostic improvements.

>  
>      return 0;
>  }
> @@ -495,11 +493,44 @@ static int parse_buildid(const struct livepatch_elf_sec 
> *sec,
>     return 0;
>  }
>  
> +static int check_xen_buildid(const struct livepatch_elf *elf)
> +{
> +    struct livepatch_build_id id;
> +    const struct livepatch_elf_sec *sec =
> +        livepatch_elf_sec_by_name(elf, ELF_LIVEPATCH_XEN_DEPENDS);
> +    int rc;
> +
> +    if ( !sec )
> +    {
> +        printk(XENLOG_ERR LIVEPATCH "%s: %s is missing\n",

"%s: Section: '%s' missing\n".

(Maybe no single quotes around the section as we know it's non-empty.)

> +               elf->name, ELF_LIVEPATCH_XEN_DEPENDS);
> +        return -EINVAL;
> +    }
> +
> +    rc = parse_buildid(sec, &id);
> +    if ( rc )
> +    {
> +        printk(XENLOG_ERR LIVEPATCH "%s: failed to parse build-id at %s: 
> %d\n",

"%s: Failed to parse section '%s' as build id: %d\n".

> +               elf->name, ELF_LIVEPATCH_XEN_DEPENDS, rc);
> +        return -EINVAL;
> +    }
> +
> +    rc = xen_build_id_dep(&id);
> +    if ( rc )
> +    {
> +        printk(XENLOG_ERR LIVEPATCH
> +               "%s: check against hypervisor build-id failed: %d\n",

"%s: build-id mismatch:\n"
"  livepatch: %*phN\n"
"        xen: %*phN\n"

This needs xen_build_id_dep() inlining in order to have the xen build-id
string, but the end result is much more informative.

I think I'm happy doing all of this on commit, but it might be a better
idea not to.

~Andrew