Re: [PATCH v2] livepatch: account for patch offset when applying NOP patch

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Mar 30, 2022 at 01:05:31PM +0200, Jan Beulich wrote:
> While not triggered by the trivial xen_nop in-tree patch on
> staging/master, that patch exposes a problem on the stable trees, where
> all functions have ENDBR inserted. When NOP-ing out a range, we need to
> account for this. Handle this right in livepatch_insn_len().
> 
> This requires livepatch_insn_len() to be called _after_ ->patch_offset
> was set. Note however that the earlier call cannot be deleted. In fact
> its result should have been used to guard the is_endbr64() /
> is_endbr64_poison() invocations - add the missing check now. While
> making that adjustment, also use the local variable "old_ptr"
> consistently.
> 
> Fixes: 6974c75180f1 ("xen/x86: Livepatch: support patching CET-enhanced 
> functions")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v2: Re-issue livepatch_insn_len(). Fix buffer overrun.
> ---
> Only build tested, as I don't have a live patching environment available.
> 
> For Arm this assumes that the patch_offset field starts out as zero; I
> think we can make such an assumption, yet otoh on x86 explicit
> initialization was added by the cited commit.
> 
> Note that the other use of is_endbr64() / is_endbr64_poison() in
> arch_livepatch_verify_func() would need the extra check only for
> cosmetic reasons, because ARCH_PATCH_INSN_SIZE > ENDBR64_LEN (5 > 4).
> Hence I'm not altering the code there.
> 
> --- a/xen/arch/x86/livepatch.c
> +++ b/xen/arch/x86/livepatch.c
> @@ -157,9 +157,15 @@ void noinline arch_livepatch_apply(struc
>       * loaded hotpatch (to avoid racing against other fixups adding/removing
>       * ENDBR64 or similar instructions).
>       */
> -    if ( is_endbr64(old_ptr) || is_endbr64_poison(func->old_addr) )
> +    if ( len >= ENDBR64_LEN &&

Sorry, didn't realize before, but shouldn't this check be using
old_size instead of len (which is based on new_size)?

Thanks, Roger.