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

[Jan Beulich <jbeulich@xxxxxxxx>]

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 &&
+         (is_endbr64(old_ptr) || is_endbr64_poison(old_ptr)) )
         func->patch_offset += ENDBR64_LEN;
 
+    /* This call must be re-issued once ->patch_offset has its final value. */
+    len = livepatch_insn_len(func);
+    if ( !len )
+        return;
+
     memcpy(func->opaque, old_ptr + func->patch_offset, len);
     if ( func->new_addr )
     {
--- a/xen/include/xen/livepatch.h
+++ b/xen/include/xen/livepatch.h
@@ -90,7 +90,7 @@ static inline
 unsigned int livepatch_insn_len(const struct livepatch_func *func)
 {
     if ( !func->new_addr )
-        return func->new_size;
+        return func->new_size - func->patch_offset;
 
     return ARCH_PATCH_INSN_SIZE;
 }