[PATCH] xen/x86: Livepatch: support patching CET-enhanced functions

[Bjoern Doebel <doebel@xxxxxxxxx>]

Xen enabled CET for supporting architectures. The control flow aspect of
CET expects functions that can be called indirectly (i.e., via function
pointers) to start with an ENDBR64 instruction. Otherwise a control flow
exception is raised.

This expectation breaks livepatching flows because we patch functions by
overwriting their first 5 bytes with a JMP + <offset>, thus breaking the
ENDBR64. We fix this by checking the start of a patched function for
being ENDBR64. In the positive case we move the livepatch JMP to start
behind the ENDBR64 instruction.

To avoid having to guess the ENDBR64 offset again on patch reversal
(which might race with other mechanisms adding/removing ENDBR
dynamically), use the livepatch metadata to store the computed offset
along with the saved bytes of the overwritten function.

Signed-off-by: Bjoern Doebel <doebel@xxxxxxxxx>
CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
CC: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>

Note that on top of livepatching functions, Xen supports an additional
mode where we can "remove" a function by overwriting it with NOPs. This
is only supported for functions up to 31 bytes in size and this patch
reduces this limit to 27 bytes.
---
 xen/arch/x86/livepatch.c | 39 +++++++++++++++++++++++++++++++++------
 1 file changed, 33 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/livepatch.c b/xen/arch/x86/livepatch.c
index 65530c1e57..bb7ecf36a6 100644
--- a/xen/arch/x86/livepatch.c
+++ b/xen/arch/x86/livepatch.c
@@ -14,11 +14,28 @@
 #include <xen/vm_event.h>
 #include <xen/virtual_region.h>
 
+#include <asm/endbr.h>
 #include <asm/fixmap.h>
 #include <asm/nmi.h>
 #include <asm/livepatch.h>
 #include <asm/setup.h>
 
+/*
+ * CET hotpatching support: We may have functions starting with an ENDBR64 
instruction
+ * that MUST remain the first instruction of the function, hence we need to 
move any
+ * hotpatch trampoline further into the function. For that we need to keep 
track of the
+ * patching offset used for any loaded hotpatch (to avoid racing against other 
fixups
+ * adding/removing ENDBR64 or similar instructions).
+ *
+ * We do so by making use of the existing opaque metadata area. We use its 
first 4 bytes
+ * to track the offset into the function used for patching and the remainder 
of the data
+ * to store overwritten code bytes.
+ */
+struct __packed x86_livepatch_meta {
+    int32_t patch_offset;
+    uint8_t instruction[LIVEPATCH_OPAQUE_SIZE - sizeof(int32_t)];
+};
+
 static bool has_active_waitqueue(const struct vm_event_domain *ved)
 {
     /* ved may be xzalloc()'d without INIT_LIST_HEAD() yet. */
@@ -104,11 +121,14 @@ void noinline arch_livepatch_revive(void)
 
 int arch_livepatch_verify_func(const struct livepatch_func *func)
 {
+    BUILD_BUG_ON(sizeof(struct x86_livepatch_meta) != LIVEPATCH_OPAQUE_SIZE);
+
     /* If NOPing.. */
     if ( !func->new_addr )
     {
+        struct x86_livepatch_meta *lp = (struct 
x86_livepatch_meta*)func->opaque;
         /* Only do up to maximum amount we can put in the ->opaque. */
-        if ( func->new_size > sizeof(func->opaque) )
+        if ( func->new_size > sizeof(lp->instruction) )
             return -EOPNOTSUPP;
 
         if ( func->old_size < func->new_size )
@@ -127,15 +147,20 @@ int arch_livepatch_verify_func(const struct 
livepatch_func *func)
 void noinline arch_livepatch_apply(struct livepatch_func *func)
 {
     uint8_t *old_ptr;
-    uint8_t insn[sizeof(func->opaque)];
+    struct x86_livepatch_meta *lp = (struct x86_livepatch_meta*)func->opaque;
+    uint8_t insn[sizeof(lp->instruction)];
     unsigned int len;
 
+    lp->patch_offset = 0;
     old_ptr = func->old_addr;
     len = livepatch_insn_len(func);
     if ( !len )
         return;
 
-    memcpy(func->opaque, old_ptr, len);
+    if ( is_endbr64( old_ptr ) )
+        lp->patch_offset += ENDBR64_LEN;
+
+    memcpy(lp->instruction, old_ptr + lp->patch_offset, len);
     if ( func->new_addr )
     {
         int32_t val;
@@ -143,14 +168,14 @@ void noinline arch_livepatch_apply(struct livepatch_func 
*func)
         BUILD_BUG_ON(ARCH_PATCH_INSN_SIZE != (1 + sizeof(val)));
 
         insn[0] = 0xe9; /* Relative jump. */
-        val = func->new_addr - func->old_addr - ARCH_PATCH_INSN_SIZE;
+        val = func->new_addr - (func->old_addr + lp->patch_offset + 
ARCH_PATCH_INSN_SIZE);
 
         memcpy(&insn[1], &val, sizeof(val));
     }
     else
         add_nops(insn, len);
 
-    memcpy(old_ptr, insn, len);
+    memcpy(old_ptr + lp->patch_offset, insn, len);
 }
 
 /*
@@ -159,7 +184,9 @@ void noinline arch_livepatch_apply(struct livepatch_func 
*func)
  */
 void noinline arch_livepatch_revert(const struct livepatch_func *func)
 {
-    memcpy(func->old_addr, func->opaque, livepatch_insn_len(func));
+    struct x86_livepatch_meta *lp = (struct x86_livepatch_meta*)func->opaque;
+
+    memcpy(func->old_addr + lp->patch_offset, lp->instruction, 
livepatch_insn_len(func));
 }
 
 /*
-- 
2.32.0




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