[xen master] x86/altcall: Optimise away endbr64 instruction where possible

[patchbot@xxxxxxx]

commit 37ed5da851b867ae7133720dba3cf96e83bef2e1
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu Nov 4 19:36:23 2021 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed Feb 23 15:33:43 2022 +0000

    x86/altcall: Optimise away endbr64 instruction where possible
    
    With altcall, we convert indirect branches into direct ones.  With that
    complete, none of the potential targets need an endbr64 instruction.
    
    Furthermore, removing the endbr64 instructions is a security 
defence-in-depth
    improvement, because it limits the options available to an attacker who has
    managed to hijack a function pointer.
    
    Introduce new .init.{ro,}data.cf_clobber sections.  Have 
_apply_alternatives()
    walk over this, looking for any pointers into .text, and clobber an endbr64
    instruction if found.  This is some minor structure (ab)use but it works
    alarmingly well.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/alternative.c | 38 ++++++++++++++++++++++++++++++++++++++
 xen/arch/x86/xen.lds.S     |  6 ++++++
 xen/include/xen/init.h     |  3 +++
 3 files changed, 47 insertions(+)

diff --git a/xen/arch/x86/alternative.c b/xen/arch/x86/alternative.c
index ae7e646074..e19dfc6b36 100644
--- a/xen/arch/x86/alternative.c
+++ b/xen/arch/x86/alternative.c
@@ -173,6 +173,9 @@ text_poke(void *addr, const void *opcode, size_t len)
     return memcpy(addr, opcode, len);
 }
 
+extern void *const __initdata_cf_clobber_start[];
+extern void *const __initdata_cf_clobber_end[];
+
 /*
  * Replace instructions with better alternatives for this CPU type.
  * This runs before SMP is initialized to avoid SMP problems with
@@ -330,6 +333,41 @@ static void init_or_livepatch _apply_alternatives(struct 
alt_instr *start,
         add_nops(buf + a->repl_len, total_len - a->repl_len);
         text_poke(orig, buf, total_len);
     }
+
+    /*
+     * Clobber endbr64 instructions now that altcall has finished optimising
+     * all indirect branches to direct ones.
+     */
+    if ( force && cpu_has_xen_ibt )
+    {
+        void *const *val;
+        unsigned int clobbered = 0;
+
+        /*
+         * This is some minor structure (ab)use.  We walk the entire contents
+         * of .init.{ro,}data.cf_clobber as if it were an array of pointers.
+         *
+         * If the pointer points into .text, and at an endbr64 instruction,
+         * nop out the endbr64.  This causes the pointer to no longer be a
+         * legal indirect branch target under CET-IBT.  This is a
+         * defence-in-depth measure, to reduce the options available to an
+         * adversary who has managed to hijack a function pointer.
+         */
+        for ( val = __initdata_cf_clobber_start;
+              val < __initdata_cf_clobber_end;
+              val++ )
+        {
+            void *ptr = *val;
+
+            if ( !is_kernel_text(ptr) || !is_endbr64(ptr) )
+                continue;
+
+            add_nops(ptr, ENDBR64_LEN);
+            clobbered++;
+        }
+
+        printk("altcall: Optimised away %u endbr64 instructions\n", clobbered);
+    }
 }
 
 void init_or_livepatch apply_alternatives(struct alt_instr *start,
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 7ffecd4630..7b655df63f 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -223,6 +223,12 @@ SECTIONS
        *(.initcall1.init)
        __initcall_end = .;
 
+       . = ALIGN(POINTER_ALIGN);
+       __initdata_cf_clobber_start = .;
+       *(.init.data.cf_clobber)
+       *(.init.rodata.cf_clobber)
+       __initdata_cf_clobber_end = .;
+
        *(.init.data)
        *(.init.data.rel)
        *(.init.data.rel.*)
diff --git a/xen/include/xen/init.h b/xen/include/xen/init.h
index bfe789e93f..0af0e234ec 100644
--- a/xen/include/xen/init.h
+++ b/xen/include/xen/init.h
@@ -18,6 +18,9 @@
 #define __init_call(lvl)  __used_section(".initcall" lvl ".init")
 #define __exit_call       __used_section(".exitcall.exit")
 
+#define __initdata_cf_clobber  __section(".init.data.cf_clobber")
+#define __initconst_cf_clobber __section(".init.rodata.cf_clobber")
+
 /* These macros are used to mark some functions or 
  * initialized data (doesn't apply to uninitialized data)
  * as `initialization' functions. The kernel can take this
--
generated by git-patchbot for /home/xen/git/xen.git#master