x86: Build check for embedded endbr64 instructions

An interesting corner case occurs when the byte sequence making up endbr64
ends up on a non-instruction boundary.  Such embedded instructions mark legal
indirect branch targets as far as the CPU is concerned, which aren't legal as
far as the logic is concerned.

When CET-IBT is active, check for embedded byte sequences.  Example failures
look like:

  check-endbr.sh xen-syms Fail: Found 2 embedded endbr64 instructions
  0xffff82d040325677: test_endbr64 at /local/xen.git/xen/arch/x86/x86_64/entry.S:28
  0xffff82d040352da6: init_done at /local/xen.git/xen/arch/x86/setup.c:675

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

README

@@ -68,6 +68,7 @@ provided by your OS distributor:
 In addition to the above there are a number of optional build
 prerequisites. Omitting these will cause the related features to be
 disabled at compile time:
+    * Binary-search capable grep (if building Xen with CET support)
     * Development install of Ocaml (e.g. ocaml-nox and
       ocaml-findlib). Required to build ocaml components which
       includes the alternative ocaml xenstored.

xen/arch/x86/Makefile

@@ -142,6 +142,9 @@ $(TARGET)-syms: $(BASEDIR)/prelink.o $(obj)/xen.lds
 		| $(BASEDIR)/tools/symbols --all-symbols --xensyms --sysv --sort \
 		>$(@D)/$(@F).map
 	rm -f $(@D)/.$(@F).[0-9]* $(@D)/..$(@F).[0-9]*
+ifeq ($(CONFIG_XEN_IBT),y)
+	$(SHELL) $(BASEDIR)/tools/check-endbr.sh $@
+endif
 
 $(obj)/note.o: $(TARGET)-syms
 	$(OBJCOPY) -O binary --only-section=.note.gnu.build-id $< $@.bin
@@ -212,6 +215,9 @@ endif
 	$(NM) -pa --format=sysv $(@D)/$(@F) \
 		| $(BASEDIR)/tools/symbols --all-symbols --xensyms --sysv --sort >$(@D)/$(@F).map
 	rm -f $(@D)/.$(@F).[0-9]* $(@D)/..$(@F).[0-9]*
+ifeq ($(CONFIG_XEN_IBT),y)
+	$(SHELL) $(BASEDIR)/tools/check-endbr.sh $@
+endif
 else
 $(TARGET).efi: FORCE
 	rm -f $@

xen/tools/check-endbr.sh

@@ -0,0 +1,86 @@
+#!/bin/sh
+#
+# Usage ./$0 xen-syms
+#
+set -e
+
+# Pretty-print parameters a little for message
+MSG_PFX="${0##*/} ${1##*/}"
+
+OBJCOPY="${OBJCOPY:-objcopy}"
+OBJDUMP="${OBJDUMP:-objdump}"
+ADDR2LINE="${ADDR2LINE:-addr2line}"
+
+D=$(mktemp -d)
+trap "rm -rf $D" EXIT
+
+TEXT_BIN=$D/xen-syms.text
+VALID=$D/valid-addrs
+ALL=$D/all-addrs
+BAD=$D/bad-addrs
+
+# Check that grep can do binary searches.  Some, e.g. busybox, can't.  Leave a
+# warning but don't fail the build.
+echo "X" | grep -aob "X" -q 2>/dev/null ||
+    { echo "$MSG_PFX Warning: grep can't do binary searches" >&2; exit 0; }
+
+#
+# First, look for all the valid endbr64 instructions.
+# A worst-case disassembly, viewed through cat -A, may look like:
+#
+# ffff82d040337bd4 <endbr64>:$
+# ffff82d040337bd4:^If3 0f 1e fa          ^Iendbr64 $
+# ffff82d040337bd8:^Ieb fe                ^Ijmp    ffff82d040337bd8 <endbr64+0x4>$
+# ffff82d040337bda:^Ib8 f3 0f 1e fa       ^Imov    $0xfa1e0ff3,%eax$
+#
+# Want to grab the address of endbr64 instructions only, ignoring function
+# names/jump labels/etc, so look for 'endbr64' preceded by a tab and with any
+# number of trailing spaces before the end of the line.
+#
+${OBJDUMP} -j .text $1 -d -w | grep '	endbr64 *$' | cut -f 1 -d ':' > $VALID &
+
+#
+# Second, look for any endbr64 byte sequence
+# This has a couple of complications:
+#
+# 1) Grep binary search isn't VMA aware.  Copy .text out as binary, causing
+#    the grep offset to be from the start of .text.
+#
+# 2) dash's printf doesn't understand hex escapes, hence the use of octal.
+#
+# 3) AWK can't add 64bit integers, because internally all numbers are doubles.
+#    When the upper bits are set, the exponents worth of precision is lost in
+#    the lower bits, rounding integers to the nearest 4k.
+#
+#    Instead, use the fact that Xen's .text is within a 1G aligned region, and
+#    split the VMA in half so AWK's numeric addition is only working on 32 bit
+#    numbers, which don't lose precision.
+#
+eval $(${OBJDUMP} -j .text $1 -h |
+    awk '$2 == ".text" {printf "vma_hi=%s\nvma_lo=%s\n", substr($4, 1, 8), substr($4, 9, 16)}')
+
+${OBJCOPY} -j .text $1 -O binary $TEXT_BIN
+grep -aob "$(printf '\363\17\36\372')" $TEXT_BIN |
+    awk -F':' '{printf "%s%x\n", "'$vma_hi'", int(0x'$vma_lo') + $1}' > $ALL
+
+# Wait for $VALID to become complete
+wait
+
+# Sanity check $VALID and $ALL, in case the string parsing bitrots
+val_sz=$(stat -c '%s' $VALID)
+all_sz=$(stat -c '%s' $ALL)
+[ "$val_sz" -eq 0 ]         && { echo "$MSG_PFX Error: Empty valid-addrs" >&2; exit 1; }
+[ "$all_sz" -eq 0 ]         && { echo "$MSG_PFX Error: Empty all-addrs" >&2; exit 1; }
+[ "$all_sz" -lt "$val_sz" ] && { echo "$MSG_PFX Error: More valid-addrs than all-addrs" >&2; exit 1; }
+
+# $BAD = $ALL - $VALID
+sort $VALID $ALL | uniq -u > $BAD
+nr_bad=$(wc -l < $BAD)
+
+# Success
+[ "$nr_bad" -eq 0 ] && exit 0
+
+# Failure
+echo "$MSG_PFX Fail: Found ${nr_bad} embedded endbr64 instructions" >&2
+${ADDR2LINE} -afip -e $1 < $BAD >&2
+exit 1