Re: [PATCH 59.5/65] x86: Introduce helpers/checks for endbr64 instructions

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Fri, Nov 26, 2021 at 04:33:40PM +0000, Andrew Cooper wrote:
> ... to prevent the optimiser creating unsafe code.  See the code comment for
> full details.
> 
> Also add a build time check for endbr64 embedded in imm32 operands, which
> catches the obvious cases where the optimiser has done an unsafe thing.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
> ---
>  xen/arch/x86/Makefile       |  4 ++++
>  xen/include/asm-x86/endbr.h | 55 
> +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 59 insertions(+)
>  create mode 100644 xen/include/asm-x86/endbr.h
> 
> diff --git a/xen/arch/x86/Makefile b/xen/arch/x86/Makefile
> index 69b6cfaded25..64a5c0d20018 100644
> --- a/xen/arch/x86/Makefile
> +++ b/xen/arch/x86/Makefile
> @@ -190,6 +190,10 @@ $(TARGET)-syms: prelink.o xen.lds
>       $(MAKE) -f $(BASEDIR)/Rules.mk efi-y= $(@D)/.$(@F).1.o
>       $(LD) $(XEN_LDFLAGS) -T xen.lds -N prelink.o $(build_id_linker) \
>           $(@D)/.$(@F).1.o -o $@
> +ifeq ($(CONFIG_XEN_IBT),y)
> +     $(OBJDUMP) -d $@ | grep 0xfa1e0ff3 >/dev/null && \
> +             { echo "Found embedded endbr64 instructions" >&2; false; } || :
> +endif

Some more robust check can be done this way (warning, PoC quality bash):

    objcopy -j .text xen-syms xen-syms.text
    offset=$(objdump -h xen-syms -j .text | tail -2|head -1|awk '{printf 
"%x\n", (strtonum("0x" $4) - strtonum("0x" $6))}')
    objdump --adjust-vma=-0x$offset -d xen-syms.text|grep endbr | cut -f 1 -d 
':' | tr -d ' ' > valid-addrs
    grep -aob $'\xf3\x0f\x1e\xfa' xen-syms.text|cut -f 1 -d :|xargs printf 
'%x\n' > all-addrs
    join -v 2 <(sort valid-addrs) <(sort all-addrs) | awk '{ printf "%x\n", 
0x'$offset' + strtonum("0x" $1)}' | addr2line -e xen-syms

Currently it finds just one match:
xen/arch/x86/alternative.c:145

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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