hardening: Introduce CONFIG_ZERO_CALL_USED_REGS

When CONFIG_ZERO_CALL_USED_REGS is enabled, build the kernel with
"-fzero-call-used-regs=used-gpr" (in GCC 11). This option will zero any
caller-used register contents just before returning from a function,
ensuring that temporary values are not leaked beyond the function
boundary. This means that register contents are less likely to be
available for side channel attacks and information exposures.

Additionally this helps reduce the number of useful ROP gadgets in the
kernel image by about 20%:

$ ROPgadget.py --nosys --nojop --binary vmlinux.stock | tail -n1
Unique gadgets found: 337245

$ ROPgadget.py --nosys --nojop --binary vmlinux.zero-call-regs | tail -n1
Unique gadgets found: 267175

and more notably removes simple "write-what-where" gadgets:

$ ROPgadget.py --ropchain --binary vmlinux.stock | sed -n '/Step 1/,/Step 2/p'
- Step 1 -- Write-what-where gadgets

        [+] Gadget found: 0xffffffff8102d76c mov qword ptr [rsi], rdx ; ret
        [+] Gadget found: 0xffffffff81000cf5 pop rsi ; ret
        [+] Gadget found: 0xffffffff8104d7c8 pop rdx ; ret
        [-] Can't find the 'xor rdx, rdx' gadget. Try with another 'mov [reg], reg'

        [+] Gadget found: 0xffffffff814c2b4c mov qword ptr [rsi], rdi ; ret
        [+] Gadget found: 0xffffffff81000cf5 pop rsi ; ret
        [+] Gadget found: 0xffffffff81001e51 pop rdi ; ret
        [-] Can't find the 'xor rdi, rdi' gadget. Try with another 'mov [reg], reg'

        [+] Gadget found: 0xffffffff81540d61 mov qword ptr [rsi], rdi ; pop rbx ; pop rbp ; ret
        [+] Gadget found: 0xffffffff81000cf5 pop rsi ; ret
        [+] Gadget found: 0xffffffff81001e51 pop rdi ; ret
        [-] Can't find the 'xor rdi, rdi' gadget. Try with another 'mov [reg], reg'

        [+] Gadget found: 0xffffffff8105341e mov qword ptr [rsi], rax ; ret
        [+] Gadget found: 0xffffffff81000cf5 pop rsi ; ret
        [+] Gadget found: 0xffffffff81029a11 pop rax ; ret
        [+] Gadget found: 0xffffffff811f1c3b xor rax, rax ; ret

- Step 2 -- Init syscall number gadgets

$ ROPgadget.py --ropchain --binary vmlinux.zero* | sed -n '/Step 1/,/Step 2/p'
- Step 1 -- Write-what-where gadgets

        [-] Can't find the 'mov qword ptr [r64], r64' gadget

For an x86_64 parallel build tests, this has a less than 1% performance
impact, and grows the image size less than 1%:

$ size vmlinux.stock vmlinux.zero-call-regs
   text    data     bss     dec     hex filename
22437676   8559152 14127340 45124168 2b08a48 vmlinux.stock
22453184   8563248 14110956 45127388 2b096dc vmlinux.zero-call-regs

Impact for other architectures may vary. For example, arm64 sees a 5.5%
image size growth, mainly due to needing to always clear x16 and x17:
https://lore.kernel.org/lkml/20210510134503.GA88495@C02TD0UTHF1T.local/Signed-off-by: NKees Cook <keescook@chromium.org>

Makefile

@@ -842,6 +842,11 @@ endif
 # for the randomize_kstack_offset feature. Disable it for all compilers.
 KBUILD_CFLAGS	+= $(call cc-option, -fno-stack-clash-protection)
 
+# Clear used registers at func exit (to reduce data lifetime and ROP gadgets).
+ifdef CONFIG_ZERO_CALL_USED_REGS
+KBUILD_CFLAGS	+= -fzero-call-used-regs=used-gpr
+endif
+
 DEBUG_CFLAGS	:=
 
 # Workaround for GCC versions < 5.0

security/Kconfig.hardening

@@ -217,6 +217,25 @@ config INIT_ON_FREE_DEFAULT_ON
 	  touching "cold" memory areas. Most cases see 3-5% impact. Some
 	  synthetic workloads have measured as high as 8%.
 
+config CC_HAS_ZERO_CALL_USED_REGS
+	def_bool $(cc-option,-fzero-call-used-regs=used-gpr)
+
+config ZERO_CALL_USED_REGS
+	bool "Enable register zeroing on function exit"
+	depends on CC_HAS_ZERO_CALL_USED_REGS
+	help
+	  At the end of functions, always zero any caller-used register
+	  contents. This helps ensure that temporary values are not
+	  leaked beyond the function boundary. This means that register
+	  contents are less likely to be available for side channels
+	  and information exposures. Additionally, this helps reduce the
+	  number of useful ROP gadgets by about 20% (and removes compiler
+	  generated "write-what-where" gadgets) in the resulting kernel
+	  image. This has a less than 1% performance impact on most
+	  workloads. Image size growth depends on architecture, and should
+	  be evaluated for suitability. For example, x86_64 grows by less
+	  than 1%, and arm64 grows by about 5%.
+
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