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: Kees Cook <keescook@chromium.org>