[PATCH] ARM: decompressor: Avoid UNPREDICTABLE NOP encoding

[Ard Biesheuvel]

On Wed, 8 Sept 2021 at 18:26, Andre Przywara <andre.przywara at arm.com> wrote:
>
> In the decompressor's head.S we need to start with an instruction that
> is some kind of NOP, but also mimics as the PE/COFF header, when the
> kernel is linked as an UEFI application. The clever solution here is
> "tstne r0, #0x4d000", which in the worst case just clobbers the
> condition flags, and bears the magic "MZ" signature in the lowest 16 bits.
>
> However the encoding used (0x13105a4d) is actually not valid, since bits
> [15:12] are supposed to be 0 (written as "(0)" in the ARM ARM).
> Violating this is UNPREDICTABLE, and *can* trigger an UNDEFINED
> exception. Common Cortex cores seem to ignore those bits, but QEMU
> chooses to trap, so the code goes fishing because of a missing exception
> handler at this point. We are just saved by the fact that commonly (with
> -kernel or when running from U-Boot) the "Z" bit is set, so the
> instruction is never executed. See [0] for more details.
>
> To make things more robust and avoid UNPREDICTABLE behaviour in the
> kernel code, lets replace this with a "two-instruction NOP":
> The first instruction is an exclusive OR, the effect of which the second
> instruction reverts. This does not leave any trace, neither in a
> register nor in the condition flags. Also it's a perfectly valid
> encoding. Kudos to Peter Maydell for coming up with this gem.
>
> [0] https://lore.kernel.org/qemu-devel/YTPIdbUCmwagL5%2FD@os.inf.tu-dresden.de/T/
>
> Signed-off-by: Andre Przywara <andre.przywara at arm.com>
> Reported-by: Adam Lackorzynski <adam at l4re.org>
> Suggested-by: Peter Maydell <peter.maydell at linaro.org>

Thanks a lot for fixing this! I had no idea the encoding was invalid.

Reviewed-by: Ard Biesheuvel <ardb at kernel.org>

> ---
>  arch/arm/boot/compressed/efi-header.S | 22 ++++++++++++++--------
>  arch/arm/boot/compressed/head.S       |  3 ++-
>  2 files changed, 16 insertions(+), 9 deletions(-)
>
> diff --git a/arch/arm/boot/compressed/efi-header.S b/arch/arm/boot/compressed/efi-header.S
> index c0e7a745103e..230030c13085 100644
> --- a/arch/arm/boot/compressed/efi-header.S
> +++ b/arch/arm/boot/compressed/efi-header.S
> @@ -9,16 +9,22 @@
>  #include <linux/sizes.h>
>
>                 .macro  __nop
> -#ifdef CONFIG_EFI_STUB
> -               @ This is almost but not quite a NOP, since it does clobber the
> -               @ condition flags. But it is the best we can do for EFI, since
> -               @ PE/COFF expects the magic string "MZ" at offset 0, while the
> -               @ ARM/Linux boot protocol expects an executable instruction
> -               @ there.
> -               .inst   MZ_MAGIC | (0x1310 << 16)       @ tstne r0, #0x4d000
> -#else
>   AR_CLASS(     mov     r0, r0          )
>    M_CLASS(     nop.w                   )
> +               .endm
> +
> +               .macro __initial_nops
> +#ifdef CONFIG_EFI_STUB
> +               @ This is a two-instruction NOP, which happens to bear the
> +               @ PE/COFF signature "MZ" in the first two bytes, so the kernel
> +               @ is accepted as an EFI binary. Booting via the UEFI stub
> +               @ will not execute those instructions, but the ARM/Linux
> +               @ boot protocol does, so we need some NOPs here.
> +               .inst   MZ_MAGIC | (0xe225 << 16)       @ eor r5, r5, 0x4d000
> +               eor     r5, r5, 0x4d000                 @ undo previous insn
> +#else
> +               __nop
> +               __nop
>  #endif
>                 .endm
>
> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> index b1cb1972361b..bf79f2f78d23 100644
> --- a/arch/arm/boot/compressed/head.S
> +++ b/arch/arm/boot/compressed/head.S
> @@ -203,7 +203,8 @@ start:
>                  * were patching the initial instructions of the kernel, i.e
>                  * had started to exploit this "patch area".
>                  */
> -               .rept   7
> +               __initial_nops
> +               .rept   5
>                 __nop
>                 .endr
>  #ifndef CONFIG_THUMB2_KERNEL
> --
> 2.17.1
>