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    Subject[PATCH 5.15 261/846] ARM: 9159/1: decompressor: Avoid UNPREDICTABLE NOP encoding
    Date
    From: Andre Przywara <andre.przywara@arm.com>

    [ Upstream commit a92882a4d270fbcc021ee6848de5e48b7f0d27f3 ]

    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/

    Link: https://lore.kernel.org/linux-arm-kernel/20210908162617.104962-1-andre.przywara@arm.com/T/

    Fixes: 81a0bc39ea19 ("ARM: add UEFI stub support")
    Signed-off-by: Andre Przywara <andre.przywara@arm.com>
    Reported-by: Adam Lackorzynski <adam@l4re.org>
    Suggested-by: Peter Maydell <peter.maydell@linaro.org>
    Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
    Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
    Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
    Signed-off-by: Sasha Levin <sashal@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 c0e7a745103e2..230030c130853 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 b1cb1972361b8..bf79f2f78d232 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
    --
    2.34.1


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