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From: Linus Walleij <>
To: Andre Przywara <>
Cc: Russell King <>,
	Ard Biesheuvel <>,
	linux-efi <>,
	Linux ARM <>,
	linux-kernel <>,
	Catalin Marinas <>,
	Adam Lackorzynski <>,
	Peter Maydell <>
Subject: Re: [PATCH] ARM: decompressor: Avoid UNPREDICTABLE NOP encoding
Date: Tue, 14 Sep 2021 00:43:46 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Wed, Sep 8, 2021 at 6:26 PM Andre Przywara <> 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]
> Signed-off-by: Andre Przywara <>
> Reported-by: Adam Lackorzynski <>
> Suggested-by: Peter Maydell <>

Looks like a correct and clever solution to me!
Reviewed-by: Linus Walleij <>

Linus Walleij

      parent reply	other threads:[~2021-09-13 23:01 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-08 16:26 Andre Przywara
2021-09-09 10:48 ` Ard Biesheuvel
2021-09-13 22:43 ` Linus Walleij [this message]

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