linux-kernel - Re: [PATCH qemu v3] x86: don't let decompressed kernel image clobber setup

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Message-ID: <ee8ff694-4a6b-b4da-9a80-c456fda9c99d@grsecurity.net>
Date:   Tue, 10 Jan 2023 13:10:13 +0100
From:   Mathias Krause <minipli@...ecurity.net>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     qemu-devel@...gnu.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        ebiggers@...nel.org, ardb@...nel.org, kraxel@...hat.com,
        hpa@...or.com, bp@...en8.de, philmd@...aro.org
Subject: Re: [PATCH qemu v3] x86: don't let decompressed kernel image clobber
 setup_data

Hi Jason!

Am 30.12.22 um 23:07 schrieb Jason A. Donenfeld:
> The setup_data links are appended to the compressed kernel image. Since
> the kernel image is typically loaded at 0x100000, setup_data lives at
> `0x100000 + compressed_size`, which does not get relocated during the
> kernel's boot process.
> 
> The kernel typically decompresses the image starting at address
> 0x1000000 (note: there's one more zero there than the compressed image
> above). This usually is fine for most kernels.
> 
> However, if the compressed image is actually quite large, then
> setup_data will live at a `0x100000 + compressed_size` that extends into
> the decompressed zone at 0x1000000. In other words, if compressed_size
> is larger than `0x1000000 - 0x100000`, then the decompression step will
> clobber setup_data, resulting in crashes.
> 
> Visually, what happens now is that QEMU appends setup_data to the kernel
> image:
> 
>           kernel image            setup_data
>    |--------------------------||----------------|
> 0x100000                  0x100000+l1     0x100000+l1+l2
> 
> The problem is that this decompresses to 0x1000000 (one more zero). So
> if l1 is > (0x1000000-0x100000), then this winds up looking like:
> 
>           kernel image            setup_data
>    |--------------------------||----------------|
> 0x100000                  0x100000+l1     0x100000+l1+l2
> 
>                                  d e c o m p r e s s e d   k e r n e l
>                      |-------------------------------------------------------------|
>                 0x1000000                                                     0x1000000+l3
> 
> The decompressed kernel seemingly overwriting the compressed kernel
> image isn't a problem, because that gets relocated to a higher address
> early on in the boot process, at the end of startup_64. setup_data,
> however, stays in the same place, since those links are self referential
> and nothing fixes them up.  So the decompressed kernel clobbers it.

I just ran into this very issue yesterday when trying to boot a 6.1
kernel. pipacs pointed me to some changes of yours[1] which confirmed,
the issue is related to the additional setup_data entries, as adding,
e.g., '-M pc-i440fx-7.0' to the QEMU command line made the bug vanish
(as QEMU then omits adding the random seed setup_data entries) .

[1] https://github.com/qemu/qemu/commit/67f7e426e538

After digging a while I found this thread and it fixes the issue for me,
thereby:

Tested-by: Mathias Krause <minipli@...ecurity.net>

Thanks,
Mathias

> [snip]