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Date:   Wed, 28 Dec 2022 23:30:39 -0800
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
CC:     pbonzini@redhat.com, ebiggers@kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, qemu-devel@nongnu.org,
        ardb@kernel.org, kraxel@redhat.com, bp@alien8.de, philmd@linaro.org
Subject: =?US-ASCII?Q?Re=3A_=5BPATCH_qemu=5D_x86=3A_don=27t_let_decomp?= =?US-ASCII?Q?ressed_kernel_image_clobber_setup=5Fdata?=
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On December 28, 2022 6:31:07 PM PST, "Jason A=2E Donenfeld" <Jason@zx2c4=2E=
com> wrote:
>Hi,
>
>Read this message in a fixed width text editor with a lot of columns=2E
>
>On Wed, Dec 28, 2022 at 03:58:12PM -0800, H=2E Peter Anvin wrote:
>> Glad you asked=2E
>>=20
>> So the kernel load addresses are parameterized in the kernel image
>> setup header=2E One of the things that are so parameterized are the siz=
e
>> and possible realignment of the kernel image in memory=2E
>>=20
>> I'm very confused where you are getting the 64 MB number from=2E There
>> should not be any such limitation=2E
>
>Currently, QEMU appends it to the kernel image, not to the initramfs as
>you suggest below=2E So, that winds up looking, currently, like:
>
>          kernel image            setup_data
>   |--------------------------||----------------|
>0x100000                  0x100000+l1     0x100000+l1+l2
>
>The problem is that this decompresses to 0x1000000 (one more zero)=2E 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=20
>
>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=2E setup_data, however, stays in the same
>place, since those links are self referential and nothing fixes them up=
=2E
>So the decompressed kernel clobbers it=2E
>
>The solution in this commit adds a bunch of padding between the kernel
>image and setup_data to avoid this=2E That looks like this:
>
>          kernel image                            padding                =
               setup_data
>   |--------------------------||-----------------------------------------=
----------||----------------|
>0x100000                  0x100000+l1                                    =
     0x1000000+l3      0x1000000+l3+l2
>
>                                 d e c o m p r e s s e d   k e r n e l
>		     |-------------------------------------------------------------|
>                0x1000000                                                =
     0x1000000+l3=20
>
>This way, the decompressed kernel doesn't clobber setup_data=2E
>
>The problem is that if 0x1000000+l3-0x100000 is around 62 megabytes,
>then the bootloader crashes when trying to dereference setup_data's
>->len param at the end of initialize_identity_maps() in ident_map_64=2Ec=
=2E
>I don't know why it does this=2E If I could remove the 62 megabyte
>restriction, then I could keep with this technique and all would be
>well=2E
>
>> In general, setup_data should be able to go anywhere the initrd can
>> go, and so is subject to the same address cap (896 MB for old kernels,
>> 4 GB on newer ones; this address too is enumerated in the header=2E)
>
>It would be theoretically possible to attach it to the initrd image
>instead of to the kernel image=2E As a last resort, I guess I can look
>into doing that=2E However, that's going to require some serious rework
>and plumbing of a lot of different components=2E So if I can make it work
>as is, that'd be ideal=2E However, I need to figure out this weird 62 meg
>limitation=2E
>
>Any ideas on that?
>
>Jason

Ok, the code I sent will figure out the minimum amount of padding that you=
 need (min_initrd_addr) as well=2E