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    SubjectRe: [PATCH qemu] x86: don't let decomp ressed kernel image clobber setup data
    On December 28, 2022 6:31:07 PM PST, "Jason A. Donenfeld" <Jason@zx2c4.com> wrote:
    >Hi,
    >
    >Read this message in a fixed width text editor with a lot of columns.
    >
    >On Wed, Dec 28, 2022 at 03:58:12PM -0800, H. Peter Anvin wrote:
    >> Glad you asked.
    >>
    >> So the kernel load addresses are parameterized in the kernel image
    >> setup header. One of the things that are so parameterized are the size
    >> and possible realignment of the kernel image in memory.
    >>
    >> I'm very confused where you are getting the 64 MB number from. There
    >> should not be any such limitation.
    >
    >Currently, QEMU appends it to the kernel image, not to the initramfs as
    >you suggest below. 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). 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. 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.
    >
    >The solution in this commit adds a bunch of padding between the kernel
    >image and setup_data to avoid this. 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
    >
    >This way, the decompressed kernel doesn't clobber setup_data.
    >
    >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.c.
    >I don't know why it does this. If I could remove the 62 megabyte
    >restriction, then I could keep with this technique and all would be
    >well.
    >
    >> 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.)
    >
    >It would be theoretically possible to attach it to the initrd image
    >instead of to the kernel image. As a last resort, I guess I can look
    >into doing that. However, that's going to require some serious rework
    >and plumbing of a lot of different components. So if I can make it work
    >as is, that'd be ideal. However, I need to figure out this weird 62 meg
    >limitation.
    >
    >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.

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