Messages in this thread Patch in this message | | | Date | Fri, 30 Dec 2022 16:59:30 +0100 | From | "Jason A. Donenfeld" <> | Subject | Re: [PATCH qemu] x86: don't let decompressed kernel image clobber setup_data |
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Hi,
On Wed, Dec 28, 2022 at 11:31:34PM -0800, H. Peter Anvin wrote: > 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 > > As far as a crash... that sounds like a big and a pretty serious one at that. > > Could you let me know what kernel you are using and how *exactly* you are booting it?
I'll attach a .config file. Apply the patch at the top of this thread to qemu, except make one modification:
diff --git a/hw/i386/x86.c b/hw/i386/x86.c index 628fd2b2e9..a61ee23e13 100644 --- a/hw/i386/x86.c +++ b/hw/i386/x86.c @@ -1097,7 +1097,7 @@ void x86_load_linux(X86MachineState *x86ms, /* The early stage can't address past around 64 MB from the original * mapping, so just give up in that case. */ - if (padded_size < 62 * 1024 * 1024) + if (true || padded_size < 62 * 1024 * 1024) kernel_size = padded_size; else { fprintf(stderr, "qemu: Kernel image too large to hold setup_data\n"); Then build qemu. Run it with `-kernel bzImage`, based on the kernel built with the .config I attached.
You'll see that the CPU triple faults when hitting this line:
sd = (struct setup_data *)boot_params->hdr.setup_data; while (sd) { unsigned long sd_addr = (unsigned long)sd; kernel_add_identity_map(sd_addr, sd_addr + sizeof(*sd) + sd->len); <---- sd = (struct setup_data *)sd->next; } , because it dereferences *sd. This does not happen if the decompressed size of the kernel is < 62 megs.
So that's the "big and pretty serious" bug that might be worthy of investigation.
Jason
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