Messages in this thread | | | Date | Fri, 26 May 2023 08:07:44 -0500 | Subject | Re: [PATCH] x86/head/64: Switch to KERNEL_CS as soon as new GDT is installed | From | Tom Lendacky <> |
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On 5/25/23 18:17, Dave Hansen wrote: > On 5/17/23 09:26, Tom Lendacky wrote: >> However, a recent patchset that looked to avoid using the legacy >> decompressor during an EFI boot exposed this bug. At entry to startup_64, >> the CS value is that of EFI and is not mapped in the new kernel GDT. So >> when a #VC exception occurs, the CS value used by IRETQ is not valid and >> the guest boot crashes. > > This confused me a bit. Nobody merged that patchset yet, right? You > just happened across this issue when debugging a crash in that *other* set?
Correct, it was Ard's EFI decompressor patchset series that he submitted, but has not yet been accepted, that I was testing:
[PATCH 0/6] efi/x86: Avoid legacy decompressor during EFI boot https://lore.kernel.org/lkml/20230424165726.2245548-1-ardb@kernel.org/
I reported the problem to Ard and submitted this patch before I realized that he also included a patch in his next version of the series:
https://lore.kernel.org/lkml/20230508070330.582131-16-ardb@kernel.org/
> >> Fix this issue by moving the block that switches to the KERNEL_CS value to >> be done immediately after returning from startup_64_setup_env(). >> >> Fixes: bcce82908333 ("x86/sev: Detect/setup SEV/SME features earlier in boot") >> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com> > > Any thoughts on whether we want this in stable@?
It probably doesn't need to go to stable since the current decompressor code switches to a new GDT and uses the same kernel CS value that is being setup in arch/x86/kernel/head_64.S (which is why it hasn't been a problem).
> > I also wonder whether we need a comment in that little chunk of code > something along the lines of: > > /* > * Do not add anything which might take a fault or exception. > * IRET does not work here. > */ > > Michael, do you think you would have spotted something like this had it > been in the code when you were patching it?
Let me know if you would like a v2 with that comment. I'm also fine if you just want to add it as part of applying the patch, too.
Thanks, Tom
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