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Re: [PATCH 3/4] x86/kexec: Invalidate the IDT earlier in kexec_reloc()
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 24 Mar 2026 14:10:29 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 24 Mar 2026 13:10:39 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 19.03.2026 13:25, Andrew Cooper wrote:
> After switching stack, it is not safe to run any exception handlers, because
> attempts to access the cpu_info block are out-of-bounds and will generate wild
> accesses.
>
> Invalidating the IDT in the common path means there's no need to do so again
> in the 32bit path, so drop compat_mode_idt entirely.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Yet still two suggestions:
> --- a/xen/arch/x86/x86_64/kexec_reloc.S
> +++ b/xen/arch/x86/x86_64/kexec_reloc.S
> @@ -44,6 +44,16 @@ FUNC(kexec_reloc, PAGE_SIZE)
>
> movq %rcx, %rbp
>
> + /*
> + * Invalidate the IDT. After switching off Xen's stacks, the
> + * exception handlers are unsafe to use, because there's no way to
> + * perform arithmetic on the stack pointer to find the cpu_info
> block.
> + */
> + push $0
> + pushw $0
> + lidt (%rsp)
> + add $10, %rsp
Deliberately misaligning the stack is odd (but not technically a problem of
course). How about using two full PUSHes instead, possibly adding a displacement
of 6 to the LIDT in exchange?
Furthermore, don't we still have access to the full Xen image at this point? If
so, why not do the above without using the stack, using zero_page(%rip) (or
zero_page+6(%rip))?
Jan
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