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Re: [PATCH 4/8] Revert "x86/traps: 'Fix' safety of read_registers() in #DF path"
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 17 Mar 2025 12:03:20 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 17 Mar 2025 11:03:26 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 11.03.2025 22:10, Andrew Cooper wrote:
> This reverts commit 6065a05adf152a556fb9f11a5218c89e41b62893.
>
> The discussed "proper fix" has now been implemented, and the #DF path no
> longer writes out-of-bounds. Restore the proper #DF IST pointer.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> --- a/xen/arch/x86/cpu/common.c
> +++ b/xen/arch/x86/cpu/common.c
> @@ -847,13 +847,7 @@ void load_system_tables(void)
> tss->ist[IST_MCE - 1] = stack_top + (1 + IST_MCE) * PAGE_SIZE;
> tss->ist[IST_NMI - 1] = stack_top + (1 + IST_NMI) * PAGE_SIZE;
> tss->ist[IST_DB - 1] = stack_top + (1 + IST_DB) * PAGE_SIZE;
> - /*
> - * Gross bodge. The #DF handler uses the vm86 fields of cpu_user_regs
> - * beyond the hardware frame. Adjust the stack entrypoint so this
> - * doesn't manifest as an OoB write which hits the guard page.
> - */
> - tss->ist[IST_DF - 1] = stack_top + (1 + IST_DF) * PAGE_SIZE -
> - (sizeof(struct cpu_user_regs) - offsetof(struct cpu_user_regs,
> es));
> + tss->ist[IST_DF - 1] = stack_top + (1 + IST_DF) * PAGE_SIZE;
And one of these "es is special" also gone.
Jan
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