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Date: Wed, 1 Feb 2017 10:15:34 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
"H . Peter Anvin" <hpa@...or.com>,
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Subject: Re: [PATCH v2 3/3] x86: Make the GDT remapping read-only on 64 bit
* Thomas Garnier <thgarnie@...gle.com> wrote:
> This patch makes the GDT remapped pages read-only to prevent corruption.
> This change is done only on 64 bit.
Please spell '64-bit' consistently through the series. I've seen two variants:
64 bit
64bit
> +/*
> + * The LTR instruction marks the TSS GDT entry as busy. In 64bit, the GDT is
> + * a read-only remapping. To prevent a page fault, the GDT is switched to the
> + * original writeable version when needed.
s/In 64bit,
/On 64-bit kernels,
> + */
> +#ifdef CONFIG_X86_64
> +static inline void native_load_tr_desc(void)
> +{
> + struct desc_ptr gdt;
> + int cpu = raw_smp_processor_id();
> + bool restore = false;
> + struct desc_struct *fixmap_gdt;
> +
> + native_store_gdt(&gdt);
> + fixmap_gdt = get_cpu_fixmap_gdt(cpu);
> +
> + /*
> + * If the current GDT is the read-only fixmap, swap to the original
> + * writeable version. Swap back at the end.
> + */
> + if (gdt.address == (unsigned long)fixmap_gdt) {
> + load_direct_gdt(cpu);
> + restore = true;
> + }
> + asm volatile("ltr %w0"::"q" (GDT_ENTRY_TSS*8));
> + if (restore)
> + load_fixmap_gdt(cpu);
Please use bool plus 0/1, it's more readable (to me) than the true/false notation.
> extern void switch_to_new_gdt(int);
> +extern void load_direct_gdt(int);
> extern void load_fixmap_gdt(int);
> +/* Load the original GDT from the per-cpu structure */
> +void load_direct_gdt(int cpu)
> +{
> + struct desc_ptr gdt_descr;
> +
> + gdt_descr.address = (long)get_cpu_direct_gdt(cpu);
Please name the functions in an easier to understand way, such as:
get_cpu_gdt_rw()
get_cpu_gdt_ro()
that the GDT is in the direct mappings is less important than the fact the the
address is writable ...
> +}
> +EXPORT_SYMBOL(load_direct_gdt);
EXPORT_SYMBOL_GPL(), or no export at all.
> +EXPORT_SYMBOL(load_fixmap_gdt);
ditto.
> * VT restores TR but not its size. Useless.
> */
> - struct desc_ptr *gdt = this_cpu_ptr(&host_gdt);
> struct desc_struct *descs;
>
> - descs = (void *)gdt->address;
> + descs = (void *)get_current_direct_gdt();
Couldn't the type cast be dropped?
>
> - table_base = gdt->address;
> + table_base = (unsigned long)get_current_direct_gdt();
Instead of spreading these type casts far and wide please introduce another
accessor the returns 'unsigned long':
get_cpu_gdt_rw_vaddr()
or such.
Thanks,
Ingo
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