Re: [PATCH v3] x86: use a read-only IDT alias on all CPUs

[Ingo Molnar]

* Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> Make a copy of the IDT (as seen via the "sidt" instruction) read-only.
> This primarily removes the IDT from being a target for arbitrary memory
> write attacks, and has the added benefit of also not leaking the kernel
> base offset, if it has been relocated.
> 
> We already did this on vendor == Intel and family == 5 because of the
> F0 0F bug -- regardless of if a particular CPU had the F0 0F bug or
> not.  Since the workaround was so cheap, there simply was no reason to
> be very specific.  This patch extends the readonly alias to all CPUs,
> but does not activate the #PF to #UD conversion code needed to deliver
> the proper exception in the F0 0F case except on Intel family 5
> processors.
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Eric Northup <digitaleric@xxxxxxxxxx>
> ---
> v3:
>  - clarify commit, thanks to HPA
>  - add missing header file, thanks to buildbot. :)
> v2:
>  - clarify commit and comments
> ---
>  arch/x86/include/asm/fixmap.h |    4 +---
>  arch/x86/kernel/cpu/intel.c   |   18 +-----------------
>  arch/x86/kernel/traps.c       |    9 +++++++++
>  arch/x86/xen/mmu.c            |    4 +---
>  4 files changed, 12 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
> index a09c285..51b9e32 100644
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -104,9 +104,7 @@ enum fixed_addresses {
>  	FIX_LI_PCIA,	/* Lithium PCI Bridge A */
>  	FIX_LI_PCIB,	/* Lithium PCI Bridge B */
>  #endif
> -#ifdef CONFIG_X86_F00F_BUG
> -	FIX_F00F_IDT,	/* Virtual mapping for IDT */
> -#endif
> +	FIX_RO_IDT,	/* Virtual mapping for read-only IDT */
>  #ifdef CONFIG_X86_CYCLONE_TIMER
>  	FIX_CYCLONE_TIMER, /*cyclone timer register*/
>  #endif
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 1905ce9..7170024 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -164,20 +164,6 @@ int __cpuinit ppro_with_ram_bug(void)
>  	return 0;
>  }
>  
> -#ifdef CONFIG_X86_F00F_BUG
> -static void __cpuinit trap_init_f00f_bug(void)
> -{
> -	__set_fixmap(FIX_F00F_IDT, __pa_symbol(idt_table), PAGE_KERNEL_RO);
> -
> -	/*
> -	 * Update the IDT descriptor and reload the IDT so that
> -	 * it uses the read-only mapped virtual address.
> -	 */
> -	idt_descr.address = fix_to_virt(FIX_F00F_IDT);
> -	load_idt(&idt_descr);
> -}
> -#endif
> -
>  static void __cpuinit intel_smp_check(struct cpuinfo_x86 *c)
>  {
>  	/* calling is from identify_secondary_cpu() ? */
> @@ -206,8 +192,7 @@ static void __cpuinit intel_workarounds(struct cpuinfo_x86 *c)
>  	/*
>  	 * All current models of Pentium and Pentium with MMX technology CPUs
>  	 * have the F0 0F bug, which lets nonprivileged users lock up the
> -	 * system.
> -	 * Note that the workaround only should be initialized once...
> +	 * system. Announce that the fault handler will be checking for it.
>  	 */
>  	c->f00f_bug = 0;
>  	if (!paravirt_enabled() && c->x86 == 5) {
> @@ -215,7 +200,6 @@ static void __cpuinit intel_workarounds(struct cpuinfo_x86 *c)
>  
>  		c->f00f_bug = 1;
>  		if (!f00f_workaround_enabled) {
> -			trap_init_f00f_bug();
>  			printk(KERN_NOTICE "Intel Pentium with F0 0F bug - workaround enabled.\n");
>  			f00f_workaround_enabled = 1;
>  		}
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 68bda7a..10e2446 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -56,6 +56,7 @@
>  #include <asm/fpu-internal.h>
>  #include <asm/mce.h>
>  #include <asm/context_tracking.h>
> +#include <asm/fixmap.h>
>  
>  #include <asm/mach_traps.h>
>  
> @@ -753,6 +754,14 @@ void __init trap_init(void)
>  #endif
>  
>  	/*
> +	 * Set the IDT descriptor to a fixed read-only location, so that the
> +	 * "sidt" instruction will not leak the location of the kernel, and
> +	 * to defend the IDT against arbitrary memory write vulnerabilities.
> +	 * It will be reloaded in cpu_init() */
> +	__set_fixmap(FIX_RO_IDT, __pa_symbol(idt_table), PAGE_KERNEL_RO);
> +	idt_descr.address = fix_to_virt(FIX_RO_IDT);
> +
> +	/*
>  	 * Should be a barrier for any external CPU state:
>  	 */
>  	cpu_init();
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index 6afbb2c..8bc4dec 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -2039,9 +2039,7 @@ static void xen_set_fixmap(unsigned idx, phys_addr_t phys, pgprot_t prot)
>  
>  	switch (idx) {
>  	case FIX_BTMAP_END ... FIX_BTMAP_BEGIN:
> -#ifdef CONFIG_X86_F00F_BUG
> -	case FIX_F00F_IDT:
> -#endif
> +	case FIX_RO_IDT:
>  #ifdef CONFIG_X86_32
>  	case FIX_WP_TEST:
>  	case FIX_VDSO:

This looks very nice to me now. Peter, any objections?

Thanks,

	Ingo
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