Re: [PATCH v5] x86/dom0: disable SMAP for PV domain building only

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.08.2024 14:39, Roger Pau Monne wrote:
> --- a/xen/arch/x86/dom0_build.c
> +++ b/xen/arch/x86/dom0_build.c
> @@ -612,7 +612,24 @@ int __init construct_dom0(struct domain *d, const 
> module_t *image,
>      if ( is_hvm_domain(d) )
>          rc = dom0_construct_pvh(d, image, image_headroom, initrd, cmdline);
>      else if ( is_pv_domain(d) )
> +    {
> +        /*
> +         * Temporarily clear SMAP in CR4 to allow user-accesses in
> +         * construct_dom0().  This saves a large number of corner cases
> +         * interactions with copy_from_user().
> +         */
> +        if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
> +        {
> +            cr4_pv32_mask &= ~X86_CR4_SMAP;
> +            write_cr4(read_cr4() & ~X86_CR4_SMAP);
> +        }
>          rc = dom0_construct_pv(d, image, image_headroom, initrd, cmdline);
> +        if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
> +        {
> +            write_cr4(read_cr4() | X86_CR4_SMAP);
> +            cr4_pv32_mask |= X86_CR4_SMAP;
> +        }
> +    }

Andrew, looking at this code I really wonder what benefit you request to
move this further is going to have. Afaict no matter where we put it, it'll
be an #ifdef around each of the two accesses to cr4_pv32_mask when we make
that variable CONFIG_PV32-only.

> --- a/xen/arch/x86/include/asm/setup.h
> +++ b/xen/arch/x86/include/asm/setup.h
> @@ -64,6 +64,8 @@ extern bool opt_dom0_verbose;
>  extern bool opt_dom0_cpuid_faulting;
>  extern bool opt_dom0_msr_relaxed;
>  
> +extern unsigned long cr4_pv32_mask;

With this ...

> --- a/xen/arch/x86/setup.c
> +++ b/xen/arch/x86/setup.c
> @@ -80,7 +80,7 @@ int8_t __initdata opt_probe_port_aliases = -1;
>  boolean_param("probe-port-aliases", opt_probe_port_aliases);
>  
>  /* Only used in asm code and within this source file */
> -unsigned long asmlinkage __read_mostly cr4_pv32_mask;
> +unsigned long asmlinkage __ro_after_init cr4_pv32_mask;

... both the comment and the asmlinkage become stale now.

Jan