diff mbox series

[v2,4.15] x86: mirror compat argument translation area for 32-bit PV

Message ID 2357b6ef-a452-13c8-8656-e42642e80d99@suse.com (mailing list archive)
State Superseded
Headers show
Series [v2,4.15] x86: mirror compat argument translation area for 32-bit PV | expand

Commit Message

Jan Beulich Feb. 22, 2021, 3:50 p.m. UTC
Now that we guard the entire Xen VA space against speculative abuse
through hypervisor accesses to guest memory, the argument translation
area's VA also needs to live outside this range, at least for 32-bit PV
guests. To avoid extra is_hvm_*() conditionals, use the alternative VA
uniformly.

While this could be conditionalized upon CONFIG_PV32 &&
CONFIG_SPECULATIVE_HARDEN_GUEST_ACCESS, omitting such extra conditionals
keeps the code more legible imo.

Fixes: 4dc181599142 ("x86/PV: harden guest memory accesses against speculative abuse")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>
---
v2: Rename PERDOMAIN2_VIRT_START to PERDOMAIN_ALT_VIRT_START.

Comments

Roger Pau Monné Feb. 22, 2021, 4:11 p.m. UTC | #1
On Mon, Feb 22, 2021 at 04:50:14PM +0100, Jan Beulich wrote:
> Now that we guard the entire Xen VA space against speculative abuse
> through hypervisor accesses to guest memory, the argument translation
> area's VA also needs to live outside this range, at least for 32-bit PV
> guests. To avoid extra is_hvm_*() conditionals, use the alternative VA
> uniformly.
> 
> While this could be conditionalized upon CONFIG_PV32 &&
> CONFIG_SPECULATIVE_HARDEN_GUEST_ACCESS, omitting such extra conditionals
> keeps the code more legible imo.
> 
> Fixes: 4dc181599142 ("x86/PV: harden guest memory accesses against speculative abuse")
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Release-Acked-by: Ian Jackson <iwj@xenproject.org>

Acked-by: Roger Pau Monné <roger.pau@citrix.com>

One comment nit below.

> ---
> v2: Rename PERDOMAIN2_VIRT_START to PERDOMAIN_ALT_VIRT_START.
> 
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -1727,6 +1727,11 @@ void init_xen_l4_slots(l4_pgentry_t *l4t
>                 (ROOT_PAGETABLE_FIRST_XEN_SLOT + slots -
>                  l4_table_offset(XEN_VIRT_START)) * sizeof(*l4t));
>      }
> +
> +    /* Slot 511: Per-domain mappings mirror. */
> +    if ( !is_pv_64bit_domain(d) )
> +        l4t[l4_table_offset(PERDOMAIN_ALT_VIRT_START)] =
> +            l4e_from_page(d->arch.perdomain_l3_pg, __PAGE_HYPERVISOR_RW);
>  }
>  
>  bool fill_ro_mpt(mfn_t mfn)
> --- a/xen/include/asm-x86/config.h
> +++ b/xen/include/asm-x86/config.h
> @@ -159,11 +159,11 @@ extern unsigned char boot_edid_info[128]
>   *    1:1 direct mapping of all physical memory.
>  #endif
>   *  0xffff880000000000 - 0xffffffffffffffff [120TB,             PML4:272-511]
> - *    PV: Guest-defined use.
> + *    PV (64-bit): Guest-defined use.
>   *  0xffff880000000000 - 0xffffff7fffffffff [119.5TB,           PML4:272-510]
>   *    HVM/idle: continuation of 1:1 mapping
>   *  0xffffff8000000000 - 0xffffffffffffffff [512GB, 2^39 bytes  PML4:511]
> - *    HVM/idle: unused
> + *    HVM / 32-bit PV: Secondary per-domain mappings.

I would use "Mirrored per-domain mappings." to make it clear this is
indented to be backed up by the same physical range as the original
per-domain mappings.

Thanks, Roger.
diff mbox series

Patch

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1727,6 +1727,11 @@  void init_xen_l4_slots(l4_pgentry_t *l4t
                (ROOT_PAGETABLE_FIRST_XEN_SLOT + slots -
                 l4_table_offset(XEN_VIRT_START)) * sizeof(*l4t));
     }
+
+    /* Slot 511: Per-domain mappings mirror. */
+    if ( !is_pv_64bit_domain(d) )
+        l4t[l4_table_offset(PERDOMAIN_ALT_VIRT_START)] =
+            l4e_from_page(d->arch.perdomain_l3_pg, __PAGE_HYPERVISOR_RW);
 }
 
 bool fill_ro_mpt(mfn_t mfn)
--- a/xen/include/asm-x86/config.h
+++ b/xen/include/asm-x86/config.h
@@ -159,11 +159,11 @@  extern unsigned char boot_edid_info[128]
  *    1:1 direct mapping of all physical memory.
 #endif
  *  0xffff880000000000 - 0xffffffffffffffff [120TB,             PML4:272-511]
- *    PV: Guest-defined use.
+ *    PV (64-bit): Guest-defined use.
  *  0xffff880000000000 - 0xffffff7fffffffff [119.5TB,           PML4:272-510]
  *    HVM/idle: continuation of 1:1 mapping
  *  0xffffff8000000000 - 0xffffffffffffffff [512GB, 2^39 bytes  PML4:511]
- *    HVM/idle: unused
+ *    HVM / 32-bit PV: Secondary per-domain mappings.
  *
  * Compatibility guest area layout:
  *  0x0000000000000000 - 0x00000000f57fffff [3928MB,            PML4:0]
@@ -242,6 +242,9 @@  extern unsigned char boot_edid_info[128]
 #endif
 #define DIRECTMAP_VIRT_END      (DIRECTMAP_VIRT_START + DIRECTMAP_SIZE)
 
+/* Slot 511: secondary per-domain mappings (for compat xlat area accesses). */
+#define PERDOMAIN_ALT_VIRT_START PML4_ADDR(511)
+
 #ifndef __ASSEMBLY__
 
 #ifdef CONFIG_PV32
--- a/xen/include/asm-x86/x86_64/uaccess.h
+++ b/xen/include/asm-x86/x86_64/uaccess.h
@@ -1,7 +1,17 @@ 
 #ifndef __X86_64_UACCESS_H
 #define __X86_64_UACCESS_H
 
-#define COMPAT_ARG_XLAT_VIRT_BASE ((void *)ARG_XLAT_START(current))
+/*
+ * With CONFIG_SPECULATIVE_HARDEN_GUEST_ACCESS (apparent) PV guest accesses
+ * are prohibited to touch the Xen private VA range.  The compat argument
+ * translation area, therefore, can't live within this range.  Domains
+ * (potentially) in need of argument translation (32-bit PV, possibly HVM) get
+ * a secondary mapping installed, which needs to be used for such accesses in
+ * the PV case, and will also be used for HVM to avoid extra conditionals.
+ */
+#define COMPAT_ARG_XLAT_VIRT_BASE ((void *)ARG_XLAT_START(current) + \
+                                   (PERDOMAIN_ALT_VIRT_START - \
+                                    PERDOMAIN_VIRT_START))
 #define COMPAT_ARG_XLAT_SIZE      (2*PAGE_SIZE)
 struct vcpu;
 int setup_compat_arg_xlat(struct vcpu *v);