[PATCH RFC] x86/HVM: also stuff RSB upon exit to guest

["Jan Beulich" <JBeulich@suse.com>]

In order to mostly eliminate abuse of what Xen leaves in the RSB by
guest level attackers, fill the RSB with almost-NULL pointers right
before entering guest context.

The placement of the initialization code is intentional: If it was put
in e.g. hvm_enable(), we'd have to be more careful wrt. changing the
low L4 entry of the idle page tables (I didn't check whether boot time
low mappings have disappeared by then), and get_random() couldn't be
used either. Furthermore this way, if no HVM guest gets ever started,
no setup would ever occur.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
TBD: In the end I'm not sure the (pseudo-)randomness is worth it.
     Placing the stub uniformly at a fixed address would allow to get
     rid of the variable, slightly streamlining the call sites.
TBD: Obviously using NULL here has the downside of reads through NULL
     not going to fault anymore.

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -85,6 +85,10 @@ integer_param("hvm_debug", opt_hvm_debug
 
 struct hvm_function_table hvm_funcs __read_mostly;
 
+extern void do_overwrite_rsb(void);
+extern const char do_overwrite_rsb_end[];
+void (* __read_mostly hvm_overwrite_rsb)(void) = do_overwrite_rsb;
+
 /*
  * The I/O permission bitmap is globally shared by all HVM guests except
  * the hardware domain which needs a more permissive one.
@@ -583,6 +587,49 @@ int hvm_domain_initialise(struct domain
         return -EINVAL;
     }
 
+    if ( boot_cpu_has(X86_FEATURE_SC_RSB_HVM) &&
+         unlikely((unsigned long)hvm_overwrite_rsb >= PAGE_SIZE) )
+    {
+        /*
+         * Map an RSB stuffing routine at a random, 16-byte aligned address
+         * in the first linear page, to allow filling the RSB with almost-NULL
+         * pointers before entering HVM guest context.  This builds on the
+         * assumption that no sane OS will place anything there which could be
+         * abused as an exploit gadget.
+         */
+        unsigned long addr = (get_random() << 4) & ~PAGE_MASK;
+        unsigned int size = do_overwrite_rsb_end -
+                            (const char *)do_overwrite_rsb;
+        struct page_info *pg = alloc_domheap_page(NULL, 0);
+        void *ptr;
+
+        if ( !pg ||
+             map_pages_to_xen(0, page_to_mfn(pg), 1, PAGE_HYPERVISOR_RX) )
+        {
+            if ( pg )
+                free_domheap_page(pg);
+            return -ENOMEM;
+        }
+
+        /*
+         * Avoid NULL itself, so that branches there will hit the all-ones
+         * pattern installed below.
+         */
+        if ( !addr )
+            addr = 0x10;
+        while ( addr + size > PAGE_SIZE )
+            addr -= 0x10;
+
+        ptr = __map_domain_page(pg);
+        memset(ptr, -1, PAGE_SIZE);
+        memcpy(ptr + addr, do_overwrite_rsb, size);
+        unmap_domain_page(ptr);
+
+        smp_wmb();
+        hvm_overwrite_rsb = (void *)addr;
+        printk(XENLOG_INFO "RSB stuffing stub at %p\n", hvm_overwrite_rsb);
+    }
+
     spin_lock_init(&d->arch.hvm_domain.irq_lock);
     spin_lock_init(&d->arch.hvm_domain.uc_lock);
     spin_lock_init(&d->arch.hvm_domain.write_map.lock);
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1661,6 +1661,10 @@ void init_xen_l4_slots(l4_pgentry_t *l4t
                (ROOT_PAGETABLE_FIRST_XEN_SLOT + slots -
                 l4_table_offset(XEN_VIRT_START)) * sizeof(*l4t));
     }
+
+    /* Make sure the RSB stuffing stub is accessible. */
+    if ( is_hvm_domain(d) )
+        l4t[0] = idle_pg_table[0];
 }
 
 bool fill_ro_mpt(mfn_t mfn)
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -552,6 +552,13 @@ ENTRY(dom_crash_sync_extable)
         jmp   asm_domain_crash_synchronous /* Does not return */
         .popsection
 
+#ifdef CONFIG_HVM
+ENTRY(do_overwrite_rsb)
+        DO_OVERWRITE_RSB tmp=rdx
+        ret
+GLOBAL(do_overwrite_rsb_end)
+#endif
+
         .section .text.entry, "ax", @progbits
 
 ENTRY(common_interrupt)
--- a/xen/include/asm-x86/spec_ctrl_asm.h
+++ b/xen/include/asm-x86/spec_ctrl_asm.h
@@ -249,6 +249,8 @@
 
 /* Use when exiting to HVM guest context. */
 #define SPEC_CTRL_EXIT_TO_HVM                                           \
+    mov hvm_overwrite_rsb(%rip), %rcx;                                  \
+    ALTERNATIVE "", "INDIRECT_CALL %rcx", X86_FEATURE_SC_RSB_HVM;       \
     ALTERNATIVE "",                                                     \
         DO_SPEC_CTRL_EXIT_TO_GUEST, X86_FEATURE_SC_MSR_HVM
 




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