[xen staging] x86/PV: further harden guest memory accesses against speculative abuse

[patchbot@xxxxxxx]

commit 8306d773b03acec6062c0547ac05e3dd4a6960f6
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Jan 27 15:23:59 2025 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Jan 27 15:23:59 2025 +0100

    x86/PV: further harden guest memory accesses against speculative abuse
    
    The original implementation has two issues: For one it doesn't preserve
    non-canonical-ness of inputs in the range 0x8000000000000000 through
    0x80007fffffffffff. Bogus guest pointers in that range would not cause a
    (#GP) fault upon access, when they should.
    
    And then there is an AMD-specific aspect, where only the low 48 bits of
    an address are used for speculative execution; the architecturally
    mandated #GP for non-canonical addresses would be raised at a later
    execution stage. Therefore to prevent Xen controlled data to make it
    into any of the caches in a guest controllable manner, we need to
    additionally ensure that for non-canonical inputs bit 47 would be clear.
    
    See the code comment for how addressing both is being achieved.
    
    Fixes: 4dc181599142 ("x86/PV: harden guest memory accesses against 
speculative abuse")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/include/asm/asm-defns.h | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/include/asm/asm-defns.h 
b/xen/arch/x86/include/asm/asm-defns.h
index d55dd3bbc3..32d6b44910 100644
--- a/xen/arch/x86/include/asm/asm-defns.h
+++ b/xen/arch/x86/include/asm/asm-defns.h
@@ -1,3 +1,5 @@
+#include <asm/page-bits.h>
+
 #ifndef HAVE_AS_CLAC_STAC
 .macro clac
     .byte 0x0f, 0x01, 0xca
@@ -65,17 +67,36 @@
 .macro guest_access_mask_ptr ptr:req, scratch1:req, scratch2:req
 #if defined(CONFIG_SPECULATIVE_HARDEN_GUEST_ACCESS)
     /*
-     * Here we want
-     *
-     * ptr &= ~0ull >> (ptr < HYPERVISOR_VIRT_END);
-     *
+     * Here we want to adjust \ptr such that
+     * - if it's within Xen range, it becomes non-canonical,
+     * - otherwise if it's (non-)canonical on input, it retains that property,
+     * - if the result is non-canonical, bit 47 is clear (to avoid
+     *   potentially populating the cache with Xen data on AMD-like hardware),
      * but guaranteed without any conditional branches (hence in assembly).
+     *
+     * To achieve this we determine which bit to forcibly clear: Either bit 47
+     * (in case the address is below HYPERVISOR_VIRT_END) or bit 63.  Further
+     * we determine whether for forcably set bit 63: In case we first cleared
+     * it, we'll merely restore the original address.  In case we ended up
+     * clearing bit 47 (i.e. the address was either non-canonical or within Xen
+     * range), setting the bit will yield a guaranteed non-canonical address.
+     * If we didn't clear a bit, we also won't set one: The address was in the
+     * low half of address space in that case with bit 47 already clear.  The
+     * address can thus be left unchanged, whether canonical or not.
      */
     mov $(HYPERVISOR_VIRT_END - 1), \scratch1
-    mov $~0, \scratch2
+    mov $(VADDR_BITS - 1), \scratch2
     cmp \ptr, \scratch1
+    /*
+     * Not needed: The value we have in \scratch1 will be truncated to 6 bits,
+     * thus yielding the value we need.
+    mov $63, \scratch1
+     */
+    cmovnb \scratch2, \scratch1
+    xor \scratch2, \scratch2
+    btr \scratch1, \ptr
     rcr $1, \scratch2
-    and \scratch2, \ptr
+    or \scratch2, \ptr
 #elif defined(CONFIG_DEBUG) && defined(CONFIG_PV)
     xor $~\@, \scratch1
     xor $~\@, \scratch2
--
generated by git-patchbot for /home/xen/git/xen.git#staging