[xen master] x86/xlat: fix UB pointer arithmetic in COMPAT_ARG_XLAT_VIRT_BASE

[patchbot@xxxxxxx]

commit fc302866f42f552337ae7d8d78877aec36e6e2ff
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Tue Mar 18 09:20:41 2025 +0100
Commit:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
CommitDate: Thu Mar 20 12:28:30 2025 +0100

    x86/xlat: fix UB pointer arithmetic in COMPAT_ARG_XLAT_VIRT_BASE
    
    UBSAN complains with:
    
    UBSAN: Undefined behaviour in common/compat/memory.c:90:9
    pointer operation overflowed ffff820080000000 to 0000020080000000
    [...]
    Xen call trace:
        [<ffff82d040303782>] R common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xc0
        [<ffff82d040304bc3>] F __ubsan_handle_pointer_overflow+0xcb/0x100
        [<ffff82d0402a6259>] F compat_memory_op+0xf1/0x4d20
        [<ffff82d04041532d>] F hvm_memory_op+0x55/0xe0
        [<ffff82d040416150>] F hvm_hypercall+0xae8/0x21b0
        [<ffff82d0403b24ca>] F svm_vmexit_handler+0x1252/0x2450
        [<ffff82d0402049c0>] F svm_stgi_label+0x5/0x15
    
    Adjust the calculations in COMPAT_ARG_XLAT_VIRT_BASE to subtract from the
    per-domain area to obtain the mirrored linear address in the 4th slot,
    instead of overflowing the per-domain linear address.
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/include/asm/x86_64/uaccess.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/include/asm/x86_64/uaccess.h 
b/xen/arch/x86/include/asm/x86_64/uaccess.h
index c6fa3fd381..9f515b2638 100644
--- a/xen/arch/x86/include/asm/x86_64/uaccess.h
+++ b/xen/arch/x86/include/asm/x86_64/uaccess.h
@@ -9,9 +9,9 @@
  * 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_VIRT_BASE ((void *)ARG_XLAT_START(current) - \
+                                   PERDOMAIN_VIRT_START + \
+                                   PERDOMAIN_ALT_VIRT_START)
 #define COMPAT_ARG_XLAT_SIZE      (2*PAGE_SIZE)
 struct vcpu;
 int setup_compat_arg_xlat(struct vcpu *v);
--
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