[xen staging] x86/hvm: Fix UB on defensive default case in handle_pio()

[patchbot@xxxxxxx]

commit 2b3f56ab514d46bed652527974b74d1cef76b759
Author:     Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
AuthorDate: Wed Oct 14 09:27:28 2026 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Oct 14 09:27:28 2026 +0200

    x86/hvm: Fix UB on defensive default case in handle_pio()
    
    There's a potential 32bit left-shift (when size is 4) to a 32bit value,
    which would trigger UB and tickle UBSAN the wrong way. Shifting right an
    all-ones mask fixes the issue.
    
    Fixes: 4609fc8eb04e ("x86/hvm: Improve error information in handle_pio()")
    Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hvm/io.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/io.c b/xen/arch/x86/hvm/io.c
index 912e55d416..91cc2e1dcb 100644
--- a/xen/arch/x86/hvm/io.c
+++ b/xen/arch/x86/hvm/io.c
@@ -135,7 +135,7 @@ bool handle_pio(uint16_t port, unsigned int size, int dir)
     default:
         gprintk(XENLOG_ERR, "Unexpected PIO status %d, port %#x %s 0x%0*x\n",
                 rc, port, dir == IOREQ_WRITE ? "write" : "read",
-                size * 2, data & ((1u << (size * 8)) - 1));
+                size * 2, data & (UINT32_MAX >> (32 - size * 8)));
         domain_crash(curr->domain);
         return false;
     }
--
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