x86/ioremap: prevent additions against the NULL pointer

This was reported by clang UBSAN as:

UBSAN: Undefined behaviour in arch/x86/mm.c:6297:40
applying zero offset to null pointer
[...]
Xen call trace:
    [<ffff82d040303662>] R common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xc0
    [<ffff82d040304aa3>] F __ubsan_handle_pointer_overflow+0xcb/0x100
    [<ffff82d0406ebbc0>] F ioremap_wc+0xc8/0xe0
    [<ffff82d0406c3728>] F video_init+0xd0/0x180
    [<ffff82d0406ab6f5>] F console_init_preirq+0x3d/0x220
    [<ffff82d0406f1876>] F __start_xen+0x68e/0x5530
    [<ffff82d04020482e>] F __high_start+0x8e/0x90

Fix bt_ioremap() and ioremap{,_wc}() to not add the offset if the returned
pointer from __vmap() is NULL.

Fixes: d0d4635d03 ('implement vmap()')
Fixes: f390941a92 ('x86/DMI: fix table mapping when one lives above 1Mb')
Fixes: 81d195c6c0 ('x86: introduce ioremap_wc()')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 9a6f2c52f7
master date: 2025-03-17 13:33:29 +0100

xen/arch/x86/dmi_scan.c

@@ -112,6 +112,7 @@ static const void *__init bt_ioremap(paddr_t addr, unsigned int len)
 {
     mfn_t mfn = _mfn(PFN_DOWN(addr));
     unsigned int offs = PAGE_OFFSET(addr);
+    void *va;
 
     if ( addr + len <= MB(1) )
         return __va(addr);
@@ -119,8 +120,10 @@ static const void *__init bt_ioremap(paddr_t addr, unsigned int len)
     if ( system_state < SYS_STATE_boot )
         return __acpi_map_table(addr, len);
 
-    return __vmap(&mfn, PFN_UP(offs + len), 1, 1, PAGE_HYPERVISOR_RO,
-                  VMAP_DEFAULT) + offs;
+    va = __vmap(&mfn, PFN_UP(offs + len), 1, 1, PAGE_HYPERVISOR_RO,
+                VMAP_DEFAULT);
+
+    return va ? va + offs : NULL;
 }
 
 static void __init bt_iounmap(const void *ptr, unsigned int len)

xen/arch/x86/mm.c

@@ -6274,7 +6274,9 @@ void __iomem *ioremap(paddr_t pa, size_t len)
         unsigned int offs = pa & (PAGE_SIZE - 1);
         unsigned int nr = PFN_UP(offs + len);
 
-        va = __vmap(&mfn, nr, 1, 1, PAGE_HYPERVISOR_UCMINUS, VMAP_DEFAULT) + offs;
+        va = __vmap(&mfn, nr, 1, 1, PAGE_HYPERVISOR_UCMINUS, VMAP_DEFAULT);
+        if ( va )
+            va += offs;
     }
 
     return (void __force __iomem *)va;
@@ -6291,7 +6293,7 @@ void __iomem *__init ioremap_wc(paddr_t pa, size_t len)
 
     va = __vmap(&mfn, nr, 1, 1, PAGE_HYPERVISOR_WC, VMAP_DEFAULT);
 
-    return (void __force __iomem *)(va + offs);
+    return (void __force __iomem *)(va ? va + offs : NULL);
 }
 
 int create_perdomain_mapping(struct domain *d, unsigned long va,