[PATCH v1 3/3] xen/libfdt: fix UBSAN null pointer in fdt_property()

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

fdt_property() unconditionally calls memcpy(ptr, val, len) even when
len is zero and val is NULL.  This is a legitimate calling convention
for adding empty FDT properties such as "interrupt-controller", which
carry no payload.

In Xen, memcpy() maps to __builtin_memcpy(). The compiler treats
__builtin_memcpy as nonnull on its pointer arguments, so UBSAN fires
before it can observe that len is zero:
  UBSAN: Undefined behaviour in common/libfdt/fdt_sw.c:333:2
         null pointer passed as argument 2, declared with nonnull
         attribute

Guard the memcpy() with a check on len so it is skipped entirely when
there is no payload to copy, bringing the code in line with the
nonnull contract.

Fixes: f0ea06558068 ("libfdt: add version 1.3.0")
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
Reviewed-by: Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>
---
 xen/common/libfdt/fdt_sw.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/common/libfdt/fdt_sw.c b/xen/common/libfdt/fdt_sw.c
index 4c569ee7eb0d..96d4cf571319 100644
--- a/xen/common/libfdt/fdt_sw.c
+++ b/xen/common/libfdt/fdt_sw.c
@@ -330,7 +330,8 @@ int fdt_property(void *fdt, const char *name, const void 
*val, int len)
        ret = fdt_property_placeholder(fdt, name, len, &ptr);
        if (ret)
                return ret;
-       memcpy(ptr, val, len);
+       if (len)
+               memcpy(ptr, val, len);
        return 0;
 }
 
-- 
2.54.0