[Xen-devel] [PATCH 1/2] libfdt: Fix undefined behaviour in fdt_offset_ptr()

[Jan Beulich <jbeulich@xxxxxxxx>]

From: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>

Using pointer arithmetic to generate a pointer outside a known object is,
technically, undefined behaviour in C.  Unfortunately, we were using that
in fdt_offset_ptr() to detect overflows.

To fix this we need to do our bounds / overflow checking on the offsets
before constructing pointers from them.

Reported-by: David Binderman <dcb314@xxxxxxxxxxx>
Signed-off-by: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
[upstream commit d0b3ab0a0f46ac929b4713da46f7fdcd893dd3bd]
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/libfdt/fdt.c
+++ b/xen/common/libfdt/fdt.c
@@ -74,18 +74,19 @@ int fdt_check_header(const void *fdt)
 
 const void *fdt_offset_ptr(const void *fdt, int offset, unsigned int len)
 {
-       const char *p;
+       unsigned absoffset = offset + fdt_off_dt_struct(fdt);
+
+       if ((absoffset < offset)
+           || ((absoffset + len) < absoffset)
+           || (absoffset + len) > fdt_totalsize(fdt))
+               return NULL;
 
        if (fdt_version(fdt) >= 0x11)
                if (((offset + len) < offset)
                    || ((offset + len) > fdt_size_dt_struct(fdt)))
                        return NULL;
 
-       p = _fdt_offset_ptr(fdt, offset);
-
-       if (p + len < p)
-               return NULL;
-       return p;
+       return _fdt_offset_ptr(fdt, offset);
 }
 
 uint32_t fdt_next_tag(const void *fdt, int startoffset, int *nextoffset)


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