[xen stable-4.17] ocaml/libs/xc: Fix NULL dereference with physinfo_arch_caps()

[patchbot@xxxxxxx]

commit db8d48abd38e20abe162fcb0cba2c244b9c6adba
Author:     Edwin Török <edwin.torok@xxxxxxxxx>
AuthorDate: Thu Aug 3 16:29:05 2023 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Aug 3 16:29:05 2023 +0200

    ocaml/libs/xc: Fix NULL dereference with physinfo_arch_caps()
    
    `Tag_cons` is `0` and is meant to be used as the tag argument for
    `caml_alloc`/`caml_alloc_small` when constructing a non-empty list.
    
    The empty list is `Val_emptylist` instead, which is really just 
`Val_int(0)`.
    
    Assigning `0` to a list value like this is equivalent to assigning the naked
    pointer `NULL` to the field.  Naked pointers are not valid in OCaml 5, 
however
    even in OCaml <5.x any attempt to iterate on the list will lead to a 
segfault.
    
    The list currently only has an opaque type, so no code would have reason to
    iterate on it currently, but we shouldn't construct invalid OCaml values 
that
    might lead to a crash when exploring the type.
    
    `Val_emptylist` is available since OCaml 3.01 as a constant.
    
    Fixes: e5ac68a0110c ("x86/hvm: Revert per-domain APIC acceleration support")
    Signed-off-by: Edwin Török <edwin.torok@xxxxxxxxx>
    Acked-by: Christian Lindig <christian.lindig@xxxxxxxxx>
    master commit: 99e45548934923f0d2c1d52ae1239ffe4ef17a06
    master date: 2023-07-13 11:06:07 +0100
---
 tools/ocaml/libs/xc/xenctrl_stubs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/ocaml/libs/xc/xenctrl_stubs.c 
b/tools/ocaml/libs/xc/xenctrl_stubs.c
index facb561577..7de2ff5444 100644
--- a/tools/ocaml/libs/xc/xenctrl_stubs.c
+++ b/tools/ocaml/libs/xc/xenctrl_stubs.c
@@ -748,7 +748,7 @@ CAMLprim value stub_xc_physinfo(value xch)
        Store_field(physinfo, 9, Val_int(c_physinfo.max_cpu_id + 1));
 
 #if defined(__i386__) || defined(__x86_64__)
-       arch_cap_list = Tag_cons;
+       arch_cap_list = Val_emptylist;
 
        arch_cap_flags_tag = 1; /* tag x86 */
 #else
--
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