[PATCH] kho: fix signed shift UB in kho_preserved_memory_reserve()

[Kiarash Azarnia <kiarash.azarnia@gmail.com>]

kho_preserved_memory_reserve() computes the size of a preserved
reservation as:

	sz = 1 << (order + PAGE_SHIFT);

`1` is a signed int, so the shift is signed-int arithmetic. For order
19 (a 2 GiB region) it produces 1 << 31, which is unrepresentable in
int and is undefined behavior; in practice it yields 0x80000000,
sign-extended on the assignment to the u64 sz. For order >= 20 the
shift count exceeds the width of int, which is also undefined. The
return value of memblock_reserve() is ignored and memblock_cap_size()
clamps the bogus size, so the kernel silently reserves the wrong
amount of memory for the preserved region.

kho_alloc_preserve() caps order at MAX_PAGE_ORDER and cannot reach
order 19, but a boot-time reserve_mem= region of at least 2 GiB drives
kho_preserve_pages() to compute order 19, and kho_preserve_pages() is
EXPORT_SYMBOL_GPL(), so the path is reachable.

Cast the shift operand to u64 so the arithmetic is done in 64 bits:

	sz = (u64)1 << (order + PAGE_SHIFT);

Fixes: 3f2ad90060f6 ("kho: adopt radix tree for preserved memory tracking")
Cc: stable@vger.kernel.org
Signed-off-by: Kiarash Azarnia <kiarash.azarnia@gmail.com>
---
 kernel/liveupdate/kexec_handover.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index 175c08a6e41e..c79f48bd64ac 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -501,7 +501,7 @@ static int __init kho_preserved_memory_reserve(phys_addr_t phys,
 	struct page *page;
 	u64 sz;
 
-	sz = 1 << (order + PAGE_SHIFT);
+	sz = (u64)1 << (order + PAGE_SHIFT);
 	page = kho_get_preserved_page(phys, order);
 
 	/* Reserve the memory preserved in KHO in memblock */
-- 
2.53.0