[PATCH 4.14 32/74] x86/cpu_entry_area: Prevent wraparound in setup_cpu_entry_area_ptes() on 32bit

[Greg Kroah-Hartman]

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

commit f6c4fd506cb626e4346aa81688f255e593a7c5a0 upstream.

The loop which populates the CPU entry area PMDs can wrap around on 32bit
machines when the number of CPUs is small.

It worked wonderful for NR_CPUS=64 for whatever reason and the moron who
wrote that code did not bother to test it with !SMP.

Check for the wraparound to fix it.

Fixes: 92a0f81d8957 ("x86/cpu_entry_area: Move it out of the fixmap")
Reported-by: kernel test robot <fengguang.wu@xxxxxxxxx>
Signed-off-by: Thomas "Feels stupid" Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/x86/mm/cpu_entry_area.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/arch/x86/mm/cpu_entry_area.c
+++ b/arch/x86/mm/cpu_entry_area.c
@@ -122,7 +122,8 @@ static __init void setup_cpu_entry_area_
 	start = CPU_ENTRY_AREA_BASE;
 	end = start + CPU_ENTRY_AREA_MAP_SIZE;
 
-	for (; start < end; start += PMD_SIZE)
+	/* Careful here: start + PMD_SIZE might wrap around */
+	for (; start < end && start >= CPU_ENTRY_AREA_BASE; start += PMD_SIZE)
 		populate_extra_pte(start);
 #endif
 }