x86/cpu_entry_area: Prevent wraparound in setup_cpu_entry_area_ptes() on 32bit

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@intel.com>
Signed-off-by: Thomas "Feels stupid" Gleixner <tglx@linutronix.de>
Tested-by: Borislav Petkov <bp@alien8.de>

1 files changed, 2 insertions, 1 deletions

diff --git a/arch/x86/mm/cpu_entry_area.c b/arch/x86/mm/cpu_entry_area.c
index 21e8b595cbb1..fe814fd5e014 100644
--- 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_ptes(void)
         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
 }