x86/e820: Discard high memory that can't be addressed by 32-bit systems

Dave Hansen reports the following crash on a 32-bit system with
CONFIG_HIGHMEM=y and CONFIG_X86_PAE=y:

  > 0xf75fe000 is the mem_map[] entry for the first page >4GB. It
  > obviously wasn't allocated, thus the oops.

  BUG: unable to handle page fault for address: f75fe000
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  *pdpt = 0000000002da2001 *pde = 000000000300c067 *pte = 0000000000000000
  Oops: Oops: 0002 [#1] SMP NOPTI
  CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 6.15.0-rc1-00288-ge618ee89561b-dirty #311 PREEMPT(undef)
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
  EIP: __free_pages_core+0x3c/0x74
  ...
  Call Trace:
   memblock_free_pages+0x11/0x2c
   memblock_free_all+0x2ce/0x3a0
   mm_core_init+0xf5/0x320
   start_kernel+0x296/0x79c
   i386_start_kernel+0xad/0xb0
   startup_32_smp+0x151/0x154

The mem_map[] is allocated up to the end of ZONE_HIGHMEM which is defined
by max_pfn.

The bug was introduced by this recent commit:

  6faea34 ("arch, mm: streamline HIGHMEM freeing")

Previously, freeing of high memory was also clamped to the end of
ZONE_HIGHMEM but after this change, memblock_free_all() tries to
free memory above the of ZONE_HIGHMEM as well and that causes
access to mem_map[] entries beyond the end of the memory map.

To fix this, discard the memory after max_pfn from memblock on
32-bit systems so that core MM would be aware only of actually
usable memory.

Fixes: 6faea34 ("arch, mm: streamline HIGHMEM freeing")
Reported-by: Dave Hansen <[email protected]>
Tested-by: Arnd Bergmann <[email protected]>
Signed-off-by: Mike Rapoport (Microsoft) <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Davide Ciminaghi <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected] # discussion and submission

Author: rppt

arch/x86/kernel/e820.c

@@ -1299,6 +1299,14 @@ void __init e820__memblock_setup(void)
 		memblock_add(entry->addr, entry->size);
 	}
 
+	/*
+	 * 32-bit systems are limited to 4BG of memory even with HIGHMEM and
+	 * to even less without it.
+	 * Discard memory after max_pfn - the actual limit detected at runtime.
+	 */
+	if (IS_ENABLED(CONFIG_X86_32))
+		memblock_remove(PFN_PHYS(max_pfn), -1);
+
 	/* Throw away partial pages: */
 	memblock_trim_memory(PAGE_SIZE);