From: Mike Rapoport (Microsoft) <rppt@kernel.org>
Date: Sun, 13 Apr 2025 08:08:58 +0000 (+0300)
Subject: x86/e820: Discard high memory that can't be addressed by 32-bit systems
X-Git-Tag: v6.15-rc4~3^2~3
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=83b2d345e1786fdab96fc2b52942eebde125e7cd;p=thirdparty%2Fkernel%2Flinux.git

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:

  6faea3422e3b ("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: 6faea3422e3b ("arch, mm: streamline HIGHMEM freeing")
Reported-by: Dave Hansen <dave.hansen@intel.com>
Tested-by: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Shevchenko <andy@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Davide Ciminaghi <ciminaghi@gnudd.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: kvm@vger.kernel.org
Link: https://lore.kernel.org/r/20250413080858.743221-1-rppt@kernel.org # discussion and submission
---

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 9d8dd8deb2a70..9920122018a0b 100644
--- a/arch/x86/kernel/e820.c
+++ b/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);