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

[tip-bot2 for Mike Rapoport (Microsoft)]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     e71b6094c20f5dc9c43dc89af8a569ffa511d676
Gitweb:        https://git.kernel.org/tip/e71b6094c20f5dc9c43dc89af8a569ffa511d676
Author:        Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
AuthorDate:    Sun, 13 Apr 2025 11:08:58 +03:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Wed, 16 Apr 2025 09:16:02 +02:00

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@xxxxxxxxx>
Tested-by: Arnd Bergmann <arnd@xxxxxxxxxx>
Tested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Andy Shevchenko <andy@xxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Davide Ciminaghi <ciminaghi@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
Cc: kvm@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20250413080858.743221-1-rppt@xxxxxxxxxx
---
 arch/x86/kernel/e820.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 9d8dd8d..de62388 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -1299,6 +1299,13 @@ void __init e820__memblock_setup(void)
 		memblock_add(entry->addr, entry->size);
 	}
 
+	/*
+	 * Discard memory above 4GB because 32-bit systems are limited to 4GB
+	 * of memory even with HIGHMEM.
+	 */
+	if (IS_ENABLED(CONFIG_X86_32))
+		memblock_remove(PFN_PHYS(MAX_NONPAE_PFN), -1);
+
 	/* Throw away partial pages: */
 	memblock_trim_memory(PAGE_SIZE);