[PATCH] x86/hibernate: Ignore page-zero RAM in E820 checksum

[Matthias Goergens <matthias.goergens@gmail.com>]

The legacy kexec_load path reconstructs the E820 map exported through
sysfs.  kexec-tools leaves the first 1 KiB unavailable for the real-mode
transition, so a kernel entered through kexec_load can see conventional
RAM starting at 0x400.  A subsequent firmware boot reports the same RAM
range starting at zero.

The hibernation E820 checksum compares those byte representations and
rejects the image, even though the maps agree from page one onwards.  The
reproducer observed this exact transition:

  firmware:     RAM [0-0x9fbff]
  kexec_load:   gap [0-0x3ff], RAM [0x400-0x9fbff]
  firmware:     RAM [0-0x9fbff]

Common x86 setup already converts conventional RAM in page zero to
reserved memory in trim_bios_range() before registering hibernation
nosave regions.  Page zero therefore cannot occur in the image.

Canonicalise only the conventional-RAM portion below PAGE_SIZE before
calculating the checksum.  Preserve RESERVED, ACPI, NVS, UNUSABLE, PMEM
and all other E820 types so that changes to exceptional mappings remain
detectable.  Maps without conventional RAM intersecting page zero retain
the previous checksum byte stream.

This is deliberately narrower than the June proposal to checksum only RAM
and its opt-in relaxed_memmap successor.  Rafael noted that ignoring non-RAM
changes could hide moved ACPI or UEFI regions still used by the resumed
kernel.  This patch preserves every non-RAM entry and ignores only RAM within
page zero, which common setup already reserves and excludes from the image.

Changing the checksum semantics means an image made by an unpatched
kernel can fail to resume under a patched kernel, or vice versa, when its
raw map contains page-zero RAM.  Such cross-version attempts remain
fail-closed; normal same-kernel hibernation is unaffected.

With the reproduced raw-map difference retained, both the direct-boot
control and legacy kexec_load hibernation/resume tests passed.  The test
kernel also completed a clean full bzImage build.

Fixes: 62a03defeabd ("PM / hibernate: Verify the consistent of e820 memory map by md5 digest")
Reported-by: Roberto Ricci <io@r-ricci.it>
Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
Link: https://lore.kernel.org/all/Z-hYWc9LtBU1Yhtg@desktop0a/
Link: https://lists.openwall.net/linux-kernel/2025/04/04/1372
Link: https://lore.kernel.org/all/CAJZ5v0jmOj0WBtMTvbnaD+2b0bTFowA=JWrqRzaaCYpHpai1Nw@mail.gmail.com/
Link: https://lore.kernel.org/all/20260623165724.10753-1-scardracs@disroot.org/
---
 arch/x86/power/hibernate.c | 47 ++++++++++++++++++++++++++++++++++----
 1 file changed, 43 insertions(+), 4 deletions(-)

diff --git a/arch/x86/power/hibernate.c b/arch/x86/power/hibernate.c
index a2294c1649f65..ec53c970c92e6 100644
--- a/arch/x86/power/hibernate.c
+++ b/arch/x86/power/hibernate.c
@@ -63,6 +63,25 @@ struct restore_data_record {
 	unsigned long e820_checksum;
 };
 
+static bool trim_e820_page_zero_ram(struct e820_entry *entry)
+{
+	u64 lowmem_size;
+
+	/*
+	 * Page zero is BIOS-owned and registered as nosave.  Boot loaders may
+	 * therefore omit part of its conventional RAM entry without changing
+	 * any memory available to the image.  Preserve all other E820 types.
+	 */
+	if (entry->type != E820_TYPE_RAM || entry->addr >= PAGE_SIZE)
+		return true;
+
+	lowmem_size = min_t(u64, entry->size, PAGE_SIZE - entry->addr);
+	entry->addr += lowmem_size;
+	entry->size -= lowmem_size;
+
+	return entry->size;
+}
+
 /**
  * compute_e820_crc32 - calculate crc32 of a given e820 table
  *
@@ -70,12 +89,32 @@ struct restore_data_record {
  *
  * Return: the resulting checksum
  */
-static inline u32 compute_e820_crc32(struct e820_table *table)
+static u32 compute_e820_crc32(struct e820_table *table)
 {
-	int size = offsetof(struct e820_table, entries) +
-		sizeof(struct e820_entry) * table->nr_entries;
+	struct e820_entry entry;
+	u32 crc = ~0;
+	u32 nr_entries = 0;
+	u32 i;
+
+	for (i = 0; i < table->nr_entries; i++) {
+		entry = table->entries[i];
+		if (trim_e820_page_zero_ram(&entry))
+			nr_entries++;
+	}
+
+	crc = crc32_le(crc, (unsigned char const *)&nr_entries,
+		       sizeof(nr_entries));
+
+	for (i = 0; i < table->nr_entries; i++) {
+		entry = table->entries[i];
+		if (!trim_e820_page_zero_ram(&entry))
+			continue;
+
+		crc = crc32_le(crc, (unsigned char const *)&entry,
+			       sizeof(entry));
+	}
 
-	return ~crc32_le(~0, (unsigned char const *)table, size);
+	return ~crc;
 }
 
 #ifdef CONFIG_X86_64
-- 
2.55.0