[PATCH v5 03/13] ACPI: extlog: Validate elog record length before walking sections

extlog_print() copies a fixed ELOG_ENTRY_LEN (4096) bytes from the elog
record into elog_buf, then walks the sections using the firmware-controlled
data_length. Nothing keeps data_length inside the buffer, so a malformed
record walks the section pointer past elog_buf and reads adjacent memory.
Unlike the GHES paths, extlog never calls cper_estatus_check().

Reject a record longer than ELOG_ENTRY_LEN and run cper_estatus_check()
before walking the sections. The length test alone is not enough: a wrapped
length reads back short and passes it, which cper_estatus_check() catches
via the header check added earlier. Drop a malformed record with
NOTIFY_DONE and without MCE_HANDLED_EXTLOG, since extlog did not consume
it.

Reported-by: [email protected]
Closes: https://sashiko.dev/#/patchset/[email protected]?part=6
Fixes: f6ec01da40e4 ("ACPI: extlog: Handle multiple records")
Reviewed-by: Alison Schofield <[email protected]>
Reviewed-by: Shuai Xue <[email protected]>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Dave Jiang <[email protected]>
---
v5:
- Expanded the comment to say why the length bound runs before
  cper_estatus_check() rather than after (Jonathan Cameron).
---
 drivers/acpi/acpi_extlog.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/acpi/acpi_extlog.c b/drivers/acpi/acpi_extlog.c
index 7ad3b36013cc..046e92ee195f 100644
--- a/drivers/acpi/acpi_extlog.c
+++ b/drivers/acpi/acpi_extlog.c
@@ -208,6 +208,15 @@ static int extlog_print(struct notifier_block *nb, unsigned long val,
 
 	tmp = (struct acpi_hest_generic_status *)elog_buf;
 
+	/*
+	 * Bound the length before cper_estatus_check() walks the sections: it
+	 * iterates over data_length, which is not yet known to fit elog_buf.
+	 * cper_estatus_check_header() then rejects a length that wrapped, which
+	 * the bound cannot see.
+	 */
+	if (cper_estatus_len(tmp) > ELOG_ENTRY_LEN || cper_estatus_check(tmp))
+		return NOTIFY_DONE;
+
 	if (!ras_userspace_consumers()) {
 		print_extlog_rcd(NULL, tmp, cpu);
 		goto out;
-- 
2.54.0