[PATCH 1/3 v4] x86/kdump: always reserve the low 1MiB when the crashkernel option is specified

[Lianbo Jiang]

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=204793

Kdump kernel will reuse the first 640k region because of some reasons,
for example: the trampline and conventional PC system BIOS region may
require to allocate memory in this area. Obviously, kdump kernel will
also overwrite the first 640k region, therefore, kernel has to copy
the contents of the first 640k area to a backup area, which is done in
purgatory(), because vmcore may need the old memory. When vmcore is
dumped, kdump kernel will read the old memory from the backup area of
the first 640k area.

Basically, the main reason should be clear, kernel does not correctly
handle the first 640k region when SME is active, which causes that
kernel does not properly copy these old memory to the backup area in
purgatory(). Therefore, kdump kernel reads out the incorrect contents
from the backup area when dumping vmcore. Finally, the phenomenon is
as follow:

[root linux]$ crash vmlinux /var/crash/127.0.0.1-2019-09-19-08\:31\:27/vmcore
WARNING: kernel relocated [240MB]: patching 97110 gdb minimal_symbol values

      KERNEL: /var/crash/127.0.0.1-2019-09-19-08:31:27/vmlinux
    DUMPFILE: /var/crash/127.0.0.1-2019-09-19-08:31:27/vmcore  [PARTIAL DUMP]
        CPUS: 128
        DATE: Thu Sep 19 08:31:18 2019
      UPTIME: 00:01:21
LOAD AVERAGE: 0.16, 0.07, 0.02
       TASKS: 1343
    NODENAME: amd-ethanol
     RELEASE: 5.3.0-rc7+
     VERSION: #4 SMP Thu Sep 19 08:14:00 EDT 2019
     MACHINE: x86_64  (2195 Mhz)
      MEMORY: 127.9 GB
       PANIC: "Kernel panic - not syncing: sysrq triggered crash"
         PID: 9789
     COMMAND: "bash"
        TASK: "ffff89711894ae80  [THREAD_INFO: ffff89711894ae80]"
         CPU: 83
       STATE: TASK_RUNNING (PANIC)

crash> kmem -s|grep -i invalid
kmem: dma-kmalloc-512: slab:ffffd77680001c00 invalid freepointer:a6086ac099f0c5a4
kmem: dma-kmalloc-512: slab:ffffd77680001c00 invalid freepointer:a6086ac099f0c5a4
crash>

BTW: I also tried to fix the above problem in purgatory(), but there
are too many restricts in purgatory() context, for example: i can't
allocate new memory to create the identity mapping page table for SME
situation.

Currently, there are two places where the first 640k area is needed,
the first one is in the find_trampoline_placement(), another one is
in the reserve_real_mode(), and their content doesn't matter.

To avoid the above error, when the crashkernel kernel command line
option is specified, lets reserve the remaining low 1MiB memory(
after reserving real mode memroy) so that the allocated memory does
not fall into the low 1MiB area, which makes us not to copy the first
640k content to a backup region in purgatory(). This indicates that
it does not need to be included in crash dumps or used for anything
execept the processor trampolines that must live in the low 1MiB.

In addition, also need to clean all the code related to the backup
region later.

Signed-off-by: Lianbo Jiang <lijiang@xxxxxxxxxx>
---
 arch/x86/realmode/init.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
index 7dce39c8c034..1f0492830f2c 100644
--- a/arch/x86/realmode/init.c
+++ b/arch/x86/realmode/init.c
@@ -34,6 +34,17 @@ void __init reserve_real_mode(void)
 
 	memblock_reserve(mem, size);
 	set_real_mode_mem(mem);
+
+#ifdef CONFIG_KEXEC_CORE
+	/*
+	 * When the crashkernel option is specified, only use the low
+	 * 1MiB for the real mode trampoline.
+	 */
+	if (strstr(boot_command_line, "crashkernel=")) {
+		memblock_reserve(0, 1<<20);
+		pr_info("Reserving the low 1MiB of memory for crashkernel\n");
+	}
+#endif /* CONFIG_KEXEC_CORE */
 }
 
 static void __init setup_real_mode(void)
-- 
2.17.1