Re: [PATCH v8 6/7] x86/crash: pass dm crypt keys to kdump kernel
From: Coiby Xu
Date: Thu May 01 2025 - 20:15:44 EST
On Wed, Apr 30, 2025 at 04:48:25PM +0200, Arnaud Lefebvre wrote:
On Tue, Apr 29, 2025 at 05:40:21PM +0800, Coiby Xu wrote:
On Wed, Apr 23, 2025 at 10:59:06PM +0200, Arnaud Lefebvre wrote:
diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
index 68530fad05f7..5604a5109858 100644
--- a/arch/x86/kernel/kexec-bzimage64.c
+++ b/arch/x86/kernel/kexec-bzimage64.c
@@ -76,6 +76,10 @@ static int setup_cmdline(struct kimage *image, struct boot_params *params,
if (image->type == KEXEC_TYPE_CRASH) {
len = sprintf(cmdline_ptr,
"elfcorehdr=0x%lx ", image->elf_load_addr);
+
+ if (image->dm_crypt_keys_addr != 0)
+ len += sprintf(cmdline_ptr + len,
+ "dmcryptkeys=0x%lx ", image->dm_crypt_keys_addr);
sprintf will return the length of dmcryptkey=xxx which will be added to
len.
}
memcpy(cmdline_ptr + len, cmdline, cmdline_len);
cmdline_len += len;
Then cmdline_len will included the new len.
Yes, the cmdline_len is correct. No issue there.
Thanks for confirming it!
You are adding another kernel parameter but I believe without taking its
length into account. See the MAX_ELFCOREHDR_STR_LEN constant which is added to the
params_cmdline_sz variable for the elfcorehdr= parameter.
Thanks for raising the concern! I believe this issue has already been
took care of. Please check the above two inline comments:)
I'm sorry but I don't think it is. If you look at my comments below:
This will (at least during my tests) truncate the cmdline given to the crash kernel because
the next section (efi_map_offset) will have an offset starting inside the cmdline section
and it might overwrite the end of it:
kexec-bzimage64.c:480:
params_cmdline_sz = sizeof(struct boot_params) + cmdline_len +
MAX_ELFCOREHDR_STR_LEN; <<< Should have + 31 here for "dmcryptkeys=0x<ptr> "
params_cmdline_sz = ALIGN(params_cmdline_sz, 16);
kbuf.bufsz = params_cmdline_sz + ALIGN(efi_map_sz, 16) +
sizeof(struct setup_data) +
sizeof(struct efi_setup_data) +
sizeof(struct setup_data) +
RNG_SEED_LENGTH;
And I believe the buffer might be too small.
Also, there is another check a few lines above that needs to take the size into account:
/*
* In case of crash dump, we will append elfcorehdr=<addr> to
* command line. Make sure it does not overflow
*/
if (cmdline_len + MAX_ELFCOREHDR_STR_LEN > header->cmdline_size) {
pr_err("Appending elfcorehdr=<addr> to command line exceeds maximum allowed length\n");
return ERR_PTR(-EINVAL);
}
To try to explain a bit more, we pass a lot of arguments to the crash kernel so
the initrd (dracut) can mount the encrypted disk. When I run kexec using
the following:
/usr/host/bin/kexec --debug --load-panic /linux-hv '--append=maxcpus=1
reset_devices rd.info rd.cc.kdump root=UUID=d039277c-2ee
3-466a-85eb-db9524398135 console=ttyS0 rd.timeout=10 rd.shell=1
rd.cc.kdump.encrypted
rd.cc.kdump.device=UUID=908234b1-c1f3-4150-bfdf-c260907a2447
rd.cc.kdump.keyring=cryptsetup:908234b1-c1f3-4150-bfdf-c260907a2447' --initrd
/crash-initrd
kexec debug print those logs:
<snip>
[ 53.642483] kexec-bzImage64: Loaded purgatory at 0xb6ffb000
[ 53.642828] kexec-bzImage64: Loaded boot_param, command line and misc at
0xb6ff9000 bufsz=0x12f0 memsz=0x2000
[ 53.643366] kexec-bzImage64: Loaded 64bit kernel at 0xb1000000
bufsz=0x16a5000 memsz=0x550d000
[ 53.643918] kexec-bzImage64: Loaded initrd at 0xaeb90000 bufsz=0x246f2a1
memsz=0x246f2a1
[ 53.644363] kexec-bzImage64: Final command line is: elfcorehdr=0x77000000
dmcryptkeys=0xa81fc000 maxcpus=1 reset_devices rd.info rd.cc.kdump
root=UUID=d039277c-2ee3-466a-85eb-db9524398135 console=ttyS0 rd.timeout=10
rd.shell=1 rd.cc.kdump.encrypted
rd.cc.kdump.device=UUID=908234b1-c1f3-4150-bfdf-c260907a2447
rd.cc.kdump.keyring=cryptsetup:908234b1-c1f3-4150-bfdf-c260907a2447
<snip>
Here, we see the full command line, as expected. But when I trigger a panic
using `echo c > /proc/sysrq-trigger`, the first two lines of the crash kernel
loading are:
[ 0.000000] Linux version 6.12.23+ (arnaud@exherbo) (gcc (GCC) 12.3.0, GNU ld
(GNU Binutils) 2.44) #4 SMP Wed Apr 30 16:11:39 CEST 2025
[ 0.000000] Command line: elfcorehdr=0x77000000 dmcryptkeys=0x9ec14000
maxcpus=1 reset_devices rd.info rd.cc.kdump
root=UUID=d039277c-2ee3-466a-85eb-db9524398135 console=ttyS0 rd.timeout=10
rd.shell=1 rd.cc.kdump.encrypted
rd.cc.kdump.device=UUID=908234b1-c1f3-4150-bfdf-c260907a2447
rd.cc.kdump.keyring=cryptsetup:908234b1-c1f3-4150-bfdf-c26090
You can see some of it is truncated at the end. It's missing `7a2447`. This is
because I guess it gets overridden.
My comment above explains where and why it might happen. If I add the size of
the dmcryptkeys string length to the params_cmdline_sz variable, we should
allocate enough space to have it all. With the patch below, it works fine and I
get the full cmdline when my crash kernel boots:
[ 0.000000] Linux version 6.12.23+ (arnaud@exherbo) (gcc (GCC) 12.3.0, GNU ld
(GNU Binutils) 2.44) #3 SMP Thu Apr 24 16:42:18 CEST 2025
[ 0.000000] Command line: elfcorehdr=0x77000000 dmcryptkeys=0xa81fc000
maxcpus=1 reset_devices rd.info rd.cc.kdump
root=UUID=d039277c-2ee3-466a-85eb-db9524398135 console=ttyS0 rd.timeout=10
rd.shell=1 rd.cc.kdump.encrypted
rd.cc.kdump.device=UUID=908234b1-c1f3-4150-bfdf-c260907a2447
rd.cc.kdump.keyring=cryptsetup:908234b1-c1f3-4150-bfdf-c260907a2447
diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
index 5604a5109858..06fc1f412af4 100644
--- a/arch/x86/kernel/kexec-bzimage64.c
+++ b/arch/x86/kernel/kexec-bzimage64.c
@@ -27,6 +27,7 @@
#include <asm/kexec-bzimage64.h>
#define MAX_ELFCOREHDR_STR_LEN 30 /* elfcorehdr=0x<64bit-value> */
+#define MAX_DMCRYPTKEYS_STR_LEN 31
/*
* Defines lowest physical address for various segments. Not sure where
@@ -434,7 +435,7 @@ static void *bzImage64_load(struct kimage *image, char *kernel,
* In case of crash dump, we will append elfcorehdr=<addr> to
* command line. Make sure it does not overflow
*/
- if (cmdline_len + MAX_ELFCOREHDR_STR_LEN > header->cmdline_size) {
+ if (cmdline_len + MAX_ELFCOREHDR_STR_LEN + MAX_DMCRYPTKEYS_STR_LEN > header->cmdline_size) {
pr_err("Appending elfcorehdr=<addr> to command line exceeds maximum allowed length\n");
return ERR_PTR(-EINVAL);
}
@@ -478,7 +479,7 @@ static void *bzImage64_load(struct kimage *image, char *kernel,
*/
efi_map_sz = efi_get_runtime_map_size();
params_cmdline_sz = sizeof(struct boot_params) + cmdline_len +
- MAX_ELFCOREHDR_STR_LEN;
+ MAX_ELFCOREHDR_STR_LEN + MAX_DMCRYPTKEYS_STR_LEN;
params_cmdline_sz = ALIGN(params_cmdline_sz, 16);
kbuf.bufsz = params_cmdline_sz + ALIGN(efi_map_sz, 16) +
sizeof(struct setup_data) +
Let me know if it makes more sense!
Yes, thanks for providing a crystal clear explanation and also a fix! I
appreciate your elaboration to show me what the problem is! I'll fix it
in v9.
--
Best regards,
Coiby