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Message-ID: <c2mqpzhtyrd5qzove5pa44ob2iiwdcrtnj655sozy2a4pprln7@mql5bjuwjbxs>
Date: Wed, 30 Apr 2025 16:48:25 +0200
From: Arnaud Lefebvre <arnaud.lefebvre@...ver-cloud.com>
To: Coiby Xu <coxu@...hat.com>
Cc: kexec@...ts.infradead.org, Ondrej Kozina <okozina@...hat.com>,
Milan Broz <gmazyland@...il.com>, Thomas Staudt <tstaudt@...ibm.com>,
Daniel P . Berrangé <berrange@...hat.com>, Kairui Song <ryncsn@...il.com>,
Pingfan Liu <kernelfans@...il.com>, Baoquan He <bhe@...hat.com>, Dave Young <dyoung@...hat.com>,
linux-kernel@...r.kernel.org, x86@...nel.org, Dave Hansen <dave.hansen@...el.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v8 6/7] x86/crash: pass dm crypt keys to kdump kernel
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.
>
>>
>>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@...erbo) (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@...erbo) (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!
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