[PATCH v2] arm64: Add support to supply 'kaslr-seed' to secondary kernel

Bhupesh Sharma bhsharma at redhat.com
Sun Jun 17 22:56:11 PDT 2018


Hello Simon,

On 05/15/2018 12:50 PM, Bhupesh Sharma wrote:
> Hello Akashi,
> 
> On Thu, Apr 26, 2018 at 2:27 PM, Bhupesh Sharma <bhsharma at redhat.com> wrote:
>> This patch adds the support to supply 'kaslr-seed' to secondary kernel,
>> when we do a 'kexec warm reboot to another kernel' (although the
>> behaviour remains the same for the 'kdump' case as well) on arm64
>> platforms using the 'kexec_load' invocation method.
>>
>> Lets consider the case where the primary kernel working on the arm64
>> platform supports kaslr (i.e 'CONFIG_RANDOMIZE_BASE' was set to y and
>> we have a compliant EFI firmware which supports EFI_RNG_PROTOCOL and
>> hence can pass a non-zero (valid) seed to the primary kernel).
>>
>> Now the primary kernel reads the 'kaslr-seed' and wipes it to 0 and
>> uses the seed value to randomize for e.g. the module base address
>> offset.
>>
>> In the case of 'kexec_load' (or even kdump for brevity),
>> we rely on the user-space kexec-tools to pass an appropriate dtb to the
>> secondary kernel and since 'kaslr-seed' is wiped to 0 by the primary
>> kernel, the secondary will essentially work with *nokaslr* as
>> 'kaslr-seed' is set to 0 when it is passed to the secondary kernel.
>>
>> This can be true even in case the secondary kernel had
>> 'CONFIG_RANDOMIZE_BASE' and 'CONFIG_RANDOMIZE_MODULE_REGION_FULL' set to
>> y.
>>
>> This patch addresses this issue by first checking if the device tree
>> provided by the firmware to the kernel supports the 'kaslr-seed'
>> property and verifies that it is really wiped to 0. If this condition is
>> met, it fixes up the 'kaslr-seed' property by using the getrandom()
>> syscall to get a suitable random number.
>>
>> I verified this patch on my Qualcomm arm64 board and here are some test
>> results:
>>
>> 1. Ensure that the primary kernel is boot'ed with 'kaslr-seed'
>>     dts property and it is really wiped to 0:
>>
>>     [root at qualcomm-amberwing]# dtc -I dtb -O dts /sys/firmware/fdt | grep -A 10 -i chosen
>>          chosen {
>>                  kaslr-seed = <0x0 0x0>;
>>                  ...
>>          }
>>
>> 2. Now issue 'kexec_load' to load the secondary kernel (let's assume
>>     that we are using the same kernel as the secondary kernel):
>>     # kexec -l /boot/vmlinuz-`uname -r` --initrd=/boot/initramfs-`uname
>>       -r`.img --reuse-cmdline -d
>>
>> 3. Issue 'kexec -e' to warm boot to the secondary:
>>     # kexec -e
>>
>> 4. Now after the secondary boots, confirm that the load address of the
>>     modules is randomized in every successive boot:
>>
>>     [root at qualcomm-amberwing]# cat /proc/modules
>>     sunrpc 524288 1 - Live 0xffff0307db190000
>>     vfat 262144 1 - Live 0xffff0307db110000
>>     fat 262144 1 vfat, Live 0xffff0307db090000
>>     crc32_ce 262144 0 - Live 0xffff0307d8c70000
>>     ...
>>
>> Signed-off-by: Bhupesh Sharma <bhsharma at redhat.com>
>> ---
>> Changes since v1:
>>   - Addressed Akashi's comments regarding the goto label path.
>>   - v1 can be viewed here: https://marc.info/?l=kexec&m=152373724406110&w=2
> 
> Ping. Any comments on this v2?
> 
> Thanks,
> Bhupesh
> 
>>   kexec/arch/arm64/kexec-arm64.c | 138 +++++++++++++++++++++++++++++------------
>>   1 file changed, 100 insertions(+), 38 deletions(-)
>>
>> diff --git a/kexec/arch/arm64/kexec-arm64.c b/kexec/arch/arm64/kexec-arm64.c
>> index 62f37585b788..a206c172b1aa 100644
>> --- a/kexec/arch/arm64/kexec-arm64.c
>> +++ b/kexec/arch/arm64/kexec-arm64.c
>> @@ -15,6 +15,11 @@
>>   #include <linux/elf-em.h>
>>   #include <elf.h>
>>
>> +#include <unistd.h>
>> +#include <syscall.h>
>> +#include <errno.h>
>> +#include <linux/random.h>
>> +
>>   #include "kexec.h"
>>   #include "kexec-arm64.h"
>>   #include "crashdump.h"
>> @@ -392,11 +397,13 @@ static int fdt_setprop_range(void *fdt, int nodeoffset,
>>   static int setup_2nd_dtb(struct dtb *dtb, char *command_line, int on_crash)
>>   {
>>          uint32_t address_cells, size_cells;
>> -       int range_len;
>> -       int nodeoffset;
>> +       uint64_t fdt_val64;
>> +       uint64_t *prop;
>>          char *new_buf = NULL;
>> +       int len, range_len;
>> +       int nodeoffset;
>>          int new_size;
>> -       int result;
>> +       int result, kaslr_seed;
>>
>>          result = fdt_check_header(dtb->buf);
>>
>> @@ -407,47 +414,103 @@ static int setup_2nd_dtb(struct dtb *dtb, char *command_line, int on_crash)
>>
>>          result = set_bootargs(dtb, command_line);
>>
>> -       if (on_crash) {
>> -               /* determine #address-cells and #size-cells */
>> -               result = get_cells_size(dtb->buf, &address_cells, &size_cells);
>> -               if (result) {
>> -                       fprintf(stderr,
>> -                               "kexec: cannot determine cells-size.\n");
>> -                       result = -EINVAL;
>> -                       goto on_error;
>> -               }
>> +       /* determine #address-cells and #size-cells */
>> +       result = get_cells_size(dtb->buf, &address_cells, &size_cells);
>> +       if (result) {
>> +               fprintf(stderr, "kexec: cannot determine cells-size.\n");
>> +               result = -EINVAL;
>> +               goto on_error;
>> +       }
>>
>> -               if (!cells_size_fitted(address_cells, size_cells,
>> -                                       &elfcorehdr_mem)) {
>> -                       fprintf(stderr,
>> -                               "kexec: elfcorehdr doesn't fit cells-size.\n");
>> +       if (!cells_size_fitted(address_cells, size_cells,
>> +                               &elfcorehdr_mem)) {
>> +               fprintf(stderr, "kexec: elfcorehdr doesn't fit cells-size.\n");
>> +               result = -EINVAL;
>> +               goto on_error;
>> +       }
>> +
>> +       if (!cells_size_fitted(address_cells, size_cells,
>> +                               &crash_reserved_mem)) {
>> +               fprintf(stderr, "kexec: usable memory range doesn't fit cells-size.\n");
>> +               result = -EINVAL;
>> +               goto on_error;
>> +       }
>> +
>> +       /* duplicate dt blob */
>> +       range_len = sizeof(uint32_t) * (address_cells + size_cells);
>> +       new_size = fdt_totalsize(dtb->buf)
>> +               + fdt_prop_len(PROP_ELFCOREHDR, range_len)
>> +               + fdt_prop_len(PROP_USABLE_MEM_RANGE, range_len);
>> +
>> +       new_buf = xmalloc(new_size);
>> +       result = fdt_open_into(dtb->buf, new_buf, new_size);
>> +       if (result) {
>> +               dbgprintf("%s: fdt_open_into failed: %s\n", __func__,
>> +                               fdt_strerror(result));
>> +               result = -ENOSPC;
>> +               goto on_error;
>> +       }
>> +
>> +       /* fixup 'kaslr-seed' with a random value, if supported */
>> +       nodeoffset = fdt_path_offset(new_buf, "/chosen");
>> +       prop = fdt_getprop_w(new_buf, nodeoffset,
>> +                       "kaslr-seed", &len);
>> +       if (!prop || len != sizeof(uint64_t)) {
>> +               dbgprintf("%s: no kaslr-seed found\n",
>> +                               __func__);
>> +               /* for kexec warm reboot case, we don't need to fixup
>> +                * other dtb properties
>> +                */
>> +               if (!on_crash) {
>> +                       dump_reservemap(dtb);
>> +                       if (new_buf)
>> +                               free(new_buf);
>> +
>> +                       return result;
>> +               }
>> +       } else {
>> +               kaslr_seed = fdt64_to_cpu(*prop);
>> +
>> +               /* kaslr_seed must be wiped clean by primary
>> +                * kernel during boot
>> +                */
>> +               if (kaslr_seed != 0) {
>> +                       dbgprintf("%s: kaslr-seed is not wiped to 0.\n",
>> +                                       __func__);
>>                          result = -EINVAL;
>>                          goto on_error;
>>                  }
>>
>> -               if (!cells_size_fitted(address_cells, size_cells,
>> -                                       &crash_reserved_mem)) {
>> -                       fprintf(stderr,
>> -                               "kexec: usable memory range doesn't fit cells-size.\n");
>> +               /*
>> +                * Invoke the getrandom system call with
>> +                * GRND_NONBLOCK, to make sure we
>> +                * have a valid random seed to pass to the
>> +                * secondary kernel.
>> +                */
>> +               result = syscall(SYS_getrandom, &fdt_val64,
>> +                               sizeof(fdt_val64),
>> +                               GRND_NONBLOCK);
>> +
>> +               if(result == -1) {
>> +                       dbgprintf("%s: Reading random bytes failed.\n",
>> +                                       __func__);
>>                          result = -EINVAL;
>>                          goto on_error;
>>                  }
>>
>> -               /* duplicate dt blob */
>> -               range_len = sizeof(uint32_t) * (address_cells + size_cells);
>> -               new_size = fdt_totalsize(dtb->buf)
>> -                       + fdt_prop_len(PROP_ELFCOREHDR, range_len)
>> -                       + fdt_prop_len(PROP_USABLE_MEM_RANGE, range_len);
>> -
>> -               new_buf = xmalloc(new_size);
>> -               result = fdt_open_into(dtb->buf, new_buf, new_size);
>> +               nodeoffset = fdt_path_offset(new_buf, "/chosen");
>> +               result = fdt_setprop_inplace(new_buf,
>> +                               nodeoffset, "kaslr-seed",
>> +                               &fdt_val64, sizeof(fdt_val64));
>>                  if (result) {
>> -                       dbgprintf("%s: fdt_open_into failed: %s\n", __func__,
>> -                               fdt_strerror(result));
>> -                       result = -ENOSPC;
>> +                       dbgprintf("%s: fdt_setprop failed: %s\n",
>> +                                       __func__, fdt_strerror(result));
>> +                       result = -EINVAL;
>>                          goto on_error;
>>                  }
>> +       }
>>
>> +       if (on_crash) {
>>                  /* add linux,elfcorehdr */
>>                  nodeoffset = fdt_path_offset(new_buf, "/chosen");
>>                  result = fdt_setprop_range(new_buf, nodeoffset,
>> @@ -455,7 +518,7 @@ static int setup_2nd_dtb(struct dtb *dtb, char *command_line, int on_crash)
>>                                  address_cells, size_cells);
>>                  if (result) {
>>                          dbgprintf("%s: fdt_setprop failed: %s\n", __func__,
>> -                               fdt_strerror(result));
>> +                                       fdt_strerror(result));
>>                          result = -EINVAL;
>>                          goto on_error;
>>                  }
>> @@ -467,18 +530,17 @@ static int setup_2nd_dtb(struct dtb *dtb, char *command_line, int on_crash)
>>                                  address_cells, size_cells);
>>                  if (result) {
>>                          dbgprintf("%s: fdt_setprop failed: %s\n", __func__,
>> -                               fdt_strerror(result));
>> +                                       fdt_strerror(result));
>>                          result = -EINVAL;
>>                          goto on_error;
>>                  }
>> -
>> -               fdt_pack(new_buf);
>> -               dtb->buf = new_buf;
>> -               dtb->size = fdt_totalsize(new_buf);
>>          }
>>
>> -       dump_reservemap(dtb);
>> +       fdt_pack(new_buf);
>> +       dtb->buf = new_buf;
>> +       dtb->size = fdt_totalsize(new_buf);
>>
>> +       dump_reservemap(dtb);
>>
>>          return result;
>>
>> --
>> 2.7.4
>>

Ping. Seems we don't have comments on this v2 (since the last month or so).

Having the capability to pass an appropriate 'kaslr-seed' value to the 
secondary kernel is important for booting KASLR enabled secondary 
kernels - which is an important use case for kexec for distributions 
like Fedora.

Kindly consider applying this patch which has been properly tested on a 
couple of arm64 machines both for KASLR and non-KASLR boot cases.

Right now, I am forced to keep this patch locally and refer KASLR users 
on arm64 to my public github tree which has this patch committed on top 
of the upstream kexec-tools. I would prefer to route them to the 
upstream kexec-tools itself (rather than my github tree), once this 
patch is applied there.

Thanks for your help.

Regards,
Bhupesh



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