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

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Hello Akashi,

On Thu, Apr 26, 2018 at 2:27 PM, Bhupesh Sharma <bhsharma@xxxxxxxxxx> 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@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@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@xxxxxxxxxx>
> ---
> 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
>

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