Hello Akashi,

Thanks for the review comments.

On Mon, Apr 16, 2018 at 8:00 AM, AKASHI Takahiro
<takahiro.aka...@linaro.org> wrote:
> Bhupesh,
>
> On Sun, Apr 15, 2018 at 01:49:40AM +0530, Bhupesh Sharma 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 <bhsha...@redhat.com>
>> ---
>>  kexec/arch/arm64/kexec-arm64.c | 135 
>> +++++++++++++++++++++++++++++------------
>>  1 file changed, 97 insertions(+), 38 deletions(-)
>>
>> diff --git a/kexec/arch/arm64/kexec-arm64.c b/kexec/arch/arm64/kexec-arm64.c
>> index 62f37585b788..2ab11227447a 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 +414,99 @@ 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)) {
>
> Do we need this check?
> Please note that people are allowed to provide a dtb explicitly
> at command line and may want to use kexec as bootloader on
> no-uefi platform.

I agree. Lets look at the original behaviour (before this patch). We
used to unpack and fixup dtb properties and then pack it back when
'on_crash' was true (i.e only for the kdump case). In case of 'kexec'
we do not fixup the dtb (as per my understanding, please correct me if
I am wrong here).

With this patch I wanted the dtb's kaslr-seed property to be fixed-up
(if its supported and is wiped to 0 by the primary kernel). But this
check is harmless in case we don't find the 'kaslr-seed' property in
the dtb (for e.g. on non-uefi/u-boot based arm64 platforms).

In case the property is not seen in the dtb, we just print a debug
message (if '-d' flag was used to launch kexec) and proceed to perform
fixup of other dtb properties (like 'linux, usable-memory-range) in
case 'on_crash' is true (i.e. 'kexec -p' use case). In the 'kexec -l'
case since we don't do any other fixups in the original approach so we
retain the same behavior here.

>> +             dbgprintf("%s: no kaslr-seed found: %s\n",
>> +                             __func__, fdt_strerror(result));
>> +             /* for kexec warm reboot case, we don't need to fixup
>> +              * other dtb properties
>> +              */
>> +             if (!on_crash)
>> +                     goto free_new_buf;
>> +
>> +     } 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__);
>
> Ditto
> If this is a user-provided dtb, there is no reason to reject it.
> I think all what is needed here is to feed a *sane* dtb to kexec.
>
> So along with the comment above, it may be useful to add a command line
> option for turning on or off "kaslr-seed".

Please see my comments above. Since the 'kaslr-seed' property just
needs to be read from the dtb, we probably don't need a separate
command line option for the same as we already have nokaslr available.
If we want the secondary kernel to boot with *nokaslr*, we can pass
the same to the secondary via the command line arguments.

BTW, I also tried the behaviour with --dtb being passed while invoking
the 'kexec -l' with the patch in question and the resulting behaviour
is correct, i.e. we see that if the secondary kernel supports
CONFIG_RANDOMIZE_BASE=y, we get the resulting randomization in module
load address (for e.g.):

# kexec -l /boot/vmlinuz-`uname -r` --initrd=/boot/initramfs-`uname
-r`.img --command-line="$(cat /proc/cmdline)" --dtb /sys/firmware/fdt
-d

# kexec -e

On successive kexec warm reboots I see that '/proc/kallsyms' and
'/proc/modules' have randomized addresses.

>>                       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);
>
> Why do you use syscall() here?

I found that the standard way to invokde a getrandom() call is via a
SYSCALL (please see
<https://nikmav.blogspot.in/2016/10/random-generator-linux.html>).

>> +
>> +             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 +514,7 @@ 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;
>>               }
>> @@ -467,23 +526,23 @@ 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;
>>
>>  on_error:
>>       fprintf(stderr, "kexec: %s failed.\n", __func__);
>> +free_new_buf:
>
> Well, technically correct, but it looks odd as it is placed
> on *error* return path.

I agree. I was not too comfortable with placing this label here.
I will try to find a better approach in v2.

> You also miss dump_reservemap().

Oops. Sure will fix this in v2.

Regards,
Bhupesh

> Thanks,
> -Takahiro AKASHI
>
>>       if (new_buf)
>>               free(new_buf);
>>
>> --
>> 2.7.4
>>

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