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SubjectRe: [PATCH v16 16/16] arm64: kexec_file: add kaslr support
On Tue, Dec 11, 2018 at 02:50:02PM +0900, AKASHI Takahiro wrote:
> On Fri, Nov 30, 2018 at 01:19:44PM +0000, Will Deacon wrote:
> > On Thu, Nov 15, 2018 at 02:52:55PM +0900, AKASHI Takahiro wrote:
> > > Adding "kaslr-seed" to dtb enables triggering kaslr, or kernel virtual
> > > address randomization, at secondary kernel boot. We always do this as
> > > it will have no harm on kaslr-incapable kernel.
> > >
> > > We don't have any "switch" to turn off this feature directly, but still
> > > can suppress it by passing "nokaslr" as a kernel boot argument.
> > >
> > > Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> > > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > > Cc: Will Deacon <will.deacon@arm.com>
> > > ---
> > > arch/arm64/kernel/machine_kexec_file.c | 46 +++++++++++++++++++++++++-
> > > 1 file changed, 45 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c
> > > index ab296b98d633..a0a730bd9be6 100644
> > > --- a/arch/arm64/kernel/machine_kexec_file.c
> > > +++ b/arch/arm64/kernel/machine_kexec_file.c
> > > @@ -16,6 +16,7 @@
> > > #include <linux/libfdt.h>
> > > #include <linux/memblock.h>
> > > #include <linux/of_fdt.h>
> > > +#include <linux/random.h>
> > > #include <linux/slab.h>
> > > #include <linux/string.h>
> > > #include <linux/types.h>
> > > @@ -28,6 +29,7 @@
> > > #define FDT_PSTR_INITRD_STA "linux,initrd-start"
> > > #define FDT_PSTR_INITRD_END "linux,initrd-end"
> > > #define FDT_PSTR_BOOTARGS "bootargs"
> > > +#define FDT_PSTR_KASLR_SEED "kaslr-seed"
> > >
> > > const struct kexec_file_ops * const kexec_file_loaders[] = {
> > > &kexec_image_ops,
> > > @@ -46,11 +48,38 @@ int arch_kimage_file_post_load_cleanup(struct kimage *image)
> > > return kexec_image_post_load_cleanup_default(image);
> > > }
> > >
> > > +/* crng needs to have been initialized for providing kaslr-seed */
> > > +static int random_ready;
> > > +
> > > +static void random_ready_notified(struct random_ready_callback *unused)
> > > +{
> > > + random_ready = 1;
> > > +}
> > > +
> > > +static struct random_ready_callback random_ready_cb = {
> > > + .func = random_ready_notified,
> > > +};
> > > +
> > > +static __init int init_random_ready_cb(void)
> > > +{
> > > + int ret;
> > > +
> > > + ret = add_random_ready_callback(&random_ready_cb);
> > > + if (ret == -EALREADY)
> > > + random_ready = 1;
> > > + else if (ret)
> > > + pr_warn("failed to add a callback for random_ready\n");
> > > +
> > > + return 0;
> > > +}
> > > +late_initcall(init_random_ready_cb)
> >
> > Why can't we just call crng_ready()?
>
> because crng_ready() is locally defined in drivers/char/random.c.
> Instead, I'd like to use
> wait_for_random_bytes();
> value = get_random_u64();

Correction:
After several tests, I now don't think that calling wait_for_random_bytes()
is a good idea since it can make kexec_file_load() syscall stalled.
So, I would go for
if (rng_is_initialized())
value = get_random_u64();

-Takahiro Akashi

> > > +
> > > static int setup_dtb(struct kimage *image,
> > > unsigned long initrd_load_addr, unsigned long initrd_len,
> > > char *cmdline, void *dtb)
> > > {
> > > int nodeoffset;
> > > + u64 value;
> > > int ret;
> > >
> > > nodeoffset = fdt_path_offset(dtb, "/chosen");
> > > @@ -106,12 +135,27 @@ static int setup_dtb(struct kimage *image,
> > > return -EINVAL;
> > > }
> > >
> > > + /* add kaslr-seed */
> > > + ret = fdt_delprop(dtb, nodeoffset, FDT_PSTR_KASLR_SEED);
> > > + if (ret && (ret != -FDT_ERR_NOTFOUND))
> > > + return -EINVAL;
> > > +
> > > + if (random_ready) {
> > > + get_random_bytes(&value, sizeof(value));
> >
> > get_random_u64() ?
>
> OK.
>
> > > + ret = fdt_setprop_u64(dtb, nodeoffset, FDT_PSTR_KASLR_SEED,
> > > + value);
> > > + if (ret)
> > > + return (ret == -FDT_ERR_NOSPACE ? -ENOMEM : -EINVAL);
> > > + } else {
> >
> > Wouldn't we be better off preserving the previous seed here, if it was
> > present?
>
> While there's no guarantee that dtb won't be (partially) broken
> on failure, I will let this function return successfully
> by deleting succeeding fdt_delprop().
>
>
> > > + pr_notice("kaslr-seed won't be fed\n");
> >
> > "fed" is probably not the right word here.
>
> => won't be *provided* on kexec?
>
> -Takahiro Akashi
>
> > Will

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