On Thu, Apr 01, 2021 at 04:23:44PM -0700, Kees Cook wrote: > This provides the ability for architectures to enable kernel stack base > address offset randomization. This feature is controlled by the boot > param "randomize_kstack_offset=on/off", with its default value set by > CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT. > > This feature is based on the original idea from the last public release > of PaX's RANDKSTACK feature: https://pax.grsecurity.net/docs/randkstack.txt > All the credit for the original idea goes to the PaX team. Note that > the design and implementation of this upstream randomize_kstack_offset > feature differs greatly from the RANDKSTACK feature (see below). [...] > diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h > new file mode 100644 > index 000000000000..fd80fab663a9 > --- /dev/null > +++ b/include/linux/randomize_kstack.h > @@ -0,0 +1,54 @@ > +/* SPDX-License-Identifier: GPL-2.0-only */ > +#ifndef _LINUX_RANDOMIZE_KSTACK_H > +#define _LINUX_RANDOMIZE_KSTACK_H > + > +#include <linux/kernel.h> > +#include <linux/jump_label.h> > +#include <linux/percpu-defs.h> > + > +DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, > + randomize_kstack_offset); > +DECLARE_PER_CPU(u32, kstack_offset); > + > +/* > + * Do not use this anywhere else in the kernel. This is used here because > + * it provides an arch-agnostic way to grow the stack with correct > + * alignment. Also, since this use is being explicitly masked to a max of > + * 10 bits, stack-clash style attacks are unlikely. For more details see > + * "VLAs" in Documentation/process/deprecated.rst > + */ > +void *__builtin_alloca(size_t size); > +/* > + * Use, at most, 10 bits of entropy. We explicitly cap this to keep the > + * "VLA" from being unbounded (see above). 10 bits leaves enough room for > + * per-arch offset masks to reduce entropy (by removing higher bits, since > + * high entropy may overly constrain usable stack space), and for > + * compiler/arch-specific stack alignment to remove the lower bits. > + */ > +#define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF) > + > +/* > + * These macros must be used during syscall entry when interrupts and > + * preempt are disabled, and after user registers have been stored to > + * the stack. > + */ This comment is out of date, as this is called from preemptible context on arm64. Does that matter in terms of offset randomness? Will
