[RFC PATCH 1/1] arm64: implement live patching

Thu Jun 17 02:29:22 PDT 2021

> It's my understanding that the two pieces of work required to enable live
> patching on arm are in flight upstream;
> - Reliable stack traces as implemented by Madhavan T. Venkataraman [1]
> - Objtool as implemented by Julien Thierry [2]
> 
> This is the remaining part required to enable live patching on arm.
> Based on work by Torsten Duwe [3]
> 
> Allocate a task flag used to represent the patch pending state for the
> task. Also implement generic functions klp_arch_set_pc() &
> klp_get_ftrace_location().
> 
> In klp_arch_set_pc() it is sufficient to set regs->pc as in
> ftrace_common_return() the return address is loaded from the stack.
> 
> ldr     x9, [sp, #S_PC]
> <snip>
> ret     x9
> 
> In klp_get_ftrace_location() it is necessary to advance the address by
> AARCH64_INSN_SIZE (4) to point to the BL in the callsite as 2 nops were
> placed at the start of the function, one to be patched to save the LR and
> another to be patched to branch to the ftrace call, and
> klp_get_ftrace_location() is expected to return the address of the BL. It
> may also be necessary to advance the address by another AARCH64_INSN_SIZE
> if CONFIG_ARM64_BTI_KERNEL is enabled due to the instruction placed at the
> branch target to satisfy BTI,
> 
> Signed-off-by: Suraj Jitindar Singh <surajjs at amazon.com>
> 
> [1] https://lkml.org/lkml/2021/5/26/1212
> [2] https://lkml.org/lkml/2021/3/3/1135
> [3] https://lkml.org/lkml/2018/10/26/536
> ---

AFAIU Madhavan's patch series linked in the above [1] is currently awaiting
review by Mark Rutland. It seems that not only this patch series but also the
implementation of arch_stack_walk_reliable() at the below link is required
to enable livepatch.

https://lore.kernel.org/linux-arm-kernel/bf3a5289-8199-b665-0327-ed8240dd7827@linux.microsoft.com/

>  arch/arm64/Kconfig                   |  3 ++
>  arch/arm64/include/asm/livepatch.h   | 42 ++++++++++++++++++++++++++++
>  arch/arm64/include/asm/thread_info.h |  4 ++-
>  arch/arm64/kernel/signal.c           |  4 +++
>  4 files changed, 52 insertions(+), 1 deletion(-)
>  create mode 100644 arch/arm64/include/asm/livepatch.h
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b098dabed8c2..c4636990c01d 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -187,6 +187,7 @@ config ARM64
>  	select HAVE_GCC_PLUGINS
>  	select HAVE_HW_BREAKPOINT if PERF_EVENTS
>  	select HAVE_IRQ_TIME_ACCOUNTING
> +	select HAVE_LIVEPATCH
>  	select HAVE_NMI
>  	select HAVE_PATA_PLATFORM
>  	select HAVE_PERF_EVENTS
> @@ -1946,3 +1947,5 @@ source "arch/arm64/kvm/Kconfig"
>  if CRYPTO
>  source "arch/arm64/crypto/Kconfig"
>  endif
> +
> +source "kernel/livepatch/Kconfig"

I think ` source "kernel/livepatch/Kconfig"` should be placed between
`menu "Kernel Features"` and `endmenu`.

Thanks
Keiya

> diff --git a/arch/arm64/include/asm/livepatch.h b/arch/arm64/include/asm/livepatch.h
> new file mode 100644
> index 000000000000..72d7cd86f158
> --- /dev/null
> +++ b/arch/arm64/include/asm/livepatch.h
> @@ -0,0 +1,42 @@
> +/* SPDX-License-Identifier: GPL-2.0
> + *
> + * livepatch.h - arm64-specific Kernel Live Patching Core
> + */
> +#ifndef _ASM_ARM64_LIVEPATCH_H
> +#define _ASM_ARM64_LIVEPATCH_H
> +
> +#include <linux/ftrace.h>
> +
> +static inline void klp_arch_set_pc(struct ftrace_regs *fregs, unsigned long ip)
> +{
> +	struct pt_regs *regs = ftrace_get_regs(fregs);
> +
> +	regs->pc = ip;
> +}
> +
> +/*
> + * klp_get_ftrace_location is expected to return the address of the BL to the
> + * relevant ftrace handler in the callsite. The location of this can vary based
> + * on several compilation options.
> + * CONFIG_DYNAMIC_FTRACE_WITH_REGS
> + *	- Inserts 2 nops on function entry the second of which is the BL
> + *	  referenced above. (See ftrace_init_nop() for the callsite sequence)
> + *	  (this is required by livepatch and must be selected)
> + * CONFIG_ARM64_BTI_KERNEL:
> + *	- Inserts a hint #0x22 on function entry if the function is called
> + *	  indirectly (to satisfy BTI requirements), which is inserted before
> + *	  the two nops from above.
> + */
> +#define klp_get_ftrace_location klp_get_ftrace_location
> +static inline unsigned long klp_get_ftrace_location(unsigned long faddr)
> +{
> +	unsigned long addr = faddr + AARCH64_INSN_SIZE;
> +
> +#if IS_ENABLED(CONFIG_ARM64_BTI_KERNEL)
> +	addr = ftrace_location_range(addr, addr + AARCH64_INSN_SIZE);
> +#endif
> +
> +	return addr;
> +}
> +
> +#endif /* _ASM_ARM64_LIVEPATCH_H */
> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> index 6623c99f0984..cca936d53a40 100644
> --- a/arch/arm64/include/asm/thread_info.h
> +++ b/arch/arm64/include/asm/thread_info.h
> @@ -67,6 +67,7 @@ int arch_dup_task_struct(struct task_struct *dst,
>  #define TIF_UPROBE		4	/* uprobe breakpoint or singlestep */
>  #define TIF_MTE_ASYNC_FAULT	5	/* MTE Asynchronous Tag Check Fault */
>  #define TIF_NOTIFY_SIGNAL	6	/* signal notifications exist */
> +#define TIF_PATCH_PENDING	7	/* pending live patching update */
>  #define TIF_SYSCALL_TRACE	8	/* syscall trace active */
>  #define TIF_SYSCALL_AUDIT	9	/* syscall auditing */
>  #define TIF_SYSCALL_TRACEPOINT	10	/* syscall tracepoint for ftrace */
> @@ -97,11 +98,12 @@ int arch_dup_task_struct(struct task_struct *dst,
>  #define _TIF_SVE		(1 << TIF_SVE)
>  #define _TIF_MTE_ASYNC_FAULT	(1 << TIF_MTE_ASYNC_FAULT)
>  #define _TIF_NOTIFY_SIGNAL	(1 << TIF_NOTIFY_SIGNAL)
> +#define _TIF_PATCH_PENDING	(1 << TIF_PATCH_PENDING)
> 
>  #define _TIF_WORK_MASK		(_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
>  				 _TIF_NOTIFY_RESUME | _TIF_FOREIGN_FPSTATE | \
>  				 _TIF_UPROBE | _TIF_MTE_ASYNC_FAULT | \
> -				 _TIF_NOTIFY_SIGNAL)
> +				 _TIF_NOTIFY_SIGNAL | _TIF_PATCH_PENDING)
> 
>  #define _TIF_SYSCALL_WORK	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
>  				 _TIF_SYSCALL_TRACEPOINT | _TIF_SECCOMP | \
> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index 6237486ff6bb..d1eedb0589a7 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -18,6 +18,7 @@
>  #include <linux/sizes.h>
>  #include <linux/string.h>
>  #include <linux/tracehook.h>
> +#include <linux/livepatch.h>
>  #include <linux/ratelimit.h>
>  #include <linux/syscalls.h>
> 
> @@ -932,6 +933,9 @@ asmlinkage void do_notify_resume(struct pt_regs *regs,
>  					       (void __user *)NULL, current);
>  			}
> 
> +			if (thread_flags & _TIF_PATCH_PENDING)
> +				klp_update_patch_state(current);
> +
>  			if (thread_flags & (_TIF_SIGPENDING | _TIF_NOTIFY_SIGNAL))
>  				do_signal(regs);
> 
> --
> 2.17.1