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Subject: Re: [RFC PATCH 1/1] arm64: implement live patching
From:   Suraj Jitindar Singh <sjitindarsingh@gmail.com>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        live-patching@vger.kernel.org, catalin.marinas@arm.com,
        will@kernel.org, broonie@kernel.org, madvenka@linux.microsoft.com,
        duwe@lst.de, benh@kernel.crashing.org
Date:   Tue, 08 Jun 2021 17:32:10 -0700
In-Reply-To: <20210607102058.GB97489@C02TD0UTHF1T.local>
References: <20210604235930.603-1-surajjs@amazon.com>
         <20210607102058.GB97489@C02TD0UTHF1T.local>
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On Mon, 2021-06-07 at 11:20 +0100, Mark Rutland wrote:
> On Fri, Jun 04, 2021 at 04:59:30PM -0700, Suraj Jitindar Singh wrote:
> > 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]
> 
> We also need to rework the arm64 patching code to not rely on any
> code
> which may itself be patched. Currently there's a reasonable amount of
> patching code that can either be patched directly, or can be
> instrumented by code that can be patched.

If I understand correctly you're saying that it's unsafe for patching
code to call any other code (directly or indirectly) which can itself
be patched.

While I understand it's obviously important to fix this issue in the
patching code as whole, live patching uses ftrace and as far as I can
tell the only patching functions used by ftrace (in
ftrace_modify_code()) is aarch64_insn_patch_text_nosync(). So would I
be correct in my understanding that so long as that function doesn't
call any other functions which can themselves be patched, then this
would be safe?

> 
> I have some WIP patches for that at:
> 
>   
> https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/patching/rework
> 
> ... but there's still a lot to do, and it's not clear to me if
> there's
> other stuff we need to rework to make patch-safe.

Is there any work I could contribute to this?

> 
> Do we have any infrastructure for testing LIVEPATCH?

Not currently. However I'm thinking something which attempted to
trivially patch every patchable function would provide pretty good test
coverage (while being very slow).

> 
> > 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@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
> > ---
> >  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"
> > 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)
> 
> Is `faddr` the address of the function, or the addresds of the patch
> site (the
> dyn_ftrace::ip)? Either way, I think there's a problem; see below.
> 

It is the address of the function.

> > +{
> > +	unsigned long addr = faddr + AARCH64_INSN_SIZE;
> > +
> > +#if IS_ENABLED(CONFIG_ARM64_BTI_KERNEL)
> > +	addr = ftrace_location_range(addr, addr + AARCH64_INSN_SIZE);
> > +#endif
> 
> I don't think this is right; function are not guaranteed to have a
> BTI,
> since e.g. static functions which are called directly may not be
> given
> one:

Correct, gcc only inserts the instruction on functions which it
determines are called indirectly and thus validated.

> 
> > [mark@lakrids:/mnt/data/tests/bti-patching]% cat test.c
> > #define noinline __attribute__((noinline))
> > 
> > static noinline void a(void)
> > {
> >         asm volatile("" ::: "memory");
> > }
> > 
> > void b(void)
> > {
> >         a();
> > }
> > [mark@lakrids:/mnt/data/tests/bti-patching]% usekorg 10.3.0
> > aarch64-linux-gcc -c test.c -fpatchable-function-entry=2 -mbranch-
> > protection=bti -O2
> > [mark@lakrids:/mnt/data/tests/bti-patching]% usekorg 10.3.0
> > aarch64-linux-objdump -d
> > test.o                                                     
> > 
> > test.o:     file format elf64-littleaarch64
> > 
> > 
> > Disassembly of section .text:
> > 
> > 0000000000000000 <a>:
> >    0:   d503201f        nop
> >    4:   d503201f        nop
> >    8:   d65f03c0        ret
> >    c:   d503201f        nop
> > 
> > 0000000000000010 <b>:
> >   10:   d503245f        bti     c
> >   14:   d503201f        nop
> >   18:   d503201f        nop
> >   1c:   17fffff9        b       0 <a>
> 
> If `faddr` is the address of the function, then we'll need to do
> something dynamic to skip any BTI. If it's the address of the patch
> site, then we shouldn't need to consider the BTI at all: att boot
> time
> the recorded lcoation points at the first NOP, and at init time we
> point
> dyn_ftrace::ip at the second NOP -- see ftrace_call_adjust().

faddr is the address of the function.
This concern is what my code attempts to address.

Either way we need the address of the branch which is AARCH64_INSN_SIZE
after the function address if BTI is _disabled_.
If BTI is _enabled_ the branch could be either at the address we just
computed or AARCH64_INSN_SIZE after it depending on if the instruction
was inserted by the compiler. This is why ftrace_location_range() is
used as it finds the correct ftrace branch location within the
specified range.
So the range is from
faddr + AARCH64_INSN_SIZE (BTI disabled or enabled & not inserted)
to
faddr + 2 * AARCH64_INSN_SIZE (BTI enabled and instr inserted)

> 
> Thanks,
> Mark.

Thanks for taking a look.
Suraj.

> 
> > +
> > +	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
> >