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@xxxxxxxxxx> > > > > [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 > >
