From: Peter Zijlstra peterz@infradead.org

Nicolai Stange discovered[1] that if live kernel patching is enabled, and the function tracer started tracing the same function that was patched, the conversion of the fentry call site during the translation of going from calling the live kernel patch trampoline to the iterator trampoline, would have as slight window where it didn't call anything.

As live kernel patching depends on ftrace to always call its code (to prevent the function being traced from being called, as it will redirect it). This small window would allow the old buggy function to be called, and this can cause undesirable results.

Nicolai submitted new patches[2] but these were controversial. As this is similar to the static call emulation issues that came up a while ago[3]. But after some debate[4][5] adding a gap in the stack when entering the breakpoint handler allows for pushing the return address onto the stack to easily emulate a call.

[1] http://lkml.kernel.org/r/20180726104029.7736-1-nstange@suse.de [2] http://lkml.kernel.org/r/20190427100639.15074-1-nstange@suse.de [3] http://lkml.kernel.org/r/3cf04e113d71c9f8e4be95fb84a510f085aa4afa.1541711457... [4] http://lkml.kernel.org/r/CAHk-=wh5OpheSU8Em_Q3Hg8qw_JtoijxOdPtHru6d+5K8TWM=A... [5] http://lkml.kernel.org/r/CAHk-=wjvQxY4DvPrJ6haPgAa6b906h=MwZXO6G8OtiTGe=N7_w...

Cc: Andy Lutomirski luto@kernel.org Cc: Nicolai Stange nstange@suse.de Cc: Thomas Gleixner tglx@linutronix.de Cc: Ingo Molnar mingo@redhat.com Cc: Borislav Petkov bp@alien8.de Cc: "H. Peter Anvin" hpa@zytor.com Cc: the arch/x86 maintainers x86@kernel.org Cc: Josh Poimboeuf jpoimboe@redhat.com Cc: Jiri Kosina jikos@kernel.org Cc: Miroslav Benes mbenes@suse.cz Cc: Petr Mladek pmladek@suse.com Cc: Joe Lawrence joe.lawrence@redhat.com Cc: Shuah Khan shuah@kernel.org Cc: Konrad Rzeszutek Wilk konrad.wilk@oracle.com Cc: Tim Chen tim.c.chen@linux.intel.com Cc: Sebastian Andrzej Siewior bigeasy@linutronix.de Cc: Mimi Zohar zohar@linux.ibm.com Cc: Juergen Gross jgross@suse.com Cc: Nick Desaulniers ndesaulniers@google.com Cc: Nayna Jain nayna@linux.ibm.com Cc: Masahiro Yamada yamada.masahiro@socionext.com Cc: Joerg Roedel jroedel@suse.de Cc: "open list:KERNEL SELFTEST FRAMEWORK" linux-kselftest@vger.kernel.org Cc: stable@vger.kernel.org Fixes: b700e7f03df5 ("livepatch: kernel: add support for live patching") Signed-off-by: *** Need SoB From Peter Zijlstra *** Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org --- arch/x86/kernel/ftrace.c | 25 ++++++++++++++++++++----- 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c index ef49517f6bb2..fd152f5a937b 100644 --- a/arch/x86/kernel/ftrace.c +++ b/arch/x86/kernel/ftrace.c @@ -29,6 +29,7 @@ #include <asm/kprobes.h> #include <asm/ftrace.h> #include <asm/nops.h> +#include <asm/text-patching.h>

#ifdef CONFIG_DYNAMIC_FTRACE

@@ -231,6 +232,7 @@ int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr, }

static unsigned long ftrace_update_func; +static unsigned long ftrace_update_func_call;

static int update_ftrace_func(unsigned long ip, void *new) { @@ -259,6 +261,8 @@ int ftrace_update_ftrace_func(ftrace_func_t func) unsigned char *new; int ret;

+ ftrace_update_func_call = (unsigned long)func; + new = ftrace_call_replace(ip, (unsigned long)func); ret = update_ftrace_func(ip, new);

@@ -294,13 +298,21 @@ int ftrace_int3_handler(struct pt_regs *regs) if (WARN_ON_ONCE(!regs)) return 0;

- ip = regs->ip - 1; - if (!ftrace_location(ip) && !is_ftrace_caller(ip)) - return 0; + ip = regs->ip - INT3_INSN_SIZE;

- regs->ip += MCOUNT_INSN_SIZE - 1; + if (ftrace_location(ip)) { + int3_emulate_call(regs, (unsigned long)ftrace_regs_caller); + return 1; + } else if (is_ftrace_caller(ip)) { + if (!ftrace_update_func_call) { + int3_emulate_jmp(regs, ip + CALL_INSN_SIZE); + return 1; + } + int3_emulate_call(regs, ftrace_update_func_call); + return 1; + }

- return 1; + return 0; } NOKPROBE_SYMBOL(ftrace_int3_handler);

@@ -859,6 +871,8 @@ void arch_ftrace_update_trampoline(struct ftrace_ops *ops)

func = ftrace_ops_get_func(ops);

+ ftrace_update_func_call = (unsigned long)func; + /* Do a safe modify in case the trampoline is executing */ new = ftrace_call_replace(ip, (unsigned long)func); ret = update_ftrace_func(ip, new); @@ -960,6 +974,7 @@ static int ftrace_mod_jmp(unsigned long ip, void *func) { unsigned char *new;

+ ftrace_update_func_call = 0UL; new = ftrace_jmp_replace(ip, (unsigned long)func);

return update_ftrace_func(ip, new);