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    Subject[PATCH 4.14 34/77] ftrace/x86_64: Emulate call function while updating in breakpoint handler
    Date
    From: Peter Zijlstra <peterz@infradead.org>

    commit 9e298e8604088a600d8100a111a532a9d342af09 upstream.

    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.git.jpoimboe@redhat.com
    [4] http://lkml.kernel.org/r/CAHk-=wh5OpheSU8Em_Q3Hg8qw_JtoijxOdPtHru6d+5K8TWM=A@mail.gmail.com
    [5] http://lkml.kernel.org/r/CAHk-=wjvQxY4DvPrJ6haPgAa6b906h=MwZXO6G8OtiTGe=N7_w@mail.gmail.com

    [
    Live kernel patching is not implemented on x86_32, thus the emulate
    calls are only for x86_64.
    ]

    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")
    Tested-by: Nicolai Stange <nstange@suse.de>
    Reviewed-by: Nicolai Stange <nstange@suse.de>
    Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    [ Changed to only implement emulated calls for x86_64 ]
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/kernel/ftrace.c | 32 +++++++++++++++++++++++++++-----
    1 file changed, 27 insertions(+), 5 deletions(-)

    --- a/arch/x86/kernel/ftrace.c
    +++ b/arch/x86/kernel/ftrace.c
    @@ -30,6 +30,7 @@
    #include <asm/sections.h>
    #include <asm/ftrace.h>
    #include <asm/nops.h>
    +#include <asm/text-patching.h>

    #ifdef CONFIG_DYNAMIC_FTRACE

    @@ -229,6 +230,7 @@ int ftrace_modify_call(struct dyn_ftrace
    }

    static unsigned long ftrace_update_func;
    +static unsigned long ftrace_update_func_call;

    static int update_ftrace_func(unsigned long ip, void *new)
    {
    @@ -257,6 +259,8 @@ int ftrace_update_ftrace_func(ftrace_fun
    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);

    @@ -292,13 +296,28 @@ int ftrace_int3_handler(struct pt_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;
    +#ifdef CONFIG_X86_64
    + 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;
    + }
    +#else
    + if (ftrace_location(ip) || is_ftrace_caller(ip)) {
    + int3_emulate_jmp(regs, ip + CALL_INSN_SIZE);
    + return 1;
    + }
    +#endif

    - return 1;
    + return 0;
    }

    static int ftrace_write(unsigned long ip, const char *val, int size)
    @@ -869,6 +888,8 @@ void arch_ftrace_update_trampoline(struc

    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);
    @@ -965,6 +986,7 @@ static int ftrace_mod_jmp(unsigned long
    {
    unsigned char *new;

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

    return update_ftrace_func(ip, new);

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