• Josh Poimboeuf's avatar
    ftrace/x86: Fix triple fault with graph tracing and suspend-to-ram · 1883b91b
    Josh Poimboeuf authored
    commit 34a477e5 upstream.
    
    On x86-32, with CONFIG_FIRMWARE and multiple CPUs, if you enable function
    graph tracing and then suspend to RAM, it will triple fault and reboot when
    it resumes.
    
    The first fault happens when booting a secondary CPU:
    
    startup_32_smp()
      load_ucode_ap()
        prepare_ftrace_return()
          ftrace_graph_is_dead()
            (accesses 'kill_ftrace_graph')
    
    The early head_32.S code calls into load_ucode_ap(), which has an an
    ftrace hook, so it calls prepare_ftrace_return(), which calls
    ftrace_graph_is_dead(), which tries to access the global
    'kill_ftrace_graph' variable with a virtual address, causing a fault
    because the CPU is still in real mode.
    
    The fix is to add a check in prepare_ftrace_return() to make sure it's
    running in protected mode before continuing.  The check makes sure the
    stack pointer is a virtual kernel address.  It's a bit of a hack, but
    it's not very intrusive and it works well enough.
    
    For reference, here are a few other (more difficult) ways this could
    have potentially been fixed:
    
    - Move startup_32_smp()'s call to load_ucode_ap() down to *after* paging
      is enabled.  (No idea what that would break.)
    
    - Track down load_ucode_ap()'s entire callee tree and mark all the
      functions 'notrace'.  (Probably not realistic.)
    
    - Pause graph tracing in ftrace_suspend_notifier_call() or bringup_cpu()
      or __cpu_up(), and ensure that the pause facility can be queried from
      real mode.
    Reported-by: default avatarPaul Menzel <pmenzel@molgen.mpg.de>
    Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
    Tested-by: default avatarPaul Menzel <pmenzel@molgen.mpg.de>
    Reviewed-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    Cc: "Rafael J . Wysocki" <rjw@rjwysocki.net>
    Cc: linux-acpi@vger.kernel.org
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Len Brown <lenb@kernel.org>
    Link: http://lkml.kernel.org/r/5c1272269a580660703ed2eccf44308e790c7a98.1492123841.git.jpoimboe@redhat.comSigned-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    [bwh: Backported to 3.2: adjust context]
    Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
    1883b91b
ftrace.c 11.4 KB