Messages in this thread Patch in this message | | | From | Ben Hutchings <> | Date | Sun, 16 Jul 2017 14:56:38 +0100 | Subject | [PATCH 3.2 79/95] ftrace/x86: Fix triple fault with graph tracing and suspend-to-ram |
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3.2.91-rc1 review patch. If anyone has any objections, please let me know.
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From: Josh Poimboeuf <jpoimboe@redhat.com>
commit 34a477e5297cbaa6ecc6e17c042a866e1cbe80d6 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: Paul Menzel <pmenzel@molgen.mpg.de> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> Tested-by: Paul Menzel <pmenzel@molgen.mpg.de> Reviewed-by: Steven 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.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de> [bwh: Backported to 3.2: adjust context] Signed-off-by: Ben Hutchings <ben@decadent.org.uk> --- arch/x86/kernel/ftrace.c | 12 ++++++++++++ 1 file changed, 12 insertions(+)
--- a/arch/x86/kernel/ftrace.c +++ b/arch/x86/kernel/ftrace.c @@ -404,6 +404,18 @@ void prepare_ftrace_return(unsigned long unsigned long return_hooker = (unsigned long) &return_to_handler; + /* + * When resuming from suspend-to-ram, this function can be indirectly + * called from early CPU startup code while the CPU is in real mode, + * which would fail miserably. Make sure the stack pointer is a + * virtual address. + * + * This check isn't as accurate as virt_addr_valid(), but it should be + * good enough for this purpose, and it's fast. + */ + if (unlikely((long)__builtin_frame_address(0) >= 0)) + return; + if (unlikely(atomic_read(¤t->tracing_graph_pause))) return;
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