From 519fb5c3350d1b5225b27b1cac55144f79351718 Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Tue, 23 May 2017 10:37:30 -0500 Subject: [PATCH] x86/unwind: Add end-of-stack check for ftrace handlers Dave Jones and Steven Rostedt reported unwinder warnings like the following: WARNING: kernel stack frame pointer at ffff8800bda0ff30 in sshd:1090 has bad value 000055b32abf1fa8 In both cases, the unwinder was attempting to unwind from an ftrace handler into entry code. The callchain was something like: syscall entry code C function ftrace handler save_stack_trace() The problem is that the unwinder's end-of-stack logic gets confused by the way ftrace lays out the stack frame (with fentry enabled). I was able to recreate this warning with: echo call_usermodehelper_exec_async:stacktrace > /sys/kernel/debug/tracing/set_ftrace_filter (exit login session) I considered fixing this by changing the ftrace code to rewrite the stack to make the unwinder happy. But that seemed too intrusive after I implemented it. Instead, just add another check to the unwinder's end-of-stack logic to detect this special case. Side note: We could probably get rid of these end-of-stack checks by encoding the frame pointer for syscall entry just like we do for interrupt entry. That would be simpler, but it would also be a lot more intrusive since it would slightly affect the performance of every syscall. Reported-by: Dave Jones Reported-by: Steven Rostedt Signed-off-by: Josh Poimboeuf Acked-by: Thomas Gleixner Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Petr Mladek Cc: live-patching@vger.kernel.org Fixes: c32c47c68a0a ("x86/unwind: Warn on bad frame pointer") Link: http://lkml.kernel.org/r/671ba22fbc0156b8f7e0cfa5ab2a795e08bc37e1.1495553739.git.jpoimboe@redhat.com Signed-off-by: Ingo Molnar --- arch/x86/kernel/unwind_frame.c | 49 +++++++++++++++++++++++++++------- 1 file changed, 40 insertions(+), 9 deletions(-) diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c index 82c6d7f1fd73..b9389d72b2f7 100644 --- a/arch/x86/kernel/unwind_frame.c +++ b/arch/x86/kernel/unwind_frame.c @@ -104,6 +104,11 @@ static inline unsigned long *last_frame(struct unwind_state *state) return (unsigned long *)task_pt_regs(state->task) - 2; } +static bool is_last_frame(struct unwind_state *state) +{ + return state->bp == last_frame(state); +} + #ifdef CONFIG_X86_32 #define GCC_REALIGN_WORDS 3 #else @@ -115,16 +120,15 @@ static inline unsigned long *last_aligned_frame(struct unwind_state *state) return last_frame(state) - GCC_REALIGN_WORDS; } -static bool is_last_task_frame(struct unwind_state *state) +static bool is_last_aligned_frame(struct unwind_state *state) { unsigned long *last_bp = last_frame(state); unsigned long *aligned_bp = last_aligned_frame(state); /* - * We have to check for the last task frame at two different locations - * because gcc can occasionally decide to realign the stack pointer and - * change the offset of the stack frame in the prologue of a function - * called by head/entry code. Examples: + * GCC can occasionally decide to realign the stack pointer and change + * the offset of the stack frame in the prologue of a function called + * by head/entry code. Examples: * * : * push %edi @@ -141,11 +145,38 @@ static bool is_last_task_frame(struct unwind_state *state) * push %rbp * mov %rsp,%rbp * - * Note that after aligning the stack, it pushes a duplicate copy of - * the return address before pushing the frame pointer. + * After aligning the stack, it pushes a duplicate copy of the return + * address before pushing the frame pointer. */ - return (state->bp == last_bp || - (state->bp == aligned_bp && *(aligned_bp+1) == *(last_bp+1))); + return (state->bp == aligned_bp && *(aligned_bp + 1) == *(last_bp + 1)); +} + +static bool is_last_ftrace_frame(struct unwind_state *state) +{ + unsigned long *last_bp = last_frame(state); + unsigned long *last_ftrace_bp = last_bp - 3; + + /* + * When unwinding from an ftrace handler of a function called by entry + * code, the stack layout of the last frame is: + * + * bp + * parent ret addr + * bp + * function ret addr + * parent ret addr + * pt_regs + * ----------------- + */ + return (state->bp == last_ftrace_bp && + *state->bp == *(state->bp + 2) && + *(state->bp + 1) == *(state->bp + 4)); +} + +static bool is_last_task_frame(struct unwind_state *state) +{ + return is_last_frame(state) || is_last_aligned_frame(state) || + is_last_ftrace_frame(state); } /*