From 900742d89c1b4e04bd373aec8470b88e183f08ca Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Mon, 9 Jan 2017 12:00:22 -0600 Subject: x86/unwind: Silence warnings for non-current tasks There are a handful of callers to save_stack_trace_tsk() and show_stack() which try to unwind the stack of a task other than current. In such cases, it's remotely possible that the task is running on one CPU while the unwinder is reading its stack from another CPU, causing the unwinder to see stack corruption. These cases seem to be mostly harmless. The unwinder has checks which prevent it from following bad pointers beyond the bounds of the stack. So it's not really a bug as long as the caller understands that unwinding another task will not always succeed. Since stack "corruption" on another task's stack isn't necessarily a bug, silence the warnings when unwinding tasks other than current. Reported-by: Dave Jones Signed-off-by: Josh Poimboeuf Cc: Andy Lutomirski Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: Dmitry Vyukov Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Miroslav Benes Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/00d8c50eea3446c1524a2a755397a3966629354c.1483978430.git.jpoimboe@redhat.com Signed-off-by: Ingo Molnar --- arch/x86/kernel/unwind_frame.c | 10 ++++++++++ 1 file changed, 10 insertions(+) (limited to 'arch/x86/kernel/unwind_frame.c') diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c index 4443e499f279..195eebf6da20 100644 --- a/arch/x86/kernel/unwind_frame.c +++ b/arch/x86/kernel/unwind_frame.c @@ -207,6 +207,16 @@ bool unwind_next_frame(struct unwind_state *state) return true; bad_address: + /* + * When unwinding a non-current task, the task might actually be + * running on another CPU, in which case it could be modifying its + * stack while we're reading it. This is generally not a problem and + * can be ignored as long as the caller understands that unwinding + * another task will not always succeed. + */ + if (state->task != current) + goto the_end; + if (state->regs) { printk_deferred_once(KERN_WARNING "WARNING: kernel stack regs at %p in %s:%d has bad 'bp' value %p\n", -- cgit v1.2.3 From 84936118bdf37bda513d4a361c38181a216427e0 Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Mon, 9 Jan 2017 12:00:23 -0600 Subject: x86/unwind: Disable KASAN checks for non-current tasks There are a handful of callers to save_stack_trace_tsk() and show_stack() which try to unwind the stack of a task other than current. In such cases, it's remotely possible that the task is running on one CPU while the unwinder is reading its stack from another CPU, causing the unwinder to see stack corruption. These cases seem to be mostly harmless. The unwinder has checks which prevent it from following bad pointers beyond the bounds of the stack. So it's not really a bug as long as the caller understands that unwinding another task will not always succeed. In such cases, it's possible that the unwinder may read a KASAN-poisoned region of the stack. Account for that by using READ_ONCE_NOCHECK() when reading the stack of another task. Use READ_ONCE() when reading the stack of the current task, since KASAN warnings can still be useful for finding bugs in that case. Reported-by: Dmitry Vyukov Signed-off-by: Josh Poimboeuf Cc: Andy Lutomirski Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Dave Jones Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Miroslav Benes Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/4c575eb288ba9f73d498dfe0acde2f58674598f1.1483978430.git.jpoimboe@redhat.com Signed-off-by: Ingo Molnar --- arch/x86/kernel/unwind_frame.c | 20 ++++++++++++++++++-- 1 file changed, 18 insertions(+), 2 deletions(-) (limited to 'arch/x86/kernel/unwind_frame.c') diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c index 195eebf6da20..23d15565d02a 100644 --- a/arch/x86/kernel/unwind_frame.c +++ b/arch/x86/kernel/unwind_frame.c @@ -6,6 +6,21 @@ #define FRAME_HEADER_SIZE (sizeof(long) * 2) +/* + * This disables KASAN checking when reading a value from another task's stack, + * since the other task could be running on another CPU and could have poisoned + * the stack in the meantime. + */ +#define READ_ONCE_TASK_STACK(task, x) \ +({ \ + unsigned long val; \ + if (task == current) \ + val = READ_ONCE(x); \ + else \ + val = READ_ONCE_NOCHECK(x); \ + val; \ +}) + static void unwind_dump(struct unwind_state *state, unsigned long *sp) { static bool dumped_before = false; @@ -48,7 +63,8 @@ unsigned long unwind_get_return_address(struct unwind_state *state) if (state->regs && user_mode(state->regs)) return 0; - addr = ftrace_graph_ret_addr(state->task, &state->graph_idx, *addr_p, + addr = READ_ONCE_TASK_STACK(state->task, *addr_p); + addr = ftrace_graph_ret_addr(state->task, &state->graph_idx, addr, addr_p); return __kernel_text_address(addr) ? addr : 0; @@ -162,7 +178,7 @@ bool unwind_next_frame(struct unwind_state *state) if (state->regs) next_bp = (unsigned long *)state->regs->bp; else - next_bp = (unsigned long *)*state->bp; + next_bp = (unsigned long *)READ_ONCE_TASK_STACK(state->task,*state->bp); /* is the next frame pointer an encoded pointer to pt_regs? */ regs = decode_frame_pointer(next_bp); -- cgit v1.2.3