Commit 8023e0e2 authored Dec 16, 2016 by Josh Poimboeuf Committed by Thomas Gleixner Dec 19, 2016

x86/unwind: Adjust last frame check for aligned function stacks



Somehow, CONFIG_PARAVIRT=n convinces gcc to change the
x86_64_start_kernel() prologue from:

  0000000000000129 <x86_64_start_kernel>:
   129:	55                   	push   %rbp
   12a:	48 89 e5             	mov    %rsp,%rbp

to:

  0000000000000124 <x86_64_start_kernel>:
   124:	4c 8d 54 24 08       	lea    0x8(%rsp),%r10
   129:	48 83 e4 f0          	and    $0xfffffffffffffff0,%rsp
   12d:	41 ff 72 f8          	pushq  -0x8(%r10)
   131:	55                   	push   %rbp
   132:	48 89 e5             	mov    %rsp,%rbp

This is an unusual pattern which aligns rsp (though in this case it's
already aligned) and saves the start_cpu() return address again on the
stack before storing the frame pointer.

The unwinder assumes the last stack frame header is at a certain offset,
but the above code breaks that assumption, resulting in the following
warning:

  WARNING: kernel stack frame pointer at ffffffff82e03f40 in swapper:0 has bad value           (null)

Fix it by checking for the last task stack frame at the aligned offset
in addition to the normal unaligned offset.

Fixes: acb4608a ("x86/unwind: Create stack frames for saved syscall registers")
Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/9d7b4eb8cf55a7d6002cb738f25c23e7429c99a0.1481904011.git.jpoimboe@redhat.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent 22d3c0d6

arch/x86/kernel/unwind_frame.c

+8 −1

Original line number	Diff line number	Diff line
		@@ -46,7 +46,14 @@ static bool is_last_task_frame(struct unwind_state *state)
		unsigned long bp = (unsigned long)state->bp;
		unsigned long regs = (unsigned long)task_pt_regs(state->task);

		return bp == regs - FRAME_HEADER_SIZE;
		/*
		* 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 by a word in the prologue of a
		* function called by head/entry code.
		*/
		return bp == regs - FRAME_HEADER_SIZE \|\|
		bp == regs - FRAME_HEADER_SIZE - sizeof(long);
		}

		/*