From 8023e0e2a48d45e8d5363081fad9f7ed4402f953 Mon Sep 17 00:00:00 2001 From: Josh Poimboeuf Date: Fri, 16 Dec 2016 10:05:05 -0600 Subject: [PATCH] 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 : 129: 55 push %rbp 12a: 48 89 e5 mov %rsp,%rbp to: 0000000000000124 : 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: acb4608ad186 ("x86/unwind: Create stack frames for saved syscall registers") Reported-by: Borislav Petkov Signed-off-by: Josh Poimboeuf Cc: Andy Lutomirski Link: http://lkml.kernel.org/r/9d7b4eb8cf55a7d6002cb738f25c23e7429c99a0.1481904011.git.jpoimboe@redhat.com Signed-off-by: Thomas Gleixner --- arch/x86/kernel/unwind_frame.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c index ea7b7f9..33aeaae 100644 --- a/arch/x86/kernel/unwind_frame.c +++ b/arch/x86/kernel/unwind_frame.c @@ -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); } /* -- 2.7.4