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: acb4608ad186 ("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>

diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
index ea7b7f9a3b9e29c891509429f55238e17dcdc66c..33aeaae961aa3c5eab92f56701e312841eeb48df 100644 (file)
--- 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);
 }
 
 /*