x86/unwind: Adjust last frame check for aligned function stacks
authorJosh Poimboeuf <jpoimboe@redhat.com>
Fri, 16 Dec 2016 16:05:05 +0000 (10:05 -0600)
committerThomas Gleixner <tglx@linutronix.de>
Mon, 19 Dec 2016 10:47:05 +0000 (11:47 +0100)
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>
arch/x86/kernel/unwind_frame.c

index ea7b7f9a3b9e29c891509429f55238e17dcdc66c..33aeaae961aa3c5eab92f56701e312841eeb48df 100644 (file)
@@ -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);
 }
 
 /*