[tip:x86/urgent] x86/unwind: Adjust last frame check for aligned function stacks

[tip-bot for Josh Poimboeuf]

Commit-ID:  8023e0e2a48d45e8d5363081fad9f7ed4402f953
Gitweb:     http://git.kernel.org/tip/8023e0e2a48d45e8d5363081fad9f7ed4402f953
Author:     Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate: Fri, 16 Dec 2016 10:05:05 -0600
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Mon, 19 Dec 2016 11:47:05 +0100

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@xxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/9d7b4eb8cf55a7d6002cb738f25c23e7429c99a0.1481904011.git.jpoimboe@xxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 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);
 }
 
 /*