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Subject[tip:x86/asm] x86/unwind: Ensure stack pointer is aligned
Commit-ID:  e335bb51cc15e80ac180701a0d335ef1c050828e
Gitweb: http://git.kernel.org/tip/e335bb51cc15e80ac180701a0d335ef1c050828e
Author: Josh Poimboeuf <jpoimboe@redhat.com>
AuthorDate: Mon, 17 Apr 2017 08:44:00 -0500
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 18 Apr 2017 10:30:23 +0200

x86/unwind: Ensure stack pointer is aligned

With frame pointers disabled, on some older versions of GCC (like
4.8.3), it's possible for the stack pointer to get aligned at a
half-word boundary:

00000000000004d0 <fib_table_lookup>:
4d0: 41 57 push %r15
4d2: 41 56 push %r14
4d4: 41 55 push %r13
4d6: 41 54 push %r12
4d8: 55 push %rbp
4d9: 53 push %rbx
4da: 48 83 ec 24 sub $0x24,%rsp

In such a case, the unwinder ends up reading the entire stack at the
wrong alignment. Then the last read goes past the end of the stack,
hitting the stack guard page:

BUG: stack guard page was hit at ffffc900217c4000 (stack is ffffc900217c0000..ffffc900217c3fff)
kernel stack overflow (page fault): 0000 [#1] SMP
...

Fix it by ensuring the stack pointer is properly aligned before
unwinding.

Reported-by: Jirka Hladky <jhladky@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: 7c7900f89770 ("x86/unwind: Add new unwind interface and implementations")
Link: http://lkml.kernel.org/r/cff33847cc9b02fa548625aa23268ac574460d8d.1492436590.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/kernel/dumpstack.c | 2 +-
arch/x86/kernel/unwind_guess.c | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 924f45e..dbce3cc 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -77,7 +77,7 @@ void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
* - softirq stack
* - hardirq stack
*/
- for (regs = NULL; stack; stack = stack_info.next_sp) {
+ for (regs = NULL; stack; stack = PTR_ALIGN(stack_info.next_sp, sizeof(long))) {
const char *stack_name;

/*
diff --git a/arch/x86/kernel/unwind_guess.c b/arch/x86/kernel/unwind_guess.c
index 22881dd..039f367 100644
--- a/arch/x86/kernel/unwind_guess.c
+++ b/arch/x86/kernel/unwind_guess.c
@@ -34,7 +34,7 @@ bool unwind_next_frame(struct unwind_state *state)
return true;
}

- state->sp = info->next_sp;
+ state->sp = PTR_ALIGN(info->next_sp, sizeof(long));

} while (!get_stack_info(state->sp, state->task, info,
&state->stack_mask));
@@ -49,7 +49,7 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
memset(state, 0, sizeof(*state));

state->task = task;
- state->sp = first_frame;
+ state->sp = PTR_ALIGN(first_frame, sizeof(long));

get_stack_info(first_frame, state->task, &state->stack_info,
&state->stack_mask);
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