[PATCH v3 08/13] x86/dumpstack: When OOPSing, rewind the stack before do_exit

Andy Lutomirski
2016-06-20T23:45:15Z

If we call do_exit with a clean stack, we greatly reduce the risk of
recursive oopses due to stack overflow in do_exit, and we allow
do_exit to work even if we OOPS from an IST stack. The latter gives
us a much better chance of surviving long enough after we detect a
stack overflow to write out our logs.

I intentionally separated this from the preceding patch that
disables do_exit-on-OOPS on IST stacks. This way, if we need to
revert this patch, we still end up in an acceptable state wrt stack
overflow handling.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/entry/entry_32.S | 11 +++++++++++
arch/x86/entry/entry_64.S | 11 +++++++++++
arch/x86/kernel/dumpstack.c | 13 +++++++++----
3 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 983e5d3a0d27..0b56666e6039 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -1153,3 +1153,14 @@ ENTRY(async_page_fault)
jmp error_code
END(async_page_fault)
#endif
+
+ENTRY(rewind_stack_do_exit)
+ /* Prevent any naive code from trying to unwind to our caller. */
+ xorl %ebp, %ebp
+
+ movl PER_CPU_VAR(cpu_current_top_of_stack), %esi
+ leal -TOP_OF_KERNEL_STACK_PADDING-PTREGS_SIZE(%esi), %esp
+
+ call do_exit
+1: jmp 1b
+END(rewind_stack_do_exit)
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 9ee0da1807ed..b846875aeea6 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1423,3 +1423,14 @@ ENTRY(ignore_sysret)
mov $-ENOSYS, %eax
sysret
END(ignore_sysret)
+
+ENTRY(rewind_stack_do_exit)
+ /* Prevent any naive code from trying to unwind to our caller. */
+ xorl %ebp, %ebp
+
+ movq PER_CPU_VAR(cpu_current_top_of_stack), %rax
+ leaq -TOP_OF_KERNEL_STACK_PADDING-PTREGS_SIZE(%rax), %rsp
+
+ call do_exit
+1: jmp 1b
+END(rewind_stack_do_exit)
diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 36effb39c9c9..d4d085e27d04 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -228,6 +228,8 @@ unsigned long oops_begin(void)
EXPORT_SYMBOL_GPL(oops_begin);
NOKPROBE_SYMBOL(oops_begin);

+extern void __noreturn rewind_stack_do_exit(int signr);
+
void oops_end(unsigned long flags, struct pt_regs *regs, int signr)
{
if (regs && kexec_should_crash(current))
@@ -247,12 +249,15 @@ void oops_end(unsigned long flags, struct pt_regs *regs, int signr)
return;
if (in_interrupt())
panic("Fatal exception in interrupt");
- if (((current_stack_pointer() ^ (current_top_of_stack() - 1))
- & ~(THREAD_SIZE - 1)) != 0)
- panic("Fatal exception on special stack");
if (panic_on_oops)
panic("Fatal exception");
- do_exit(signr);
+
+ /*
+ * We're not going to return, but we might be on an IST stack or
+ * have very little stack space left. Rewind the stack and kill
+ * the task.
+ */
+ rewind_stack_do_exit(signr);
}
NOKPROBE_SYMBOL(oops_end);

--
2.5.5