On Fri, Jun 24, 2016 at 11:30 AM, Josh Poimboeuf [off-list ref] wrote:

On Thu, Jun 23, 2016 at 09:23:06PM -0700, Andy Lutomirski wrote:

quoted

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

s/ebp/rbp/g/ ?

No, this quirk of the x86-64 instruction set will zero-extend to
64-bits without needing a REX prefix.

--
Brian Gerst