releases/4.19.45/sched-x86-save-flags-on-context-switch.patch

   1 From 6690e86be83ac75832e461c141055b5d601c0a6d Mon Sep 17 00:00:00 2001
   2 From: Peter Zijlstra <peterz@infradead.org>
   3 Date: Thu, 14 Feb 2019 10:30:52 +0100
   4 Subject: sched/x86: Save [ER]FLAGS on context switch
   5
   6 From: Peter Zijlstra <peterz@infradead.org>
   7
   8 commit 6690e86be83ac75832e461c141055b5d601c0a6d upstream.
   9
  10 Effectively reverts commit:
  11
  12   2c7577a75837 ("sched/x86_64: Don't save flags on context switch")
  13
  14 Specifically because SMAP uses FLAGS.AC which invalidates the claim
  15 that the kernel has clean flags.
  16
  17 In particular; while preemption from interrupt return is fine (the
  18 IRET frame on the exception stack contains FLAGS) it breaks any code
  19 that does synchonous scheduling, including preempt_enable().
  20
  21 This has become a significant issue ever since commit:
  22
  23   5b24a7a2aa20 ("Add 'unsafe' user access functions for batched accesses")
  24
  25 provided for means of having 'normal' C code between STAC / CLAC,
  26 exposing the FLAGS.AC state. So far this hasn't led to trouble,
  27 however fix it before it comes apart.
  28
  29 Reported-by: Julien Thierry <julien.thierry@arm.com>
  30 Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
  31 Acked-by: Andy Lutomirski <luto@amacapital.net>
  32 Cc: Borislav Petkov <bp@alien8.de>
  33 Cc: Josh Poimboeuf <jpoimboe@redhat.com>
  34 Cc: Linus Torvalds <torvalds@linux-foundation.org>
  35 Cc: Peter Zijlstra <peterz@infradead.org>
  36 Cc: Thomas Gleixner <tglx@linutronix.de>
  37 Cc: stable@kernel.org
  38 Fixes: 5b24a7a2aa20 ("Add 'unsafe' user access functions for batched accesses")
  39 Signed-off-by: Ingo Molnar <mingo@kernel.org>
  40 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  41
  42 ---
  43  arch/x86/entry/entry_32.S        |    2 ++
  44  arch/x86/entry/entry_64.S        |    2 ++
  45  arch/x86/include/asm/switch_to.h |    1 +
  46  arch/x86/kernel/process_32.c     |    7 +++++++
  47  arch/x86/kernel/process_64.c     |    8 ++++++++
  48  5 files changed, 20 insertions(+)
  49
  50 --- a/arch/x86/entry/entry_32.S
  51 +++ b/arch/x86/entry/entry_32.S
  52 @@ -648,6 +648,7 @@ ENTRY(__switch_to_asm)
  53         pushl   %ebx
  54         pushl   %edi
  55         pushl   %esi
  56 +       pushfl
  57
  58         /* switch stack */
  59         movl    %esp, TASK_threadsp(%eax)
  60 @@ -670,6 +671,7 @@ ENTRY(__switch_to_asm)
  61  #endif
  62
  63         /* restore callee-saved registers */
  64 +       popfl
  65         popl    %esi
  66         popl    %edi
  67         popl    %ebx
  68 --- a/arch/x86/entry/entry_64.S
  69 +++ b/arch/x86/entry/entry_64.S
  70 @@ -352,6 +352,7 @@ ENTRY(__switch_to_asm)
  71         pushq   %r13
  72         pushq   %r14
  73         pushq   %r15
  74 +       pushfq
  75
  76         /* switch stack */
  77         movq    %rsp, TASK_threadsp(%rdi)
  78 @@ -374,6 +375,7 @@ ENTRY(__switch_to_asm)
  79  #endif
  80
  81         /* restore callee-saved registers */
  82 +       popfq
  83         popq    %r15
  84         popq    %r14
  85         popq    %r13
  86 --- a/arch/x86/include/asm/switch_to.h
  87 +++ b/arch/x86/include/asm/switch_to.h
  88 @@ -40,6 +40,7 @@ asmlinkage void ret_from_fork(void);
  89   * order of the fields must match the code in __switch_to_asm().
  90   */
  91  struct inactive_task_frame {
  92 +       unsigned long flags;
  93  #ifdef CONFIG_X86_64
  94         unsigned long r15;
  95         unsigned long r14;
  96 --- a/arch/x86/kernel/process_32.c
  97 +++ b/arch/x86/kernel/process_32.c
  98 @@ -130,6 +130,13 @@ int copy_thread_tls(unsigned long clone_
  99         struct task_struct *tsk;
 100         int err;
 101
 102 +       /*
 103 +        * For a new task use the RESET flags value since there is no before.
 104 +        * All the status flags are zero; DF and all the system flags must also
 105 +        * be 0, specifically IF must be 0 because we context switch to the new
 106 +        * task with interrupts disabled.
 107 +        */
 108 +       frame->flags = X86_EFLAGS_FIXED;
 109         frame->bp = 0;
 110         frame->ret_addr = (unsigned long) ret_from_fork;
 111         p->thread.sp = (unsigned long) fork_frame;
 112 --- a/arch/x86/kernel/process_64.c
 113 +++ b/arch/x86/kernel/process_64.c
 114 @@ -300,6 +300,14 @@ int copy_thread_tls(unsigned long clone_
 115         childregs = task_pt_regs(p);
 116         fork_frame = container_of(childregs, struct fork_frame, regs);
 117         frame = &fork_frame->frame;
 118 +
 119 +       /*
 120 +        * For a new task use the RESET flags value since there is no before.
 121 +        * All the status flags are zero; DF and all the system flags must also
 122 +        * be 0, specifically IF must be 0 because we context switch to the new
 123 +        * task with interrupts disabled.
 124 +        */
 125 +       frame->flags = X86_EFLAGS_FIXED;
 126         frame->bp = 0;
 127         frame->ret_addr = (unsigned long) ret_from_fork;
 128         p->thread.sp = (unsigned long) fork_frame;