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From: Brian Gerst <brgerst@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Julien Thierry <julien.thierry@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	valentin.schneider@arm.com, Will Deacon <will.deacon@arm.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Ingo Molnar <mingo@redhat.com>, James Morse <james.morse@arm.com>,
	Andy Lutomirski <luto@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch
Date: Thu, 14 Feb 2019 11:18:01 -0500	[thread overview]
Message-ID: <CAMzpN2hg+CY+AuF=BtetS2HrMQYdxhBZL9QMvJX4LjAs7vwwAw@mail.gmail.com> (raw)
In-Reply-To: <20190214101429.GD32494@hirez.programming.kicks-ass.net>

On Thu, Feb 14, 2019 at 5:14 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Feb 13, 2019 at 02:49:47PM -0800, Andy Lutomirski wrote:
>
> > Do we need to backport this thing?
>
> Possibly, just to be safe.
>
> > The problem can’t be too widespread or we would have heard of it before.
>
> Yes, so far we've been lucky.
>
> ---
> Subject: sched/x86: Save [ER]FLAGS on context switch
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Thu Feb 14 10:30:52 CET 2019
>
> Effectively revert commit:
>
>   2c7577a75837 ("sched/x86_64: Don't save flags on context switch")
>
> Specifically because SMAP uses FLAGS.AC which invalidates the claim
> that the kernel has clean flags.
>
> In particular; while preemption from interrupt return is fine (the
> IRET frame on the exception stack contains FLAGS) it breaks any code
> that does synchonous scheduling, including preempt_enable().
>
> This has become a significant issue ever since commit:
>
>   5b24a7a2aa20 ("Add 'unsafe' user access functions for batched accesses")
>
> provided for means of having 'normal' C code between STAC / CLAC,
> exposing the FLAGS.AC state. So far this hasn't led to trouble,
> however fix it before it comes apart.
>
> Fixes: 5b24a7a2aa20 ("Add 'unsafe' user access functions for batched accesses")
> Acked-by: Andy Lutomirski <luto@amacapital.net>
> Reported-by: Julien Thierry <julien.thierry@arm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/entry/entry_32.S        |    2 ++
>  arch/x86/entry/entry_64.S        |    2 ++
>  arch/x86/include/asm/switch_to.h |    1 +
>  3 files changed, 5 insertions(+)
>
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -650,6 +650,7 @@ ENTRY(__switch_to_asm)
>         pushl   %ebx
>         pushl   %edi
>         pushl   %esi
> +       pushfl
>
>         /* switch stack */
>         movl    %esp, TASK_threadsp(%eax)
> @@ -672,6 +673,7 @@ ENTRY(__switch_to_asm)
>  #endif
>
>         /* restore callee-saved registers */
> +       popfl
>         popl    %esi
>         popl    %edi
>         popl    %ebx
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -291,6 +291,7 @@ ENTRY(__switch_to_asm)
>         pushq   %r13
>         pushq   %r14
>         pushq   %r15
> +       pushfq
>
>         /* switch stack */
>         movq    %rsp, TASK_threadsp(%rdi)
> @@ -313,6 +314,7 @@ ENTRY(__switch_to_asm)
>  #endif
>
>         /* restore callee-saved registers */
> +       popfq
>         popq    %r15
>         popq    %r14
>         popq    %r13
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -40,6 +40,7 @@ asmlinkage void ret_from_fork(void);
>   * order of the fields must match the code in __switch_to_asm().
>   */
>  struct inactive_task_frame {
> +       unsigned long flags;
>  #ifdef CONFIG_X86_64
>         unsigned long r15;
>         unsigned long r14;

flags should be initialized in copy_thread_tls().  I think the new
stack is zeroed already, but it would be better to explicitly set it.

--
Brian Gerst

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  reply	other threads:[~2019-02-14 16:18 UTC|newest]

Thread overview: 88+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-15 13:58 [PATCH v3 0/4] uaccess: Add unsafe accessors for arm64 Julien Thierry
2019-01-15 13:58 ` [PATCH v3 1/4] arm64: uaccess: Cleanup get/put_user() Julien Thierry
2019-01-15 13:58 ` [PATCH v3 2/4] arm64: uaccess: Implement unsafe accessors Julien Thierry
2019-01-15 13:58 ` [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region Julien Thierry
2019-01-30 16:58   ` Valentin Schneider
2019-02-04 13:27     ` Julien Thierry
2019-02-11 13:45   ` Ingo Molnar
2019-02-11 13:51     ` Peter Zijlstra
2019-02-12  9:15       ` Julien Thierry
2019-02-13  8:21         ` Ingo Molnar
2019-02-13 10:35         ` Peter Zijlstra
2019-02-13 10:50           ` Julien Thierry
2019-02-13 13:17             ` Peter Zijlstra
2019-02-13 13:20               ` Peter Zijlstra
2019-02-13 14:00               ` Will Deacon
2019-02-13 14:07                 ` Julien Thierry
2019-02-13 14:17                 ` Peter Zijlstra
2019-02-13 14:24                   ` Julien Thierry
2019-02-13 14:40                     ` Peter Zijlstra
2019-02-13 15:08                       ` Peter Zijlstra
2019-02-13 14:25                 ` Peter Zijlstra
2019-02-13 14:39                   ` Julien Thierry
2019-02-13 14:41                     ` Peter Zijlstra
2019-02-13 15:45                       ` Peter Zijlstra
2019-02-13 18:54                         ` Peter Zijlstra
     [not found]                         ` <D61C430D-4321-4114-AB85-671A3C7B8EAE@amacapital.net>
2019-02-13 22:21                           ` Peter Zijlstra
2019-02-13 22:49                             ` Andy Lutomirski
2019-02-14 10:14                               ` [PATCH] sched/x86: Save [ER]FLAGS on context switch Peter Zijlstra
2019-02-14 16:18                                 ` Brian Gerst [this message]
2019-02-14 19:34                                   ` Peter Zijlstra
2019-02-15 14:34                                     ` Brian Gerst
2019-02-15 17:18                                     ` Linus Torvalds
2019-02-15 17:40                                       ` Peter Zijlstra
2019-02-15 18:28                                         ` Andy Lutomirski
2019-02-15 23:34                                         ` Peter Zijlstra
2019-02-16  0:21                                           ` Linus Torvalds
2019-02-16 10:32                                             ` Peter Zijlstra
2019-02-16  4:06                                 ` hpa
2019-02-16 10:30                                   ` Peter Zijlstra
2019-02-18 22:30                                     ` H. Peter Anvin
2019-02-19  0:24                                       ` Linus Torvalds
2019-02-19  2:20                                         ` Andy Lutomirski
2019-02-19  2:46                                           ` H. Peter Anvin
2019-02-19  9:07                                             ` Julien Thierry
2019-02-19  8:53                                         ` Julien Thierry
2019-02-19  9:15                                         ` Peter Zijlstra
2019-02-19  9:19                                           ` Peter Zijlstra
2019-02-19  9:04                                       ` Peter Zijlstra
2019-02-19  9:21                                         ` hpa
2019-02-19  9:44                                         ` Peter Zijlstra
2019-02-19 11:38                                           ` Thomas Gleixner
2019-02-19 11:58                                             ` Peter Zijlstra
2019-02-19 12:48                                         ` Will Deacon
2019-02-20 22:55                                           ` H. Peter Anvin
2019-02-21 12:06                                             ` Julien Thierry
2019-02-21 21:35                                               ` Thomas Gleixner
2019-02-21 22:08                                                 ` Linus Torvalds
2019-02-22 12:58                                                   ` Peter Zijlstra
2019-02-22 18:10                                                   ` Thomas Gleixner
2019-02-22 22:26                                                     ` [RFC][PATCH] objtool: STAC/CLAC validation Peter Zijlstra
2019-02-22 23:34                                                       ` Linus Torvalds
2019-02-23  8:43                                                         ` Peter Zijlstra
2019-02-22 23:39                                                       ` hpa
2019-02-23  8:39                                                         ` Peter Zijlstra
2019-02-25  8:47                                                           ` hpa
2019-02-25 13:21                                                             ` Peter Zijlstra
2019-03-01 15:07                                                               ` Peter Zijlstra
2019-02-25  8:49                                                           ` hpa
2019-02-22 23:55                                                       ` Andy Lutomirski
2019-02-23  8:37                                                         ` Peter Zijlstra
2019-02-23 10:52                                                           ` Peter Zijlstra
2019-02-25 10:51                                                         ` Peter Zijlstra
2019-02-25 11:53                                                           ` Peter Zijlstra
2019-02-25 15:36                                                             ` Andy Lutomirski
2019-02-23  0:34                                                       ` Andy Lutomirski
2019-02-23  1:12                                                         ` Linus Torvalds
2019-02-23  1:16                                                           ` Andy Lutomirski
2019-02-23  1:33                                                             ` Linus Torvalds
2019-02-23  1:40                                                             ` Linus Torvalds
2019-02-25  8:33                                                       ` Julien Thierry
2019-02-25 11:55                                                         ` Peter Zijlstra
2019-02-21 12:46                                             ` [PATCH] sched/x86: Save [ER]FLAGS on context switch Will Deacon
2019-02-21 22:06                                               ` Andy Lutomirski
2019-02-18  9:03                                 ` [PATCH v2] " Peter Zijlstra
2019-02-13 23:19                         ` [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region Linus Torvalds
2019-01-15 13:58 ` [PATCH v3 4/4] arm64: uaccess: Implement user_access_region_active Julien Thierry
2019-01-25 14:27 ` [PATCH v3 0/4] uaccess: Add unsafe accessors for arm64 Catalin Marinas
2019-01-30 16:17 ` Julien Thierry

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