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Date: Thu, 25 Sep 2014 18:00:05 -0500
From: Chuck Ebbert <cebbert.lkml@gmail.com>
To: Anish Bhatt <anish@chelsio.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de,
        mingo@redhat.com, hpa@zytor.com, sebastian@fds-team.de
Subject: Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry
Message-ID: <20140925180005.356089de@as>
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On Thu, 25 Sep 2014 12:42:51 -0700
Anish Bhatt <anish@chelsio.com> wrote:

> The MSR_SYSCALL_MASK, which is responsible for clearing specific EFLAGS on
>  syscall entry, should also clear the nested task (NT) flag to be safe from
>  userspace injection. Without this fix the application segmentation
>  faults on syscall return because of the changed meaning of the IRET
>  instruction.
> 
> Further details can be seen here https://bugs.winehq.org/show_bug.cgi?id=33275
> 
> Signed-off-by: Anish Bhatt <anish@chelsio.com>
> Signed-off-by: Sebastian Lackner <sebastian@fds-team.de>
> ---
>  arch/x86/kernel/cpu/common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index e4ab2b4..3126558 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1184,7 +1184,7 @@ void syscall_init(void)
>  	/* Flags to clear on syscall */
>  	wrmsrl(MSR_SYSCALL_MASK,
>  	       X86_EFLAGS_TF|X86_EFLAGS_DF|X86_EFLAGS_IF|
> -	       X86_EFLAGS_IOPL|X86_EFLAGS_AC);
> +	       X86_EFLAGS_IOPL|X86_EFLAGS_AC|X86_EFLAGS_NT);
>  }
>  
>  /*

To demonstrate the bug this fixes, just strace this program, adapted
from the linked bug report:

#include <stdio.h>
#include <unistd.h>

int main()
{
	asm volatile("pushfq           \n\t" \
		     "pop %rax         \n\t" \
		     "or $0x4000,%rax  \n\t" \
		     "push %rax        \n\t" \
		     "popfq            \n\t");
	sleep(1);

	return 0;
}

I ran strace under gdb and found it dies here with SIGSEGV:

   0x00007ffff7a505c2 <+50>:	mov    $0xea,%eax
   0x00007ffff7a505c7 <+55>:	syscall 
=> 0x00007ffff7a505c9 <+57>:	cmp    $0xfffffffffffff000,%rax