Re: [PATCH 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

[Chuck Ebbert <cebbert.lkml@gmail.com>]

On Tue, 30 Sep 2014 12:40:35 -0700
Andy Lutomirski <luto@amacapital.net> wrote:

> The NT flag doesn't do anything in long mode other than causing IRET
> to #GP.  Oddly, CPL3 code can still net NT using popf.
> 
> Entry via hardware or software interrupt clears NT automatically, so
> the only relevant entries are fast syscalls.
> 
> This patch programs the CPU to clear NT on entry via SYSCALL (both
> 32-bit and 64-bit, by my reading of the AMD APM).  It also clears NT
> (and some other flags) in software on SYSENTER.
> 
> I haven't touched anything on 32-bit kernels.
> 
> If user code causes kernel code to run with NT set, then there's at
> least some (small) chance that it could cause trouble.  For example,
> user code could cause a call to EFI code with NT set, and who knows
> what would happen.  Apparently Wine sometimes does this (!), and, if
> an IRET return happens, Wine will segfault.
> 
> I think that Wine should be fixed to stop setting NT when a syscall
> happens, but handling NT more gracefully is still nice.
> 
> The syscall mask change comes from a variant of this patch by Anish
> Bhatt.
> 
> Cc: stable@kernel.org
> Reported-by: Anish Bhatt <anish@chelsio.com>
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
>  arch/x86/ia32/ia32entry.S    | 10 +++++++++-
>  arch/x86/kernel/cpu/common.c |  2 +-
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
> index 4299eb05023c..079f42a7ad58 100644
> --- a/arch/x86/ia32/ia32entry.S
> +++ b/arch/x86/ia32/ia32entry.S
> @@ -143,7 +143,15 @@ ENTRY(ia32_sysenter_target)
>  	pushq_cfi %r10
>  	CFI_REL_OFFSET rip,0
>  	pushq_cfi %rax
> -	cld
> +
> +	/*
> +	 * Sysenter doesn't filter flags, so we should filter them
> +	 * ourselves.
> +	 */
> +	pushfq_cfi
> +	andl $~(X86_EFLAGS_TF|X86_EFLAGS_DF|X86_EFLAGS_NT|X86_EFLAGS_IOPL),(%rsp)
> +	popfq_cfi

Can't you just push a constant and pop that onto flags instead? It's
not like we care what's in there on entry to the kernel.

> +
>  	SAVE_ARGS 0,1,0
>   	/* no need to do an access_ok check here because rbp has been
>   	   32bit zero extended */ 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index e4ab2b42bd6f..31265580c38a 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);
>  }
>  
>  /*