Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry

[Andy Lutomirski]

On 09/25/2014 12:42 PM, Anish Bhatt 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@xxxxxxxxxxx>
> Signed-off-by: Sebastian Lackner <sebastian@xxxxxxxxxxx>
> ---
>  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);

Something's weird here, and at the very least the changelog is
insufficiently informative.

The Intel SDM says:

If the NT flag is set and the processor is in IA-32e mode, the IRET
instruction causes a general protection exception.

Presumably interrupt delivery clears NT.  I haven't spotted where that's
documented yet.

sysret doesn't appear to care about NT at all.

So: the test code doesn't appear to do anything interesting *unless* it
goes through syscall followed by the iret exit path.  Then it receives
#GP on return, which turns into a signal.

On the premise that the slow and fast return paths ought to be
indistinguishable from userspace, I think we should fix this.  But I
want to understand it better first.

Also, 32-bit may need more care here.

--Andy
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