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

From: Andy Lutomirski
Date: Mon Sep 29 2014 - 13:40:19 EST

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
> 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 */

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.

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