Messages in this thread | | | Date | Mon, 29 Sep 2014 10:40:09 -0700 | From | Andy Lutomirski <> | Subject | Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry |
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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@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);
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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