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 <an...@chelsio.com> > Signed-off-by: Sebastian Lackner <sebast...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
