On 29.09.2014 19:40, Andy Lutomirski wrote: > 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.
Well, the best documentation I've found is something like http://www.fermimn.gov.it/linux/quarta/x86/int.htm which states: --- snip --- INTERRUPT-TO-INNER-PRIVILEGE: [...] TF := 0; NT := 0; --- snip --- (Doesn't say anything about HW interrupts though) This also makes sense at my opinion, since the interrupt handler has to know if it should return to the previous task (when NT=1) or to the same task (when NT=0). > > 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. Yep, thats also my interpretation of this issue. If the processor would be in 32-bit/protected-mode the NT flag would be interpreted as a task return, and it would probably cause a different exception, because the kernel never uses the task link property of the TSS. > > 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. A reliable way to force the slow return path is to use ptrace, see: http://lxr.free-electrons.com/source/arch/x86/kernel/entry_64.S#L544 This also matches the experience: The test application only crashes with a small probability, except you use strace, then it will always crash (because the kernel forces the slow return path). Two additional remarks: * A reliable way to let it crash without strace, is to run the fork()/clone() syscall afterwards and compile as 32-bit. * When you run exec*() afterwards, the crash will happen at the entry of the new executable. Doesn't matter if the target process is SUID or not. I don't see a way to exploit this issue, though, but probably some more people should take a look at it... > > Also, 32-bit may need more care here. That might be possible. It probably makes sense to review other parts of the code, for similar issues. > > --Andy > Regards, Sebastian -- 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/