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

[Sebastian Lackner]

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 <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.

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

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