linux-kernel - Re: [PATCH 3/9] x86/asm/entry/64: do not SAVE_EXTRA_REGS in stub

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Date:	Thu, 2 Apr 2015 15:10:31 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/9] x86/asm/entry/64: do not SAVE_EXTRA_REGS in stub_sigreturn

On Thu, Apr 2, 2015 at 11:20 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> On 04/02/2015 05:01 PM, Brian Gerst wrote:
>> On Thu, Apr 2, 2015 at 10:36 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>>> stub_sigreturn ignores old values of pt_regs->REG for all general-purpose
>>> registers, it sets them to values saved on userspace
>>> signal stack.
>>>
>>> Which is hardly surprising - it would be a bug if it would use pt_regs->REG.
>>> sigreturn must restore all registers.
>>>
>>> Therefore, SAVE_EXTRA_REGS in it ought to be redundant.
>>>
>>> It is a leftover from the time SAVE_EXTRA_REGS wasn't only saving registers,
>>> but it also was extending stack to "full" pt_regs.
>>>
>>> Delete this SAVE_EXTRA_REGS.
>>>
>>> Run-tested.
>>>
>>> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
>>> CC: Linus Torvalds <torvalds@...ux-foundation.org>
>>> CC: Steven Rostedt <rostedt@...dmis.org>
>>> CC: Ingo Molnar <mingo@...nel.org>
>>> CC: Borislav Petkov <bp@...en8.de>
>>> CC: "H. Peter Anvin" <hpa@...or.com>
>>> CC: Andy Lutomirski <luto@...capital.net>
>>> CC: Oleg Nesterov <oleg@...hat.com>
>>> CC: Frederic Weisbecker <fweisbec@...il.com>
>>> CC: Alexei Starovoitov <ast@...mgrid.com>
>>> CC: Will Drewry <wad@...omium.org>
>>> CC: Kees Cook <keescook@...omium.org>
>>> CC: x86@...nel.org
>>> CC: linux-kernel@...r.kernel.org
>>> ---
>>>  arch/x86/kernel/entry_64.S | 9 +++++++--
>>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
>>> index ec51598..1cf245d 100644
>>> --- a/arch/x86/kernel/entry_64.S
>>> +++ b/arch/x86/kernel/entry_64.S
>>> @@ -447,7 +447,12 @@ ENTRY(stub_rt_sigreturn)
>>>         CFI_STARTPROC
>>>         addq $8, %rsp
>>>         DEFAULT_FRAME 0
>>> -       SAVE_EXTRA_REGS
>>> +       /*
>>> +        * Despite RESTORE_EXTRA_REGS in return_from_stub,
>>> +        * no need to SAVE_EXTRA_REGS here:
>>> +        * sys_rt_sigreturn overwrites all general purpose pt_regs->REGs
>>> +        * on stack, for RESTORE_{EXTRA,C}_REGS to pick them up.
>>> +        */
>>>         call sys_rt_sigreturn
>>>         jmp  return_from_stub
>>>         CFI_ENDPROC
>>> @@ -458,7 +463,7 @@ ENTRY(stub_x32_rt_sigreturn)
>>>         CFI_STARTPROC
>>>         addq $8, %rsp
>>>         DEFAULT_FRAME 0
>>> -       SAVE_EXTRA_REGS
>>> +       /* No need to SAVE_EXTRA_REGS */
>>>         call sys32_x32_rt_sigreturn
>>>         jmp  return_from_stub
>>>         CFI_ENDPROC
>>
>> I had the same idea, but determined sigreturn can fault and return an
>> error code without modifying all the registers.  This would leak junk
>> from the stack.

To clarify, I remembered looking at sigreturn possibly faulting from
the 32-bit perspective, where the 6th arg is read from the user stack
and a fault there would return -EFAULT, for any syscall.

> This still can be made to work by not RESTORE'ing EXTRA_REGS either,
> if there is a way to detect the failure:
>
>         call sys_rt_sigreturn
> -       jmp  return_from_stub
> +       testl ???????????
> +       jz   return_from_stub
> +       ret
>         CFI_ENDPROC
>
> But this is not a normal syscall, off-hand I don't see an easy way
> to do the test. sys_rt_sigreturn() on failure runs this code:
>
> ...
>  segfault:
>         force_sig(SIGSEGV, current);
>         return 0;
> }
>
> Help?

I don't think you can test the return value, because in the success
case it can be any value (the restored RAX value).

--
Brian Gerst
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