On 04/02/2015 09:10 PM, Brian Gerst wrote:
> On Thu, Apr 2, 2015 at 11:20 AM, Denys Vlasenko <dvlas...@redhat.com> wrote:
>> On 04/02/2015 05:01 PM, Brian Gerst wrote:
>>> On Thu, Apr 2, 2015 at 10:36 AM, Denys Vlasenko <dvlas...@redhat.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 <dvlas...@redhat.com>
>>>> CC: Linus Torvalds <torva...@linux-foundation.org>
>>>> CC: Steven Rostedt <rost...@goodmis.org>
>>>> CC: Ingo Molnar <mi...@kernel.org>
>>>> CC: Borislav Petkov <b...@alien8.de>
>>>> CC: "H. Peter Anvin" <h...@zytor.com>
>>>> CC: Andy Lutomirski <l...@amacapital.net>
>>>> CC: Oleg Nesterov <o...@redhat.com>
>>>> CC: Frederic Weisbecker <fweis...@gmail.com>
>>>> CC: Alexei Starovoitov <a...@plumgrid.com>
>>>> CC: Will Drewry <w...@chromium.org>
>>>> CC: Kees Cook <keesc...@chromium.org>
>>>> CC: x...@kernel.org
>>>> CC: linux-kernel@vger.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).
Yeah. I think the "optimize out SAVE_EXTRA_REGS on sigreturn" idea
didn't play out.
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