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).
--
Brian Gerst
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