linux-kernel - Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32

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Message-ID: <CAMzpN2iFh75kRUUd5cpD0xoab=8g_WW-UK6ghZrse6KULjagcg@mail.gmail.com>
Date:	Tue, 24 Mar 2015 20:59:14 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, 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>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: vdso32/syscall.S: do not load __USER32_DS to %ss

On Tue, Mar 24, 2015 at 4:17 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> On 03/24/2015 05:55 PM, Brian Gerst wrote:
>>>> Might be nice to place a more generic description there, which
>>>> registers are expected to be saved by user-space calling in here, etc.
>>>
>>> __kernel_vsyscall entry point has the same ABI in any 32-bit vDSO,
>>> the good old int 0x80 calling convention:
>>>
>>> syscall# in eax,
>>> params in ebx/ecx/edx/esi/edi/ebp,
>>> all registers are preserved by the syscall.
>>>
>>> (I think we don't guarantee that all flags are preserved:
>>> I have a testcase where DF gets cleared).
>>
>> DF should always be clear on any function call per the C ABI.  But,
>> eflags should be preserved, at least the non-privileged bits.  I'd
>> like to see that testcase.
>
> The testcase is a simplistic example of how to find and use
> 32-bit vDSO to perform system calls.
>
> It also sets flags.DF before syscall, and checks whether registers
> are preserved, including flags.DF.
>
> On 32-bit kernel (on Intel CPU, where vDSO uses SYSENTER), I see this:
>
> $ ./test32_syscall_vdso
> Result:1
>
> whereas on 64-bit it is
>
> ./test32_syscall_vdso
> Result:0
>
> "Result:1" means that DF was cleared.
>
> See attached source.
>

It looks like 32-bit native does not restore eflags before sysexit,
while 64-bit compat does.

--
Brian Gerst
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