linux-kernel - Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32

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Date:	Thu, 23 Apr 2015 12:26:43 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	Andy Lutomirski <luto@...capital.net>,
	Brian Gerst <brgerst@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Will Drewry <wad@...omium.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/vdso] x86/vdso32/syscall.S: Do not load __USER32_DS
 to %ss

On 04/23/2015 12:18 PM, Borislav Petkov wrote:
> On Thu, Apr 23, 2015 at 11:56:21AM +0200, Denys Vlasenko wrote:
>> The fix can look like this (untested):
>>
>>
>> diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
>> index 0c302d0..9f4c232 100644
>> --- a/arch/x86/ia32/ia32entry.S
>> +++ b/arch/x86/ia32/ia32entry.S
>> @@ -198,6 +198,18 @@ sysexit_from_sys_call:
>>  	 * with 'sysenter' and it uses the SYSENTER calling convention.
>>  	 */
>>  	andl    $~TS_COMPAT,ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
>> +	/*
>> +	 * On AMD, SYSRET32 does not modify %ss cached descriptor;
> 
> Ok, but doc says that in both long and compat mode, SYSRET does load
> SS.sel with the value in MSR_STAR...

Yes. It loads *selector*. AMD docs say that selector is loaded as you say,
but *cached descriptor* of SS (which is a different entity) is not modified.

If *cached descriptor* is invalid, in 32-bit mode stack ops
will fail. (In 64-bit mode, CPU doesn't do those checks).

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