Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031135AbdDZVwG (ORCPT ); Wed, 26 Apr 2017 17:52:06 -0400 Received: from mga03.intel.com ([134.134.136.65]:47421 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1031114AbdDZVv6 (ORCPT ); Wed, 26 Apr 2017 17:51:58 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.37,256,1488873600"; d="scan'208";a="79334070" Message-ID: <1493243515.36058.63.camel@ranerica-desktop> Subject: Re: [v6 PATCH 07/21] x86/insn-eval: Add utility function to get segment descriptor From: Ricardo Neri To: Borislav Petkov Cc: Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , Andy Lutomirski , Peter Zijlstra , Andrew Morton , Brian Gerst , Chris Metcalf , Dave Hansen , Paolo Bonzini , Masami Hiramatsu , Huang Rui , Jiri Slaby , Jonathan Corbet , "Michael S. Tsirkin" , Paul Gortmaker , Vlastimil Babka , Chen Yucong , Alexandre Julliard , Stas Sergeev , Fenghua Yu , "Ravi V. Shankar" , Shuah Khan , linux-kernel@vger.kernel.org, x86@kernel.org, linux-msdos@vger.kernel.org, wine-devel@winehq.org, Adam Buchbinder , Colin Ian King , Lorenzo Stoakes , Qiaowei Ren , Arnaldo Carvalho de Melo , Adrian Hunter , Kees Cook , Thomas Garnier , Dmitry Vyukov Date: Wed, 26 Apr 2017 14:51:56 -0700 In-Reply-To: <20170419102636.chx5fwofysehy3yx@pd.tnic> References: <20170308003254.27833-1-ricardo.neri-calderon@linux.intel.com> <20170308003254.27833-8-ricardo.neri-calderon@linux.intel.com> <20170419102636.chx5fwofysehy3yx@pd.tnic> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.10.4-0ubuntu2 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5422 Lines: 166 On Wed, 2017-04-19 at 12:26 +0200, Borislav Petkov wrote: > On Tue, Mar 07, 2017 at 04:32:40PM -0800, Ricardo Neri wrote: > > The segment descriptor contains information that is relevant to how linear > > address need to be computed. It contains the default size of addresses as > > well as the base address of the segment. Thus, given a segment selector, > > we ought look at segment descriptor to correctly calculate the linear > > address. > > > > In protected mode, the segment selector might indicate a segment > > descriptor from either the global descriptor table or a local descriptor > > table. Both cases are considered in this function. > > > > This function is the initial implementation for subsequent functions that > > will obtain the aforementioned attributes of the segment descriptor. > > > > Cc: Dave Hansen > > Cc: Adam Buchbinder > > Cc: Colin Ian King > > Cc: Lorenzo Stoakes > > Cc: Qiaowei Ren > > Cc: Arnaldo Carvalho de Melo > > Cc: Masami Hiramatsu > > Cc: Adrian Hunter > > Cc: Kees Cook > > Cc: Thomas Garnier > > Cc: Peter Zijlstra > > Cc: Borislav Petkov > > Cc: Dmitry Vyukov > > Cc: Ravi V. Shankar > > Cc: x86@kernel.org > > Signed-off-by: Ricardo Neri > > --- > > arch/x86/lib/insn-eval.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 61 insertions(+) > > > > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c > > index 8d45df8..8608adf 100644 > > --- a/arch/x86/lib/insn-eval.c > > +++ b/arch/x86/lib/insn-eval.c > > @@ -5,9 +5,13 @@ > > */ > > #include > > #include > > +#include > > +#include > > #include > > #include > > #include > > +#include > > +#include > > #include > > > > enum reg_type { > > @@ -294,6 +298,63 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs, > > } > > > > /** > > + * get_desc() - Obtain address of segment descriptor > > + * @seg: Segment selector > > Maybe that should be > > @sel > > if it is a sel-ector. :) It makes sense. I will rename it. > > And using "sel" makes more sense then when you look at: > > desc_base = sel & ~(SEGMENT_RPL_MASK | SEGMENT_TI_MASK); > > for example: > > > + * @desc: Pointer to the selected segment descriptor > > + * > > + * Given a segment selector, obtain a memory pointer to the segment > > s/memory // Will update it. > > > + * descriptor. Both global and local descriptor tables are supported. > > + * desc will contain the address of the descriptor. > > + * > > + * Return: 0 if success, -EINVAL if failure > > Why isn't this function returning the pointer or NULL on error? Maybe > the later patches have an answer and I'll discover it if I continue > reviewing :) After revisiting the code, I don't see why the function cannot return NULL. > > > + */ > > +static int get_desc(unsigned short seg, struct desc_struct **desc) > > +{ > > + struct desc_ptr gdt_desc = {0, 0}; > > + unsigned long desc_base; > > + > > + if (!desc) > > + return -EINVAL; > > + > > + desc_base = seg & ~(SEGMENT_RPL_MASK | SEGMENT_TI_MASK); > > That looks useless as you're doing it below again. Yes, it is useless. Please see my comment below. > > > + > > +#ifdef CONFIG_MODIFY_LDT_SYSCALL > > + if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) { > > + seg >>= 3; > > + > > + mutex_lock(¤t->active_mm->context.lock); > > + if (unlikely(!current->active_mm->context.ldt || > > Is that really a fast path to complicate the if-test with an unlikely()? > If not, you don't really need it. I will remove it. > > > + seg >= current->active_mm->context.ldt->size)) { > > ldt->size is the size of the descriptor table but you've shifted seg by > 3. That selector index is shifted by 3 (to the left) to form an offset > into the descriptor table because the entries there are 8 bytes. I double-checked the ldt code and it seems to me that size refers to the number of entries in the table; it is always multiplied by LDT_ENTRY_SIZE [1], [2]. Am I missing something? > > So I *think* you wanna use the "useless" desc_base above... :) > > > + *desc = NULL; > > + mutex_unlock(¤t->active_mm->context.lock); > > + return -EINVAL; > > + } > > + > > + *desc = ¤t->active_mm->context.ldt->entries[seg]; > > ... and seg here as it is an index into the table. > > > + mutex_unlock(¤t->active_mm->context.lock); > > + return 0; > > + } > > +#endif > > + native_store_gdt(&gdt_desc); > > + > > + /* > > + * Bits [15:3] of the segment selector contain the index. Such > > + * index needs to be multiplied by 8. > > ... because . I will elaborate on the reason for this. Thanks and BR, Ricardo [1]. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/ldt.c?id=refs/tags/v4.11-rc8#n260 [2]. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/ldt.c?id=refs/tags/v4.11-rc8#n50