From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751459AbeFDTZn (ORCPT ); Mon, 4 Jun 2018 15:25:43 -0400 Received: from mga05.intel.com ([192.55.52.43]:40504 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751099AbeFDTYq (ORCPT ); Mon, 4 Jun 2018 15:24:46 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.49,476,1520924400"; d="scan'208";a="64240678" From: "Chang S. Bae" To: Andy Lutomirski , "H . Peter Anvin" , Thomas Gleixner , Ingo Molnar Cc: Andi Kleen , Dave Hansen , Markus T Metzger , "Ravi V . Shankar" , "Chang S . Bae" , LKML Subject: [PATCH 2/6] x86/fsgsbase/64: Make ptrace read FS/GS base accurately Date: Mon, 4 Jun 2018 12:24:25 -0700 Message-Id: <1528140269-26205-3-git-send-email-chang.seok.bae@intel.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1528140269-26205-1-git-send-email-chang.seok.bae@intel.com> References: <1528140269-26205-1-git-send-email-chang.seok.bae@intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Andy Lutomirski ptrace can read FS/GS base using the register access API (PTRACE_PEEKUSER, etc) or PTRACE_ARCH_PRCTL. Make both of these mechanisms return the actual FS/GS base. This will improve debuggability by providing the correct information to ptracer (GDB and etc). Signed-off-by: Andy Lutomirski [chang: Rebase and revise patch description] Signed-off-by: Chang S. Bae Reviewed-by: Andi Kleen Cc: H. Peter Anvin Cc: Dave Hansen Cc: Thomas Gleixner Cc: Ingo Molnar --- arch/x86/kernel/process_64.c | 67 +++++++++++++++++++++++++++++++++----------- 1 file changed, 51 insertions(+), 16 deletions(-) diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index ace0158..e498671 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -279,6 +279,49 @@ static __always_inline void load_seg_legacy(unsigned short prev_index, } } +static unsigned long task_seg_base(struct task_struct *task, + unsigned short selector) +{ + unsigned short idx = selector >> 3; + unsigned long base; + + if (likely((selector & SEGMENT_TI_MASK) == 0)) { + if (unlikely(idx >= GDT_ENTRIES)) + return 0; + + /* + * There are no user segments in the GDT with nonzero bases + * other than the TLS segments. + */ + if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX) + return 0; + + idx -= GDT_ENTRY_TLS_MIN; + base = get_desc_base(&task->thread.tls_array[idx]); + } else { +#ifdef CONFIG_MODIFY_LDT_SYSCALL + struct ldt_struct *ldt; + + /* + * If performance here mattered, we could protect the LDT + * with RCU. This is a slow path, though, so we can just + * take the mutex. + */ + mutex_lock(&task->mm->context.lock); + ldt = task->mm->context.ldt; + if (unlikely(idx >= ldt->nr_entries)) + base = 0; + else + base = get_desc_base(ldt->entries + idx); + mutex_unlock(&task->mm->context.lock); +#else + base = 0; +#endif + } + + return base; +} + void write_fsbase(unsigned long fsbase) { /* set the selector to 0 to not confuse __switch_to */ @@ -297,16 +340,12 @@ unsigned long read_task_fsbase(struct task_struct *task) { unsigned long fsbase; - if (task == current) { + if (task == current) fsbase = read_fsbase(); - } else { - /* - * XXX: This will not behave as expected if called - * if fsindex != 0. This preserves an existing bug - * that will be fixed. - */ + else if (task->thread.fsindex == 0) fsbase = task->thread.fsbase; - } + else + fsbase = task_seg_base(task, task->thread.fsindex); return fsbase; } @@ -315,16 +354,12 @@ unsigned long read_task_gsbase(struct task_struct *task) { unsigned long gsbase; - if (task == current) { + if (task == current) gsbase = read_inactive_gsbase(); - } else { - /* - * XXX: This will not behave as expected if called - * if gsindex != 0. Same bug preservation as above - * read_task_fsbase. - */ + else if (task->thread.gsindex == 0) gsbase = task->thread.gsbase; - } + else + gsbase = task_seg_base(task, task->thread.gsindex); return gsbase; } -- 2.7.4