From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S971856AbeCSU6g (ORCPT ); Mon, 19 Mar 2018 16:58:36 -0400 Received: from mga14.intel.com ([192.55.52.115]:62510 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965554AbeCSSIu (ORCPT ); Mon, 19 Mar 2018 14:08:50 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.48,331,1517904000"; d="scan'208";a="39330563" From: "Chang S. Bae" To: x86@kernel.org Cc: luto@kernel.org, ak@linux.intel.com, hpa@zytor.com, markus.t.metzger@intel.com, tony.luck@intel.com, ravi.v.shankar@intel.com, linux-kernel@vger.kernel.org, chang.seok.bae@intel.com Subject: [PATCH 02/15] x86/fsgsbase/64: Make ptrace read FS/GS base accurately Date: Mon, 19 Mar 2018 10:49:14 -0700 Message-Id: <1521481767-22113-3-git-send-email-chang.seok.bae@intel.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1521481767-22113-1-git-send-email-chang.seok.bae@intel.com> References: <1521481767-22113-1-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 --- arch/x86/kernel/process_64.c | 59 ++++++++++++++++++++++++++++++++++++-------- 1 file changed, 49 insertions(+), 10 deletions(-) diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index 65be0a6..2375f10 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -265,6 +265,49 @@ 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 */ @@ -285,12 +328,10 @@ unsigned long read_task_fsbase(struct task_struct *task) if (task == current) fsbase = read_fsbase(); - else - /* - * XXX: This will not behave as expected if called - * if fsindex != 0 - */ + else if (task->thread.fsindex == 0) fsbase = task->thread.fsbase; + else + fsbase = task_seg_base(task, task->thread.fsindex); return fsbase; } @@ -301,12 +342,10 @@ unsigned long read_task_gsbase(struct task_struct *task) if (task == current) gsbase = read_shadow_gsbase(); - else - /* - * XXX: This will not behave as expected if called - * if gsindex != 0 - */ + else if (task->thread.gsindex == 0) gsbase = task->thread.gsbase; + else + gsbase = task_seg_base(task, task->thread.gsindex); return gsbase; } -- 2.7.4