Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751710AbcCFRjy (ORCPT ); Sun, 6 Mar 2016 12:39:54 -0500 Received: from mail.kernel.org ([198.145.29.136]:37155 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751360AbcCFRjt (ORCPT ); Sun, 6 Mar 2016 12:39:49 -0500 From: Andy Lutomirski To: x86@kernel.org Cc: linux-kernel@vger.kernel.org, Borislav Petkov , Oleg Nesterov , Andrew Cooper , Brian Gerst , Andy Lutomirski Subject: [PATCH] x86/entry: Improve system call entry comments Date: Sun, 6 Mar 2016 09:39:45 -0800 Message-Id: <3cc149b4ce9a108a092d816c5158808c62c557f0.1457285880.git.luto@kernel.org> X-Mailer: git-send-email 2.5.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 9579 Lines: 246 Ingo suggested that the comments should explain when the various entries are used. This adds these explanations and improves other parts of the comments. Signed-off-by: Andy Lutomirski --- This applies on top of all the TF / debug / SYSENTER stuff. If this is too confusing, I'll rebase it and resend after everything settles in -tip. arch/x86/entry/entry_32.S | 59 +++++++++++++++++++++++++++- arch/x86/entry/entry_64.S | 10 +++++ arch/x86/entry/entry_64_compat.S | 85 +++++++++++++++++++++++++++------------- 3 files changed, 126 insertions(+), 28 deletions(-) diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S index 09360c445c69..846a6c478bfd 100644 --- a/arch/x86/entry/entry_32.S +++ b/arch/x86/entry/entry_32.S @@ -307,6 +307,38 @@ ENTRY(xen_sysenter_target) jmp sysenter_past_esp #endif +/* + * 32-bit SYSENTER entry. + * + * 32-bit system calls through the vDSO's __kernel_vsyscall enter here + * if X86_FEATURE_SEP is available. This is the preferred system call + * entry on 32-bit systems. + * + * The SYSENTER instruction, in principle, should *only* occur in the + * vDSO. In practice, a small number of Android devices were shipped + * with a copy of Bionic that inlined a SYSENTER instruction. This + * never happened in any of Google's Bionic versions -- it only happened + * in a narrow range of Intel-provided versions. + * + * SYSENTER loads SS, ESP, CS, and EIP from previously programmed MSRs. + * IF and VM in RFLAGS are cleared (IOW: interrupts are off). + * SYSENTER does not save anything on the stack, + * and does not save old EIP (!!!), ESP, or EFLAGS. + * + * To avoid losing track of EFLAGS.VM (and thus potentially corrupting + * user and/or vm86 state), we explicitly disable the SYSENTER + * instruction in vm86 mode by reprogramming the MSRs. + * + * Arguments: + * eax system call number + * ebx arg1 + * ecx arg2 + * edx arg3 + * esi arg4 + * edi arg5 + * ebp user stack + * 0(%ebp) arg6 + */ ENTRY(entry_SYSENTER_32) movl TSS_sysenter_sp0(%esp), %esp sysenter_past_esp: @@ -398,7 +430,32 @@ sysenter_past_esp: GLOBAL(__end_SYSENTER_singlestep_region) ENDPROC(entry_SYSENTER_32) - # system call handler stub +/* + * 32-bit legacy system call entry. + * + * 32-bit x86 Linux system calls traditionally used the INT $0x80 + * instruction. INT $0x80 lands here. + * + * This entry point can be used by 32-bit and 64-bit programs to perform + * 32-bit system calls. Instances of INT $0x80 can be found inline in + * various programs and libraries. It is also used by the vDSO's + * __kernel_vsyscall fallback for hardware that doesn't support a faster + * entry method. Restarted 32-bit system calls also fall back to INT + * $0x80 regardless of what instruction was originally used to do the + * system call. + * + * This is considered a slow path. It is not used by modern libc + * implementations on modern hardware except during process startup. + * + * Arguments: + * eax system call number + * ebx arg1 + * ecx arg2 + * edx arg3 + * esi arg4 + * edi arg5 + * ebp arg6 + */ ENTRY(entry_INT80_32) ASM_CLAC pushl %eax /* pt_regs->orig_ax */ diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index 2f61059dacc3..6f5fbc584cb1 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -106,6 +106,16 @@ ENDPROC(native_usergs_sysret64) /* * 64-bit SYSCALL instruction entry. Up to 6 arguments in registers. * + * This is the only entry point used for 64-bit system calls. The + * hardware interface is reasonably well designed and the register to + * argument mapping Linux uses fits well with the registers that are + * available when SYSCALL is used. + * + * SYSCALL instructions can be found inlined in libc implementations as + * well as some other programs and libraries. There are also a handful + * of SYSCALL instructions in the vDSO used, for example, as a + * clock_gettimeofday fallback. + * * 64-bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11, * then loads new ss, cs, and rip from previously programmed MSRs. * rflags gets masked by a value from another MSR (so CLD and CLAC diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S index aa3f79ba2e4d..1838dcdd3886 100644 --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -19,12 +19,21 @@ .section .entry.text, "ax" /* - * 32-bit SYSENTER instruction entry. + * 32-bit SYSENTER entry. * - * SYSENTER loads ss, rsp, cs, and rip from previously programmed MSRs. - * IF and VM in rflags are cleared (IOW: interrupts are off). + * 32-bit system calls through the vDSO's __kernel_vsyscall enter here + * on 64-bit kernels running on Intel CPUs. + * + * The SYSENTER instruction, in principle, should *only* occur in the + * vDSO. In practice, a small number of Android devices were shipped + * with a copy of Bionic that inlined a SYSENTER instruction. This + * never happened in any of Google's Bionic versions -- it only happened + * in a narrow range of Intel-provided versions. + * + * SYSENTER loads SS, RSP, CS, and RIP from previously programmed MSRs. + * IF and VM in RFLAGS are cleared (IOW: interrupts are off). * SYSENTER does not save anything on the stack, - * and does not save old rip (!!!) and rflags. + * and does not save old RIP (!!!), RSP, or RFLAGS. * * Arguments: * eax system call number @@ -35,10 +44,6 @@ * edi arg5 * ebp user stack * 0(%ebp) arg6 - * - * This is purely a fast path. For anything complicated we use the int 0x80 - * path below. We set up a complete hardware stack frame to share code - * with the int 0x80 path. */ ENTRY(entry_SYSENTER_compat) /* Interrupts are off on entry. */ @@ -131,17 +136,38 @@ GLOBAL(__end_entry_SYSENTER_compat) ENDPROC(entry_SYSENTER_compat) /* - * 32-bit SYSCALL instruction entry. + * 32-bit SYSCALL entry. + * + * 32-bit system calls through the vDSO's __kernel_vsyscall enter here + * on 64-bit kernels running on AMD CPUs. + * + * The SYSCALL instruction, in principle, should *only* occur in the + * vDSO. In practice, it appears that this really is the case. + * As evidence: + * + * - The calling convention for SYSCALL has changed several times without + * anyone noticing. + * + * - Prior to the in-kernel X86_BUG_SYSRET_SS_ATTRS fixup, anything + * user task that did SYSCALL without immediately reloading SS + * would randomly crash. * - * 32-bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11, - * then loads new ss, cs, and rip from previously programmed MSRs. - * rflags gets masked by a value from another MSR (so CLD and CLAC - * are not needed). SYSCALL does not save anything on the stack - * and does not change rsp. + * - Most programmers do not directly target AMD CPUs, and the 32-bit + * SYSCALL instruction does not exist on Intel CPUs. Even on AMD + * CPUs, Linux disables the SYSCALL instruction on 32-bit kernels + * because the SYSCALL instruction in legacy/native 32-bit mode (as + * opposed to compat mode) is sufficiently poorly designed as to be + * essentially unusable. * - * Note: rflags saving+masking-with-MSR happens only in Long mode + * 32-bit SYSCALL saves RIP to RCX, clears RFLAGS.RF, then saves + * RFLAGS to R11, then loads new SS, CS, and RIP from previously + * programmed MSRs. RFLAGS gets masked by a value from another MSR + * (so CLD and CLAC are not needed). SYSCALL does not save anything on + * the stack and does not change RSP. + * + * Note: RFLAGS saving+masking-with-MSR happens only in Long mode * (in legacy 32-bit mode, IF, RF and VM bits are cleared and that's it). - * Don't get confused: rflags saving+masking depends on Long Mode Active bit + * Don't get confused: RFLAGS saving+masking depends on Long Mode Active bit * (EFER.LMA=1), NOT on bitness of userspace where SYSCALL executes * or target CS descriptor's L bit (SYSCALL does not read segment descriptors). * @@ -241,7 +267,21 @@ sysret32_from_system_call: END(entry_SYSCALL_compat) /* - * Emulated IA32 system calls via int 0x80. + * 32-bit legacy system call entry. + * + * 32-bit x86 Linux system calls traditionally used the INT $0x80 + * instruction. INT $0x80 lands here. + * + * This entry point can be used by 32-bit and 64-bit programs to perform + * 32-bit system calls. Instances of INT $0x80 can be found inline in + * various programs and libraries. It is also used by the vDSO's + * __kernel_vsyscall fallback for hardware that doesn't support a faster + * entry method. Restarted 32-bit system calls also fall back to INT + * $0x80 regardless of what instruction was originally used to do the + * system call. + * + * This is considered a slow path. It is not used by modern libc + * implementations on modern hardware except during process startup. * * Arguments: * eax system call number @@ -250,17 +290,8 @@ END(entry_SYSCALL_compat) * edx arg3 * esi arg4 * edi arg5 - * ebp arg6 (note: not saved in the stack frame, should not be touched) - * - * Notes: - * Uses the same stack frame as the x86-64 version. - * All registers except eax must be saved (but ptrace may violate that). - * Arguments are zero extended. For system calls that want sign extension and - * take long arguments a wrapper is needed. Most calls can just be called - * directly. - * Assumes it is only called from user space and entered with interrupts off. + * ebp arg6 */ - ENTRY(entry_INT80_compat) /* * Interrupts are off on entry. -- 2.5.0