From 7841b408717d4c3b1b334c8f1fef7f18c98cd2bd Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Mon, 5 Oct 2015 17:48:12 -0700 Subject: [PATCH] x86/entry/compat: Implement opportunistic SYSRETL for compat syscalls If CS, SS and IP are as expected and FLAGS is compatible with SYSRETL, then return from fast compat syscalls (both SYSCALL and SYSENTER) using SYSRETL. Unlike native 64-bit opportunistic SYSRET, this is not invisible to user code: RCX and R8-R15 end up in a different state than shown saved in pt_regs. To compensate, we only do this when returning to the vDSO fast syscall return path. This won't interfere with syscall restart, as we won't use SYSRETL when returning to the INT80 restart instruction. Signed-off-by: Andy Lutomirski Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: linux-kernel@vger.kernel.org Link: http://lkml.kernel.org/r/aa15e49db33773eb10b73d73466b6d5466d7856a.1444091585.git.luto@kernel.org Signed-off-by: Ingo Molnar --- arch/x86/entry/common.c | 23 ++++++++++++++--- arch/x86/entry/entry_64_compat.S | 42 ++++++++++++++++++++++++++++++-- 2 files changed, 60 insertions(+), 5 deletions(-) diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c index 1b2606edc621..88dc5ba14d47 100644 --- a/arch/x86/entry/common.c +++ b/arch/x86/entry/common.c @@ -363,7 +363,8 @@ __visible void do_int80_syscall_32(struct pt_regs *regs) syscall_return_slowpath(regs); } -__visible void do_fast_syscall_32(struct pt_regs *regs) +/* Returns 0 to return using IRET or 1 to return using SYSRETL. */ +__visible long do_fast_syscall_32(struct pt_regs *regs) { /* * Called using the internal vDSO SYSENTER/SYSCALL32 calling @@ -395,12 +396,28 @@ __visible void do_fast_syscall_32(struct pt_regs *regs) enter_from_user_mode(); #endif prepare_exit_to_usermode(regs); - return; + return 0; /* Keep it simple: use IRET. */ } local_irq_disable(); /* Now this is just like a normal syscall. */ do_int80_syscall_32(regs); - return; + +#ifdef CONFIG_X86_64 + /* + * Opportunistic SYSRETL: if possible, try to return using SYSRETL. + * SYSRETL is available on all 64-bit CPUs, so we don't need to + * bother with SYSEXIT. + * + * Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP, + * because the ECX fixup above will ensure that this is essentially + * never the case. + */ + return regs->cs == __USER32_CS && regs->ss == __USER_DS && + regs->ip == landing_pad && + (regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)) == 0; +#else + return 0; +#endif } #endif diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S index 8f109de51d03..cf9641cd4796 100644 --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -115,7 +115,9 @@ sysenter_flags_fixed: movq %rsp, %rdi call do_fast_syscall_32 - jmp .Lsyscall_32_done + testl %eax, %eax + jz .Lsyscall_32_done + jmp sysret32_from_system_call sysenter_fix_flags: pushq $X86_EFLAGS_FIXED @@ -192,7 +194,43 @@ ENTRY(entry_SYSCALL_compat) movq %rsp, %rdi call do_fast_syscall_32 - jmp .Lsyscall_32_done + testl %eax, %eax + jz .Lsyscall_32_done + + /* Opportunistic SYSRET */ +sysret32_from_system_call: + TRACE_IRQS_ON /* User mode traces as IRQs on. */ + movq RBX(%rsp), %rbx /* pt_regs->rbx */ + movq RBP(%rsp), %rbp /* pt_regs->rbp */ + movq EFLAGS(%rsp), %r11 /* pt_regs->flags (in r11) */ + movq RIP(%rsp), %rcx /* pt_regs->ip (in rcx) */ + addq $RAX, %rsp /* Skip r8-r15 */ + popq %rax /* pt_regs->rax */ + popq %rdx /* Skip pt_regs->cx */ + popq %rdx /* pt_regs->dx */ + popq %rsi /* pt_regs->si */ + popq %rdi /* pt_regs->di */ + + /* + * USERGS_SYSRET32 does: + * GSBASE = user's GS base + * EIP = ECX + * RFLAGS = R11 + * CS = __USER32_CS + * SS = __USER_DS + * + * ECX will not match pt_regs->cx, but we're returning to a vDSO + * trampoline that will fix up RCX, so this is okay. + * + * R12-R15 are callee-saved, so they contain whatever was in them + * when the system call started, which is already known to user + * code. We zero R8-R10 to avoid info leaks. + */ + xorq %r8, %r8 + xorq %r9, %r9 + xorq %r10, %r10 + movq RSP-ORIG_RAX(%rsp), %rsp + USERGS_SYSRET32 END(entry_SYSCALL_compat) /* -- 2.34.1