From 4214a16b02971c60960afd675d03544e109e0d75 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Thu, 2 Apr 2015 17:12:12 -0700 Subject: [PATCH] x86/asm/entry/64/compat: Use SYSRETL to return from compat mode SYSENTER SYSEXIT is scary on 64-bit kernels -- SYSEXIT must be invoked with usergs and IRQs on. That means that we rely on STI to correctly mask interrupts for one instruction. This is okay by itself, but the semantics with respect to NMIs are unclear. Avoid the whole issue by using SYSRETL instead. For background, Intel CPUs don't allow SYSCALL from compat mode, but they do allow SYSRETL back to compat mode. Go figure. To avoid doing too much at once, this doesn't revamp the calling convention. We still return with EBP, EDX, and ECX on the user stack. Oddly this seems to be 30 cycles or so faster. Avoiding POPFQ and STI will account for under half of that, I think, so my best guess is that Intel just optimizes SYSRET much better than SYSEXIT. Signed-off-by: Andy Lutomirski Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/57a0bf1b5230b2716a64ebe48e9bc1110f7ab433.1428019097.git.luto@kernel.org Signed-off-by: Ingo Molnar --- arch/x86/ia32/ia32entry.S | 53 +++++++++++++++++++++++++++++---------- 1 file changed, 40 insertions(+), 13 deletions(-) diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S index 8d01cce7b6b8..5d8f987a340d 100644 --- a/arch/x86/ia32/ia32entry.S +++ b/arch/x86/ia32/ia32entry.S @@ -186,28 +186,55 @@ sysenter_dispatch: testl $_TIF_ALLWORK_MASK, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS) jnz sysexit_audit sysexit_from_sys_call: + /* + * NB: SYSEXIT is not obviously safe for 64-bit kernels -- an + * NMI between STI and SYSEXIT has poorly specified behavior, + * and and NMI followed by an IRQ with usergs is fatal. So + * we just pretend we're using SYSEXIT but we really use + * SYSRETL instead. + * + * This code path is still called 'sysexit' because it pairs + * with 'sysenter' and it uses the SYSENTER calling convention. + */ andl $~TS_COMPAT,ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS) - /* clear IF, that popfq doesn't enable interrupts early */ - andl $~0x200,EFLAGS(%rsp) - movl RIP(%rsp),%edx /* User %eip */ - CFI_REGISTER rip,rdx + movl RIP(%rsp),%ecx /* User %eip */ + CFI_REGISTER rip,rcx RESTORE_RSI_RDI - /* pop everything except ss,rsp,rflags slots */ - REMOVE_PT_GPREGS_FROM_STACK 3*8 + xorl %edx,%edx /* avoid info leaks */ xorq %r8,%r8 xorq %r9,%r9 xorq %r10,%r10 - xorq %r11,%r11 - popfq_cfi + movl EFLAGS(%rsp),%r11d /* User eflags */ /*CFI_RESTORE rflags*/ - popq_cfi %rcx /* User %esp */ - CFI_REGISTER rsp,rcx TRACE_IRQS_ON + /* - * 32bit SYSEXIT restores eip from edx, esp from ecx. - * cs and ss are loaded from MSRs. + * SYSRETL works even on Intel CPUs. Use it in preference to SYSEXIT, + * since it avoids a dicey window with interrupts enabled. */ - ENABLE_INTERRUPTS_SYSEXIT32 + movl RSP(%rsp),%esp + + /* + * USERGS_SYSRET32 does: + * gsbase = user's gs base + * eip = ecx + * rflags = r11 + * cs = __USER32_CS + * ss = __USER_DS + * + * The prologue set RIP(%rsp) to VDSO32_SYSENTER_RETURN, which does: + * + * pop %ebp + * pop %edx + * pop %ecx + * + * Therefore, we invoke SYSRETL with EDX and R8-R10 zeroed to + * avoid info leaks. R11 ends up with VDSO32_SYSENTER_RETURN's + * address (already known to user code), and R12-R15 are + * callee-saved and therefore don't contain any interesting + * kernel data. + */ + USERGS_SYSRET32 CFI_RESTORE_STATE -- 2.20.1