From: Andrew Cooper Date: Sun, 18 Dec 2016 15:42:59 +0000 Subject: [PATCH] x86/emul: Correct the handling of eflags with SYSCALL A singlestep #DB is determined by the resulting eflags value from the execution of SYSCALL, not the original eflags value. By using the original eflags value, we negate the guest kernels attempt to protect itself from a privilege escalation by masking TF. (re)introduce a singlestep boolean, defaulting to the original eflags state, but have the SYSCALL emulation recalculate it after masking has occurred. This is XSA-204 Signed-off-by: Andrew Cooper Reviewed-by: Jan Beulich --- xen/arch/x86/x86_emulate/x86_emulate.c | 24 +++++++++++++++++++++--- 1 file changed, 21 insertions(+), 3 deletions(-) diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c index f69dece..165eebb 100644 --- a/xen/arch/x86/x86_emulate/x86_emulate.c +++ b/xen/arch/x86/x86_emulate/x86_emulate.c @@ -2502,6 +2502,7 @@ x86_emulate( struct x86_emulate_state state; int rc; uint8_t b, d; + bool singlestep = ctxt->regs->eflags & EFLG_TF; struct operand src = { .reg = PTR_POISON }; struct operand dst = { .reg = PTR_POISON }; enum x86_swint_type swint_type; @@ -4678,6 +4679,23 @@ x86_emulate( (rc = ops->write_segment(x86_seg_ss, &sreg, ctxt)) ) goto done; + /* + * SYSCALL (unlike most instructions) evaluates its singlestep action + * based on the resulting EFLG_TF, not the starting EFLG_TF. + * + * As the #DB is raised after the CPL change and before the OS can + * switch stack, it is a large risk for privilege escalation. + * + * 64bit kernels should mask EFLG_TF in MSR_FMASK to avoid any + * vulnerability. Running the #DB handler on an IST stack is also a + * mitigation. + * + * 32bit kernels have no ability to mask EFLG_TF at all. Their only + * mitigation is to use a task gate for handling #DB (or to not use + * enable EFER.SCE to start with). + */ + singlestep = _regs.eflags & EFLG_TF; + break; } @@ -5580,9 +5598,9 @@ x86_emulate( if ( !mode_64bit() ) _regs.eip = (uint32_t)_regs.eip; - /* Was singestepping active at the start of this instruction? */ - if ( (rc == X86EMUL_OKAY) && (ctxt->regs->eflags & EFLG_TF) ) - ctxt->retire.singlestep = true; + /* Should a singlestep #DB be raised? */ + if ( rc == X86EMUL_OKAY ) + ctxt->retire.singlestep = singlestep; if ( rc != X86EMUL_DONE ) *ctxt->regs = _regs;