From 2e2c3efcfc9f183674a8de6ed954ffbe7188b70d Mon Sep 17 00:00:00 2001 Message-ID: <2e2c3efcfc9f183674a8de6ed954ffbe7188b70d.1695733540.git.m.a.young@durham.ac.uk> In-Reply-To: References: From: Andrew Cooper Date: Wed, 13 Sep 2023 13:53:33 +0100 Subject: [XEN PATCH 08/10] x86/spec-ctrl: Issue VERW during IST exit to Xen There is a corner case where e.g. an NMI hitting an exit-to-guest path after SPEC_CTRL_EXIT_TO_* would have run the entire NMI handler *after* the VERW flush to scrub potentially sensitive data from uarch buffers. In order to compensate, issue VERW when exiting to Xen from an IST entry. SPEC_CTRL_EXIT_TO_XEN already has two reads of spec_ctrl_flags off the stack, and we're about to add a third. Load the field into %ebx, and list the register as clobbered. %r12 has been arranged to be the ist_exit signal, so add this as an input dependency and use it to identify when to issue a VERW. Signed-off-by: Andrew Cooper Reviewed-by: Jan Beulich (cherry picked from commit 3ee6066bcd737756b0990d417d94eddc0b0d2585) --- xen/arch/x86/include/asm/spec_ctrl_asm.h | 20 +++++++++++++++----- xen/arch/x86/x86_64/entry.S | 2 +- 2 files changed, 16 insertions(+), 6 deletions(-) diff --git a/xen/arch/x86/include/asm/spec_ctrl_asm.h b/xen/arch/x86/include/asm/spec_ctrl_asm.h index 66c706496f..28a75796e6 100644 --- a/xen/arch/x86/include/asm/spec_ctrl_asm.h +++ b/xen/arch/x86/include/asm/spec_ctrl_asm.h @@ -357,10 +357,12 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise): */ .macro SPEC_CTRL_EXIT_TO_XEN /* - * Requires %r14=stack_end - * Clobbers %rax, %rcx, %rdx + * Requires %r12=ist_exit, %r14=stack_end + * Clobbers %rax, %rbx, %rcx, %rdx */ - testb $SCF_ist_sc_msr, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14) + movzbl STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14), %ebx + + testb $SCF_ist_sc_msr, %bl jz .L\@_skip_sc_msr /* @@ -371,7 +373,7 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise): */ xor %edx, %edx - testb $SCF_use_shadow, STACK_CPUINFO_FIELD(spec_ctrl_flags)(%r14) + testb $SCF_use_shadow, %bl jz .L\@_skip_sc_msr mov STACK_CPUINFO_FIELD(shadow_spec_ctrl)(%r14), %eax @@ -380,8 +382,16 @@ UNLIKELY_DISPATCH_LABEL(\@_serialise): .L\@_skip_sc_msr: - /* TODO VERW */ + test %r12, %r12 + jz .L\@_skip_ist_exit + + /* Logically DO_SPEC_CTRL_COND_VERW but without the %rsp=cpuinfo dependency */ + testb $SCF_verw, %bl + jz .L\@_skip_verw + verw STACK_CPUINFO_FIELD(verw_sel)(%r14) +.L\@_skip_verw: +.L\@_skip_ist_exit: .endm #endif /* __ASSEMBLY__ */ diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S index 4cebc4fbe3..c12e011b4d 100644 --- a/xen/arch/x86/x86_64/entry.S +++ b/xen/arch/x86/x86_64/entry.S @@ -680,7 +680,7 @@ UNLIKELY_START(ne, exit_cr3) UNLIKELY_END(exit_cr3) /* WARNING! `ret`, `call *`, `jmp *` not safe beyond this point. */ - SPEC_CTRL_EXIT_TO_XEN /* Req: %r14=end, Clob: acd */ + SPEC_CTRL_EXIT_TO_XEN /* Req: %r12=ist_exit %r14=end, Clob: abcd */ RESTORE_ALL adj=8 iretq -- 2.41.0