Messages in this thread Patch in this message | | | From | Brian Gerst <> | Subject | [PATCH v2 6/6] x86/entry/32: Clean up syscall fast exit tests | Date | Fri, 21 Jul 2023 12:10:17 -0400 |
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Merge compat and native code and clarify comments.
Signed-off-by: Brian Gerst <brgerst@gmail.com> --- arch/x86/entry/common.c | 48 +++++++++++++++----------------- arch/x86/entry/entry_64_compat.S | 5 ++-- 2 files changed, 24 insertions(+), 29 deletions(-)
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c index 4d7d6d25dda9..800664325542 100644 --- a/arch/x86/entry/common.c +++ b/arch/x86/entry/common.c @@ -241,34 +241,30 @@ __visible noinstr bool do_fast_syscall_32(struct pt_regs *regs) if (!__do_fast_syscall_32(regs)) return false; -#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. + * Check that the register state is valid for using SYSRETL/SYSEXIT + * to exit to userspace. Otherwise use the slower but fully capable + * IRET exit path. */ - return regs->cs == __USER32_CS && regs->ss == __USER_DS && - regs->ip == landing_pad && - (regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)) == 0; -#else - /* - * Opportunistic SYSEXIT: if possible, try to return using 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. - * - * We don't allow syscalls at all from VM86 mode, but we still - * need to check VM, because we might be returning from sys_vm86. - */ - return regs->cs == __USER_CS && regs->ss == __USER_DS && - regs->ip == landing_pad && - (regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)) == 0; -#endif + + /* XEN PV guests always use IRET path */ + if (cpu_feature_enabled(X86_FEATURE_XENPV)) + return false; + + /* EIP must point to the VDSO landing pad */ + if (unlikely(regs->ip != landing_pad)) + return false; + + /* CS and SS must match the values set in MSR_STAR */ + if (unlikely(regs->cs != __USER32_CS || regs->ss != __USER_DS)) + return false; + + /* If the TF, RF, or VM flags are set, use IRET */ + if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM))) + return false; + + /* Use SYSRETL/SYSEXIT to exit to userspace */ + return true; } /* Returns true to return using SYSEXIT/SYSRETL, or false to use IRET */ diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S index 27c05d08558a..84e21d1ebf10 100644 --- a/arch/x86/entry/entry_64_compat.S +++ b/arch/x86/entry/entry_64_compat.S @@ -211,9 +211,8 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_after_hwframe, SYM_L_GLOBAL) call do_fast_syscall_32 sysret32_from_system_call: - /* XEN PV guests always use IRET path */ - ALTERNATIVE "testb %al, %al; jz swapgs_restore_regs_and_return_to_usermode", \ - "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV + testb %al, %al /* Is SYSRET allowed? */ + jz swapgs_restore_regs_and_return_to_usermode /* * Opportunistic SYSRET -- 2.41.0
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