From be9afb4b529d9e3a68da1212e33be677bbfc8d2c Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Mon, 11 Nov 2019 23:03:27 +0100 Subject: [PATCH] x86/iopl: Fixup misleading comment The comment for the sys_iopl() implementation is outdated and actively misleading in some parts. Fix it up. Signed-off-by: Thomas Gleixner Acked-by: Andy Lutomirski --- arch/x86/kernel/ioport.c | 35 +++++++++++++++++++++++++++-------- 1 file changed, 27 insertions(+), 8 deletions(-) diff --git a/arch/x86/kernel/ioport.c b/arch/x86/kernel/ioport.c index f82ca1c..3548563 100644 --- a/arch/x86/kernel/ioport.c +++ b/arch/x86/kernel/ioport.c @@ -41,7 +41,7 @@ void io_bitmap_exit(void) } /* - * this changes the io permissions bitmap in the current task. + * This changes the io permissions bitmap in the current task. */ long ksys_ioperm(unsigned long from, unsigned long num, int turn_on) { @@ -136,14 +136,24 @@ SYSCALL_DEFINE3(ioperm, unsigned long, from, unsigned long, num, int, turn_on) } /* - * sys_iopl has to be used when you want to access the IO ports - * beyond the 0x3ff range: to get the full 65536 ports bitmapped - * you'd need 8kB of bitmaps/process, which is a bit excessive. + * The sys_iopl functionality depends on the level argument, which if + * granted for the task is used by the CPU to check I/O instruction and + * CLI/STI against the current priviledge level (CPL). If CPL is less than + * or equal the tasks IOPL level the instructions take effect. If not a #GP + * is raised. The default IOPL is 0, i.e. no permissions. * - * Here we just change the flags value on the stack: we allow - * only the super-user to do it. This depends on the stack-layout - * on system-call entry - see also fork() and the signal handling - * code. + * Setting IOPL to level 0-2 is disabling the userspace access. Only level + * 3 enables it. If set it allows the user space thread: + * + * - Unrestricted access to all 65535 I/O ports + * - The usage of CLI/STI instructions + * + * The advantage over ioperm is that the context switch does not require to + * update the I/O bitmap which is especially true when a large number of + * ports is accessed. But the allowance of CLI/STI in userspace is + * considered a major problem. + * + * IOPL is strictly per thread and inherited on fork. */ SYSCALL_DEFINE1(iopl, unsigned int, level) { @@ -164,9 +174,18 @@ SYSCALL_DEFINE1(iopl, unsigned int, level) security_locked_down(LOCKDOWN_IOPORT)) return -EPERM; } + /* + * Change the flags value on the return stack, which has been set + * up on system-call entry. See also the fork and signal handling + * code how this is handled. + */ regs->flags = (regs->flags & ~X86_EFLAGS_IOPL) | (level << X86_EFLAGS_IOPL_BIT); + /* Store the new level in the thread struct */ t->iopl = level << X86_EFLAGS_IOPL_BIT; + /* + * X86_32 switches immediately and XEN handles it via emulation. + */ set_iopl_mask(t->iopl); return 0; -- 2.7.4