[patch 6/9] x86/iopl: Fixup misleading comment

Thomas Gleixner
2019-11-06T20:56:57Z

The comment for the sys_iopl() implementation is outdated and actively
misleading in some parts. Fix it up.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/kernel/ioport.c | 35 +++++++++++++++++++++++++++--------
1 file changed, 27 insertions(+), 8 deletions(-)

--- a/arch/x86/kernel/ioport.c
+++ b/arch/x86/kernel/ioport.c
@@ -22,7 +22,7 @@
#include <asm/desc.h>

/*
- * 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)
{
@@ -163,14 +163,24 @@ SYSCALL_DEFINE3(ioperm, unsigned long, f
}

/*
- * 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)
{
@@ -191,9 +201,18 @@ SYSCALL_DEFINE1(iopl, unsigned int, leve
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;