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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The comment for the sys_iopl() implementation is outdated and actively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 <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
This commit is contained in:
Thomas Gleixner 2019-11-11 23:03:27 +01:00
parent 0907a09c2e
commit be9afb4b52
1 changed files with 27 additions and 8 deletions
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35
arch/x86/kernel/ioport.c
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@ -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;