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Message-ID: <157390508258.12247.14455974974055338537.tip-bot2@tip-bot2>
Date: Sat, 16 Nov 2019 11:51:22 -0000
From: "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
Andy Lutomirski <luto@...nel.org>,
Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
linux-kernel@...r.kernel.org
Subject: [tip: x86/iopl] x86/iopl: Fixup misleading comment
The following commit has been merged into the x86/iopl branch of tip:
Commit-ID: be9afb4b529d9e3a68da1212e33be677bbfc8d2c
Gitweb: https://git.kernel.org/tip/be9afb4b529d9e3a68da1212e33be677bbfc8d2c
Author: Thomas Gleixner <tglx@...utronix.de>
AuthorDate: Mon, 11 Nov 2019 23:03:27 +01:00
Committer: Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Sat, 16 Nov 2019 11:24:05 +01:00
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The comment for the sys_iopl() implementation is outdated and actively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@...utronix.de>
Acked-by: Andy Lutomirski <luto@...nel.org>
---
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;
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