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From: "tip-bot2 for Thomas Gleixner" <tip-bot2@linutronix.de>
To: linux-tip-commits@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@kernel.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
	linux-kernel@vger.kernel.org
Subject: [tip: x86/iopl] x86/iopl: Fixup misleading comment
Date: Fri, 15 Nov 2019 21:12:29 -0000
Message-ID: <157385234906.12247.16814215844130409013.tip-bot2@tip-bot2> (raw)
In-Reply-To: <20191113210105.276580787@linutronix.de>

The following commit has been merged into the x86/iopl branch of tip:

Commit-ID:     7543e168c1d3c135d21747e494d99e60550c3828
Gitweb:        https://git.kernel.org/tip/7543e168c1d3c135d21747e494d99e60550c3828
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Wed, 13 Nov 2019 21:42:56 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Thu, 14 Nov 2019 20:15:06 +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The comment for the sys_iopl() implementation is outdated and actively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@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191113210105.276580787@linutronix.de

---
 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;

  reply index

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-13 20:42 [patch V3 00/20] x86/iopl: Prevent user space from using CLI/STI with iopl(3) Thomas Gleixner
2019-11-13 20:42 ` [patch V3 01/20] x86/ptrace: Prevent truncation of bitmap size Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 02/20] x86/process: Unify copy_thread_tls() Thomas Gleixner
2019-11-13 21:10   ` Linus Torvalds
2019-11-13 21:41     ` Thomas Gleixner
2019-11-13 22:10       ` Linus Torvalds
2019-11-13 22:33         ` Thomas Gleixner
2019-11-13 21:44     ` Brian Gerst
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 03/20] x86/cpu: Unify cpu_init() Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 04/20] x86/tss: Fix and move VMX BUILD_BUG_ON() Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 05/20] x86/iopl: Cleanup include maze Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 06/20] x86/ioperm: Simplify first ioperm() invocation logic Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 07/20] x86/ioperm: Avoid bitmap allocation if no permissions are set Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 08/20] x86/io: Speedup schedule out of I/O bitmap user Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 09/20] x86/tss: Move I/O bitmap data into a seperate struct Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 10/20] x86/ioperm: Move iobitmap data into a struct Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 11/20] x86/ioperm: Add bitmap sequence number Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 12/20] x86/ioperm: Move TSS bitmap update to exit to user work Thomas Gleixner
2019-11-13 21:19   ` Linus Torvalds
2019-11-13 21:21     ` Linus Torvalds
2019-11-13 21:44       ` Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 13/20] x86/ioperm: Remove bitmap if all permissions dropped Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 14/20] x86/ioperm: Share I/O bitmap if identical Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 15/20] selftests/x86/ioperm: Extend testing so the shared bitmap is exercised Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 16/20] x86/iopl: Fixup misleading comment Thomas Gleixner
2019-11-15 21:12   ` tip-bot2 for Thomas Gleixner [this message]
2019-11-13 20:42 ` [patch V3 17/20] x86/iopl: Restrict iopl() permission scope Thomas Gleixner
2019-11-14 18:13   ` Borislav Petkov
2019-11-14 18:39     ` Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 18/20] x86/iopl: Remove legacy IOPL option Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:42 ` [patch V3 19/20] x86/ioperm: Extend IOPL config to control ioperm() as well Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-13 20:43 ` [patch V3 20/20] selftests/x86/iopl: Extend test to cover IOPL emulation Thomas Gleixner
2019-11-15 21:12   ` [tip: x86/iopl] " tip-bot2 for Thomas Gleixner
2019-11-14  8:43 ` [patch V3 00/20] x86/iopl: Prevent user space from using CLI/STI with iopl(3) Peter Zijlstra
2019-11-16 11:51 [tip: x86/iopl] x86/iopl: Fixup misleading comment tip-bot2 for Thomas Gleixner

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