[PATCH] x86/iopl: Fake iopl(3) CLI/STI usage

[Peter Zijlstra]

On Fri, Sep 17, 2021 at 10:11:31AM +0200, Ondrej Zary wrote:
> Yeah, it works!

w00t!! I've added a pr_err() to make sure people take note their
'software' is doing dodgy things.

---
Subject: x86/iopl: Fake iopl(3) CLI/STI usage
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Thu, 16 Sep 2021 23:05:09 +0200

Since commit c8137ace5638 ("x86/iopl: Restrict iopl() permission
scope") it's possible to emulate iopl(3) using ioperm(), except for
the CLI/STI usage.

Userspace CLI/STI usage is very dubious (read broken), since any
exception taken during that window can lead to rescheduling anyway (or
worse). The IOPL(2) manpage even states that usage of CLI/STI is highly
discouraged and might even crash the system.

Of course, that won't stop people and HP has the dubious honour of
being the first vendor to be found using this in their hp-health
package.

In order to enable this 'software' to still 'work', have the #GP treat
the CLI/STI instructions as NOPs when iopl(3). Warn the user that
their program is doing dubious things.

Fixes: a24ca9976843 ("x86/iopl: Remove legacy IOPL option")
Reported-by: Ondrej Zary <linux@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/kernel/traps.c |   38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -528,6 +528,41 @@ static enum kernel_gp_hint get_kernel_gp
 
 #define GPFSTR "general protection fault"
 
+bool fixup_iopl_exception(struct pt_regs *regs)
+{
+	struct thread_struct *t = &current->thread;
+	unsigned char buf[MAX_INSN_SIZE];
+	struct insn insn;
+	int nr_copied;
+
+	if (!IS_ENABLED(CONFIG_X86_IOPL_IOPERM) || t->iopl_emul != 3 || !regs)
+		return false;
+
+	nr_copied = insn_fetch_from_user(regs, buf);
+	if (nr_copied <= 0)
+		return false;
+
+	if (!insn_decode_from_regs(&insn, regs, buf, nr_copied))
+		return false;
+
+	if (insn.length != 1)
+		return false;
+
+	if (insn.opcode.bytes[0] != 0xfa &&
+	    insn.opcode.bytes[0] != 0xfb)
+		return false;
+
+	if (printk_ratelimit()) {
+		pr_err("%s[%d] attempts to use CLI/STI, pretending it's a NOP, ip:%lx",
+		       current->comm, task_pid_nr(current), regs->ip);
+		print_vma_addr(KERN_CONT " in ", regs->ip);
+		pr_cont("\n");
+	}
+
+	regs->ip += 1;
+	return true;
+}
+
 DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
 {
 	char desc[sizeof(GPFSTR) + 50 + 2*sizeof(unsigned long) + 1] = GPFSTR;
@@ -553,6 +588,9 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_pr
 	tsk = current;
 
 	if (user_mode(regs)) {
+		if (fixup_iopl_exception(regs))
+			goto exit;
+
 		tsk->thread.error_code = error_code;
 		tsk->thread.trap_nr = X86_TRAP_GP;