i386-io-linux: Prefer usage of ioperm()

Since Linux 2.6.8, it is possible to use ioperm() syscall to gain access
for all I/O ports. Because iopl() syscall before Linux 5.5 allowed
userspace to disable interrupts, prefer usage of ioperm() syscall and ask
for access only for PCI ports.

1 files changed, 46 insertions, 2 deletions

diff --git a/lib/i386-io-linux.h b/lib/i386-io-linux.h
index 731e8e3..a2fd69e 100644
--- a/lib/i386-io-linux.h
+++ b/lib/i386-io-linux.h
@@ -7,17 +7,61 @@
  */
 
 #include <sys/io.h>
+#include <errno.h>
+
+static int ioperm_enabled;
+static int iopl_enabled;
 
 static int
 intel_setup_io(struct pci_access *a UNUSED)
 {
-  return (iopl(3) < 0) ? 0 : 1;
+  if (ioperm_enabled || iopl_enabled)
+    return 1;
+
+  /*
+   * Before Linux 2.6.8, only the first 0x3ff I/O ports permissions can be
+   * modified via ioperm(). Since 2.6.8 all ports are supported.
+   * Since Linux 5.5, EFLAGS-based iopl() implementation was removed and
+   * replaced by new TSS-IOPB-map-all-based emulator. Before Linux 5.5,
+   * EFLAGS-based iopl() allowed userspace to enable/disable interrupts,
+   * which is dangerous. So prefer usage of ioperm() and fallback to iopl().
+   */
+  if (ioperm(0xcf8, 8, 1) < 0) /* conf1 + conf2 ports */
+    {
+      if (errno == EINVAL) /* ioperm() unsupported */
+        {
+          if (iopl(3) < 0)
+            return 0;
+          iopl_enabled = 1;
+          return 1;
+        }
+      return 0;
+    }
+  if (ioperm(0xc000, 0xfff, 1) < 0) /* remaining conf2 ports */
+    {
+      ioperm(0xcf8, 8, 0);
+      return 0;
+    }
+
+  ioperm_enabled = 1;
+  return 1;
 }
 
 static inline void
 intel_cleanup_io(struct pci_access *a UNUSED)
 {
-  iopl(0);
+  if (ioperm_enabled)
+    {
+      ioperm(0xcf8, 8, 0);
+      ioperm(0xc000, 0xfff, 0);
+      ioperm_enabled = 0;
+    }
+
+  if (iopl_enabled)
+    {
+      iopl(0);
+      iopl_enabled = 0;
+    }
 }
 
 static inline void intel_io_lock(void)