add SMAP helpers (#566)

* add SMAP helpers

This PR adds helpers for SMAP. SMAP is a security feature that can be
used to detect accidental access to user memory by the kernel.

Author: Freax13

src/instructions/mod.rs

@@ -6,6 +6,7 @@ pub mod interrupts;
 pub mod port;
 pub mod random;
 pub mod segmentation;
+pub mod smap;
 pub mod tables;
 pub mod tlb;
 

src/instructions/smap.rs

@@ -0,0 +1,115 @@
+//! This module provides helpers for Supervisor Mode Access Prevention (SMAP).
+//!
+//! SMAP is a security feature that helps prevent accidental accesses to user
+//! memory by a kernel. This feature can be enabled by setting
+//! [`Cr4Flags::SUPERVISOR_MODE_ACCESS_PREVENTION`]. Once enabled, accesses to
+//! user memory by the kernel will generate a page fault (#PF) unless the
+//! [`RFlags::ALIGNMENT_CHECK`] bit is set.
+//!
+//! The `stac` and `clac` instructions can be used to efficiently update the
+//! `ALIGNMENT_CHECK` flag.
+//!
+//! Not all processors support SMAP.
+
+use core::arch::asm;
+
+use bit_field::BitField;
+
+#[cfg(doc)]
+use crate::registers::control::Cr4Flags;
+use crate::registers::rflags::{self, RFlags};
+
+/// A helper type that provides SMAP related methods.
+///
+/// This type can only be instatiated if SMAP is supported by the CPU.
+#[derive(Debug, Clone, Copy)]
+pub struct Smap(());
+
+impl Smap {
+    /// Checks if the CPU supports SMAP and returns a [`Smap`] instance if
+    /// supported or `None` if not.
+    ///
+    /// This function uses CPUID to determine if SMAP is supported by the CPU.
+    ///
+    /// Note that this function does not check whether SMAP has be enabled in
+    /// CR4.
+    pub fn new() -> Option<Self> {
+        // Check if the CPU supports `stac` and `clac`.
+        let cpuid = unsafe { core::arch::x86_64::__cpuid(7) };
+        if cpuid.ebx.get_bit(20) {
+            Some(Self(()))
+        } else {
+            None
+        }
+    }
+
+    /// Returns a [`Smap`] instance.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that the CPU supports SMAP.
+    #[inline]
+    pub const unsafe fn new_unchecked() -> Self {
+        Self(())
+    }
+
+    /// Returns whether the [`RFlags::ALIGNMENT_CHECK`] flag is unset.
+    ///
+    /// Note that SMAP also requires
+    /// [`Cr4Flags::SUPERVISOR_MODE_ACCESS_PREVENTION`] to be set. This
+    /// function does not check CR4 because doing so is much slower than just
+    /// checking the AC flag.
+    #[inline]
+    pub fn is_enabled(self) -> bool {
+        !rflags::read().contains(RFlags::ALIGNMENT_CHECK)
+    }
+
+    /// Disable SMAP access checks by setting [`RFlags::ALIGNMENT_CHECK`] using
+    /// the `stac` instruction.
+    ///
+    /// This will do nothing if `SMAP` access checks are already disabled.
+    #[doc(alias = "stac")]
+    #[inline]
+    pub fn disable(self) {
+        // Technically this modifies the AC flag, but the Rust compiler doesn't
+        // care about that, so it's fine to use preserves_flags.
+        unsafe {
+            asm!("stac", options(nomem, nostack, preserves_flags));
+        }
+    }
+
+    /// Enable SMAP access checks by clearing [`RFlags::ALIGNMENT_CHECK`] using
+    /// the `clac` instruction.
+    ///
+    /// This will do nothing if `SMAP` access checks are already enabled.
+    #[doc(alias = "clac")]
+    #[inline]
+    pub fn enable(self) {
+        // Technically this modifies the AC flag, but the Rust compiler doesn't
+        // care about that, so it's fine to use preserves_flags.
+        unsafe {
+            asm!("clac", options(nomem, nostack, preserves_flags));
+        }
+    }
+
+    /// Call a closure with SMAP disabled.
+    ///
+    /// This function disables SMAP before calling the closure and restores the
+    /// SMAP state afterwards.
+    pub fn without_smap<F, R>(self, f: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        let was_enabled = self.is_enabled();
+
+        self.disable();
+
+        let result = f();
+
+        if was_enabled {
+            self.enable();
+        }
+
+        result
+    }
+}