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Add missing safety explanations for the page table node

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Zhang Junyang 2024-06-02 10:20:57 +00:00 committed by Tate, Hongliang Tian
parent 3579d88aa6
commit e8595b95fe
4 changed files with 89 additions and 54 deletions
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2
framework/aster-frame/src/arch/x86/mm/mod.rs
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@ -92,7 +92,7 @@ pub(crate) fn tlb_flush_all_including_global() {
pub struct PageTableEntry(usize);
/// Activate the given level 4 page table.
/// The cache policy of the root page table frame is controlled by `root_pt_cache`.
/// The cache policy of the root page table node is controlled by `root_pt_cache`.
///
/// ## Safety
///

2
framework/aster-frame/src/mm/page_table/cursor.rs
View File

@ -201,7 +201,7 @@ where
/// Traverse forward in the current level to the next PTE.
///
/// If reached the end of a page table frame, it leads itself up to the next frame of the parent
/// If reached the end of a page table node, it leads itself up to the next frame of the parent
/// frame if possible.
fn move_forward(&mut self) {
let page_size = page_size::<C>(self.level);

14
framework/aster-frame/src/mm/page_table/mod.rs
View File

@ -11,8 +11,8 @@ use super::{
};
use crate::arch::mm::{PageTableEntry, PagingConsts};
mod frame;
use frame::*;
mod node;
use node::*;
mod cursor;
pub(crate) use cursor::{Cursor, CursorMut, PageTableQueryResult};
#[cfg(ktest)]
@ -66,7 +66,7 @@ const fn nr_pte_index_bits<C: PagingConstsTrait>() -> usize {
nr_subpage_per_huge::<C>().ilog2() as usize
}
/// The index of a VA's PTE in a page table frame at the given level.
/// The index of a VA's PTE in a page table node at the given level.
const fn pte_index<C: PagingConstsTrait>(va: Vaddr, level: PagingLevel) -> usize {
va >> (C::BASE_PAGE_SIZE.ilog2() as usize + nr_pte_index_bits::<C>() * (level as usize - 1))
& (nr_subpage_per_huge::<C>() - 1)
@ -186,7 +186,7 @@ where
/// The physical address of the root page table.
///
/// It is dangerous to directly provide the physical address of the root page table to the
/// hardware since the page table frame may be dropped, resulting in UAF.
/// hardware since the page table node may be dropped, resulting in UAF.
pub(crate) unsafe fn root_paddr(&self) -> Paddr {
self.root.paddr()
}
@ -223,7 +223,7 @@ where
/// cursors concurrently accessing the same virtual address range, just like what
/// happens for the hardware MMU walk.
pub(crate) fn query(&self, vaddr: Vaddr) -> Option<(Paddr, PageProperty)> {
// SAFETY: The root frame is a valid page table frame so the address is valid.
// SAFETY: The root frame is a valid page table node so the address is valid.
unsafe { page_walk::<E, C>(self.root_paddr(), vaddr) }
}
@ -267,13 +267,13 @@ where
/// # Safety
///
/// The caller must ensure that the root_paddr is a valid pointer to the root
/// page table frame.
/// page table node.
///
/// # Notes on the page table free-reuse-then-read problem
///
/// Because neither the hardware MMU nor the software page walk method
/// would get the locks of the page table while reading, they can enter
/// a to-be-recycled page table frame and read the page table entries
/// a to-be-recycled page table node and read the page table entries
/// after the frame is recycled and reused.
///
/// To mitigate this problem, the page table nodes are by default not

125
framework/aster-frame/src/mm/page_table/frame.rs → framework/aster-frame/src/mm/page_table/node.rs
View File

@ -1,25 +1,26 @@
// SPDX-License-Identifier: MPL-2.0
//! This module defines page table frame abstractions and the handle.
//! This module defines page table node abstractions and the handle.
//!
//! The page table frame is also frequently referred to as a page table in many architectural
//! documentations. We also call it the page table node if emphasizing the tree structure.
//! The page table node is also frequently referred to as a page table in many architectural
//! documentations. It is essentially a page that contains page table entries (PTEs) that map
//! to child page tables nodes or mapped pages.
//!
//! This module leverages the frame metadata to manage the page table frames, which makes it
//! easier to provide the following guarantees:
//!
//! The page table frame is not freed when it is still in use by:
//! - a parent page table frame,
//! - or a handle to a page table frame,
//! The page table node is not freed when it is still in use by:
//! - a parent page table node,
//! - or a handle to a page table node,
//! - or a processor.
//! This is implemented by using a reference counter in the frame metadata. If the above
//! conditions are not met, the page table frame is ensured to be freed upon dropping the last
//! conditions are not met, the page table node is ensured to be freed upon dropping the last
//! reference.
//!
//! One can acquire exclusive access to a page table frame using merely the physical address of
//! the page table frame. This is implemented by a lock in the frame metadata. Here the
//! One can acquire exclusive access to a page table node using merely the physical address of
//! the page table node. This is implemented by a lock in the frame metadata. Here the
//! exclusiveness is only ensured for kernel code, and the processor's MMU is able to access the
//! page table frame while a lock is held. So the modification to the PTEs should be done after
//! page table node while a lock is held. So the modification to the PTEs should be done after
//! the initialization of the entity that the PTE points to. This is taken care in this module.
//!
@ -40,13 +41,13 @@ use crate::{
},
};
/// The raw handle to a page table frame.
/// The raw handle to a page table node.
///
/// This handle is a referencer of a page table frame. Thus creating and dropping it will affect
/// the reference count of the page table frame. If dropped the raw handle as the last reference,
/// the page table frame and subsequent children will be freed.
/// This handle is a referencer of a page table node. Thus creating and dropping it will affect
/// the reference count of the page table node. If dropped the raw handle as the last reference,
/// the page table node and subsequent children will be freed.
///
/// Only the CPU or a PTE can access a page table frame using a raw handle. To access the page
/// Only the CPU or a PTE can access a page table node using a raw handle. To access the page
/// table frame from the kernel code, use the handle [`PageTableNode`].
#[derive(Debug)]
pub(super) struct RawPageTableNode<E: PageTableEntryTrait, C: PagingConstsTrait>
@ -68,6 +69,9 @@ where
/// Convert a raw handle to an accessible handle by pertaining the lock.
pub(super) fn lock(self) -> PageTableNode<E, C> {
// SAFETY: The physical address in the raw handle is valid and we are
// transferring the ownership to a new handle. No increment of the reference
// count is needed.
let page = unsafe { Page::<PageTablePageMeta<E, C>>::from_raw(self.paddr()) };
debug_assert!(page.meta().level == self.level);
// Acquire the lock.
@ -86,10 +90,7 @@ where
/// Create a copy of the handle.
pub(super) fn copy_handle(&self) -> Self {
let page = unsafe { Page::<PageTablePageMeta<E, C>>::from_raw(self.paddr()) };
let inc_ref = page.clone();
core::mem::forget(page);
core::mem::forget(inc_ref);
self.inc_ref();
Self {
raw: self.raw,
level: self.level,
@ -98,8 +99,12 @@ where
}
pub(super) fn nr_valid_children(&self) -> u16 {
// SAFETY: The physical address in the raw handle is valid and we are
// accessing the page table node. We forget the handle when finished.
let page = unsafe { Page::<PageTablePageMeta<E, C>>::from_raw(self.paddr()) };
page.meta().nr_children
let nr = page.meta().nr_children;
core::mem::forget(page);
nr
}
/// Activate the page table assuming it is a root page table.
@ -133,10 +138,7 @@ where
}
// Increment the reference count of the current page table.
let page = unsafe { Page::<PageTablePageMeta<E, C>>::from_raw(self.paddr()) };
core::mem::forget(page.clone());
core::mem::forget(page);
self.inc_ref();
// Decrement the reference count of the last activated page table.
@ -159,6 +161,14 @@ where
CURRENT_IS_BOOT_PT.store(false, Ordering::Release);
}
}
fn inc_ref(&self) {
// SAFETY: The physical address in the raw handle is valid and we are
// incrementing the reference count by cloning and forgetting.
let page = unsafe { Page::<PageTablePageMeta<E, C>>::from_raw(self.paddr()) };
core::mem::forget(page.clone());
core::mem::forget(page);
}
}
impl<E: PageTableEntryTrait, C: PagingConstsTrait> Drop for RawPageTableNode<E, C>
@ -166,17 +176,19 @@ where
[(); C::NR_LEVELS as usize]:,
{
fn drop(&mut self) {
// SAFETY: The physical address in the raw handle is valid. The restored
// handle is dropped to decrement the reference count.
drop(unsafe { Page::<PageTablePageMeta<E, C>>::from_raw(self.paddr()) });
}
}
/// A mutable handle to a page table frame.
/// A mutable handle to a page table node.
///
/// The page table frame can own a set of handles to children, ensuring that the children
/// don't outlive the page table frame. Cloning a page table frame will create a deep copy
/// of the page table. Dropping the page table frame will also drop all handles if the page
/// table frame has no references. You can set the page table frame as a child of another
/// page table frame.
/// The page table node can own a set of handles to children, ensuring that the children
/// don't outlive the page table node. Cloning a page table node will create a deep copy
/// of the page table. Dropping the page table node will also drop all handles if the page
/// table frame has no references. You can set the page table node as a child of another
/// page table node.
#[derive(Debug)]
pub(super) struct PageTableNode<
E: PageTableEntryTrait = PageTableEntry,
@ -187,7 +199,7 @@ pub(super) struct PageTableNode<
pub(super) page: Page<PageTablePageMeta<E, C>>,
}
/// A child of a page table frame.
/// A child of a page table node.
#[derive(Debug)]
pub(super) enum Child<E: PageTableEntryTrait = PageTableEntry, C: PagingConstsTrait = PagingConsts>
where
@ -204,7 +216,7 @@ impl<E: PageTableEntryTrait, C: PagingConstsTrait> PageTableNode<E, C>
where
[(); C::NR_LEVELS as usize]:,
{
/// Allocate a new empty page table frame.
/// Allocate a new empty page table node.
///
/// This function returns an owning handle. The newly created handle does not
/// set the lock bit for performance as it is exclusive and unlocking is an
@ -218,9 +230,12 @@ where
// SAFETY: here the page exclusively owned by the newly created handle.
unsafe { page.meta_mut().level = level };
// Zero out the page table frame.
// Zero out the page table node.
let ptr = paddr_to_vaddr(page.paddr()) as *mut u8;
// SAFETY: The page is exclusively owned here. Pointers are valid also.
// We rely on the fact that 0 represents an absent entry to speed up `memset`.
unsafe { core::ptr::write_bytes(ptr, 0, PAGE_SIZE) };
debug_assert!(E::new_absent().as_bytes().iter().all(|&b| b == 0));
Self { page }
}
@ -261,6 +276,9 @@ where
} else {
let paddr = pte.paddr();
if !pte.is_last(self.level()) {
// SAFETY: The physical address is recorded in a valid PTE
// which would be casted from a handle. We are incrementing
// the reference count so we restore, clone, and forget both.
let node = unsafe { Page::<PageTablePageMeta<E, C>>::from_raw(paddr) };
let inc_ref = node.clone();
core::mem::forget(node);
@ -271,6 +289,9 @@ where
_phantom: PhantomData,
})
} else if tracked {
// SAFETY: The physical address is recorded in a valid PTE
// which would be casted from a handle. We are incrementing
// the reference count so we restore and forget a cloned one.
let page = unsafe { Page::<FrameMeta>::from_raw(paddr) };
core::mem::forget(page.clone());
Child::Frame(Frame { page })
@ -280,7 +301,7 @@ where
}
}
/// Make a copy of the page table frame.
/// Make a copy of the page table node.
///
/// This function allows you to control about the way to copy the children.
/// For indexes in `deep`, the children are deep copied and this function will be recursively called.
@ -398,6 +419,8 @@ where
let mut new_frame = PageTableNode::<E, C>::alloc(self.level() - 1);
for i in 0..nr_subpage_per_huge::<C>() {
let small_pa = pa + i * page_size::<C>(self.level() - 1);
// SAFETY: the index is within the bound and either physical address and
// the property are valid.
unsafe { new_frame.set_child_untracked(i, small_pa, prop) };
}
self.set_child_pt(idx, new_frame.into_raw(), true);
@ -444,24 +467,31 @@ where
.write(pte.unwrap_or(E::new_absent()))
};
// Drop the child. We must set the PTE before dropping the child. To
// drop the child just restore the handle and drop the handle.
// Drop the child. We must set the PTE before dropping the child.
// Just restore the handle and drop the handle.
let paddr = existing_pte.paddr();
if !existing_pte.is_last(self.level()) {
// This is a page table.
drop(unsafe { Page::<PageTablePageMeta<E, C>>::from_raw(paddr) });
} else if !in_untracked_range {
// This is a frame.
drop(unsafe { Page::<FrameMeta>::from_raw(paddr) });
// SAFETY: Both the `from_raw` operations here are safe as the physical
// address is valid and casted from a handle.
unsafe {
if !existing_pte.is_last(self.level()) {
// This is a page table.
drop(Page::<PageTablePageMeta<E, C>>::from_raw(paddr));
} else if !in_untracked_range {
// This is a frame.
drop(Page::<FrameMeta>::from_raw(paddr));
}
}
// Update the child count.
if pte.is_none() {
// SAFETY: Here we have an exclusive access to the page.
unsafe { self.page.meta_mut().nr_children -= 1 };
}
} else if let Some(e) = pte {
// SAFETY: This is safe as described in the above branch.
unsafe { (self.as_ptr() as *mut E).add(idx).write(e) };
// SAFETY: Here we have an exclusive access to the page.
unsafe { self.page.meta_mut().nr_children += 1 };
}
}
@ -492,24 +522,29 @@ where
let level = page.meta().level;
// Drop the children.
for i in 0..nr_subpage_per_huge::<C>() {
// SAFETY: the index is within the bound and PTE is plain-old-data. The
// SAFETY: The index is within the bound and PTE is plain-old-data. The
// address is aligned as well. We also have an exclusive access ensured
// by reference counting.
let pte_ptr = unsafe { (paddr_to_vaddr(paddr) as *const E).add(i) };
// SAFETY: The pointer is valid and the PTE is plain-old-data.
let pte = unsafe { pte_ptr.read() };
if pte.is_present() {
// Just restore the handle and drop the handle.
if !pte.is_last(level) {
// This is a page table.
// SAFETY: The physical address must be casted from a handle to a
// page table node.
drop(unsafe { Page::<Self>::from_raw(pte.paddr()) });
} else {
// This is a frame. You cannot drop a page table node that maps to
// untracked frames. This must be verified.
// SAFETY: The physical address must be casted from a handle to a
// frame.
drop(unsafe { Page::<FrameMeta>::from_raw(pte.paddr()) });
}
}
}
// Recycle this page table frame.
// Recycle this page table node.
FRAME_ALLOCATOR
.get()
.unwrap()