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Fix some page management and fault handler bugs #734

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Fix some page management and fault handler bugs #734

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6ffab3a
fix remap_pages bugs
Mar 17, 2026
f8e6710
fix some other page flag bugs and suppress panics
Mar 17, 2026
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2 changes: 2 additions & 0 deletions litebox/src/mm/linux.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1055,4 +1055,6 @@ pub enum PageFaultError {
AllocationFailed,
#[error("given page is part of an already mapped huge page")]
HugePage,
#[error("invalid frame address: {0:#x}")]
InvalidFrameAddress(u64),
}
12 changes: 12 additions & 0 deletions litebox/src/platform/page_mgmt.rs
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Expand Up @@ -235,6 +235,8 @@ pub enum DeallocationError {
Unaligned,
#[error("provided range contains unallocated pages")]
AlreadyUnallocated,
#[error("invalid frame address: {0:#x}")]
InvalidFrameAddress(u64),
}

/// Possible errors for [`PageManagementProvider::remap_pages`]
Expand All @@ -251,6 +253,12 @@ pub enum RemapError {
AlreadyAllocated,
#[error("out of memory")]
OutOfMemory,
#[error("operation encountered a huge page")]
HugePage,
#[error("invalid frame address: {0:#x}")]
InvalidFrameAddress(u64),
#[error("failed to restore original mapping after remap failure (frame leaked)")]
RestoreFailed,
}

/// Possible errors for [`PageManagementProvider::update_permissions`]
Expand All @@ -261,6 +269,10 @@ pub enum PermissionUpdateError {
Unaligned,
#[error("provided range contains unallocated pages")]
Unallocated,
#[error("operation encountered a huge page")]
HugePage,
#[error("invalid frame address: {0:#x}")]
InvalidFrameAddress(u64),
}

/// Possible errors for [`PageManagementProvider::try_allocate_cow_pages`]
Expand Down
70 changes: 53 additions & 17 deletions litebox_platform_linux_kernel/src/arch/x86/mm/paging.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -174,17 +174,36 @@ impl<M: MemoryProvider, const ALIGN: usize> X64PageTable<'_, M, ALIGN> {
Ok((frame, fl)) => {
match unsafe { inner.map_to(new_start, frame, flags, &mut allocator) } {
Ok(_) => {}
Err(e) => match e {
MapToError::PageAlreadyMapped(_) => {
return Err(page_mgmt::RemapError::AlreadyAllocated);
}
MapToError::ParentEntryHugePage => {
todo!("return Err(page_mgmt::RemapError::RemapToHugePage);")
Err(e) => {
// Restore the original mapping so the frame is not leaked.
// Safety: `start` was just unmapped so its PTE slot is free.
if let Ok(restore_flush) =
unsafe { inner.map_to(start, frame, flags, &mut allocator) }
{
if FLUSH_TLB {
restore_flush.flush();
}
} else {
// The frame is now orphaned — we cannot recover it.
debug_assert!(
false,
"remap_pages: failed to restore mapping, frame leaked"
);
return Err(page_mgmt::RemapError::RestoreFailed);
}
MapToError::FrameAllocationFailed => {
return Err(page_mgmt::RemapError::OutOfMemory);
}
},

return match e {
MapToError::PageAlreadyMapped(_) => {
Err(page_mgmt::RemapError::AlreadyAllocated)
}
MapToError::ParentEntryHugePage => {
Err(page_mgmt::RemapError::HugePage)
}
MapToError::FrameAllocationFailed => {
Err(page_mgmt::RemapError::OutOfMemory)
}
};
}
}
if FLUSH_TLB {
fl.flush();
Expand All @@ -194,15 +213,15 @@ impl<M: MemoryProvider, const ALIGN: usize> X64PageTable<'_, M, ALIGN> {
unreachable!()
}
Err(X64UnmapError::ParentEntryHugePage) => {
todo!("return Err(page_mgmt::RemapError::RemapToHugePage);")
return Err(page_mgmt::RemapError::HugePage);
}
Err(X64UnmapError::InvalidFrameAddress(pa)) => {
panic!("Invalid frame address: {:#x}", pa);
return Err(page_mgmt::RemapError::InvalidFrameAddress(pa.as_u64()));
}
},
TranslateResult::NotMapped => {}
TranslateResult::InvalidFrameAddress(pa) => {
panic!("Invalid frame address: {:#x}", pa);
return Err(page_mgmt::RemapError::InvalidFrameAddress(pa.as_u64()));
}
}
start += 1;
Expand Down Expand Up @@ -234,15 +253,18 @@ impl<M: MemoryProvider, const ALIGN: usize> X64PageTable<'_, M, ALIGN> {
offset: _,
flags,
} => {
// If it is changed to writable, we leave it to page fault handler (COW)
// If write access is requested on a read-only mapping, leave the PTE
// read-only for now and let the write-fault path enable writes lazily.
// Keep this split explicit so a future real COW implementation can hook
// into the same fault path.
let change_to_write = new_flags.contains(PageTableFlags::WRITABLE)
&& !flags.contains(PageTableFlags::WRITABLE);
let new_flags = if change_to_write {
new_flags - PageTableFlags::WRITABLE
} else {
new_flags
};
if flags != new_flags {
if flags & Self::MPROTECT_PTE_MASK != new_flags {
match unsafe {
inner.update_flags(page, (flags & !Self::MPROTECT_PTE_MASK) | new_flags)
} {
Expand Down Expand Up @@ -308,8 +330,22 @@ impl<M: MemoryProvider, const ALIGN: usize> PageTableImpl<ALIGN> for X64PageTabl
// probably set by other threads concurrently
return Ok(());
} else {
// Copy-on-Write
todo!("COW");
// This is the deferred write-enable path used by mprotect today, not a
// real COW implementation. Keep the write-fault handling separate so a
// future COW path can replace this flag update.
match unsafe { inner.update_flags(page, flags | PageTableFlags::WRITABLE) }
{
Ok(flush) => {
if FLUSH_TLB {
flush.flush();
}
return Ok(());
}
Err(FlagUpdateError::PageNotMapped) => unreachable!(),
Err(FlagUpdateError::ParentEntryHugePage) => {
return Err(PageFaultError::HugePage);
}
}
}
}

Expand Down
2 changes: 1 addition & 1 deletion litebox_platform_linux_kernel/src/lib.rs
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Expand Up @@ -466,7 +466,7 @@ impl<Host: HostInterface, const ALIGN: usize> PageManagementProvider<ALIGN> for
.ok_or(litebox::platform::page_mgmt::RemapError::Unaligned)?;
let new_range = PageRange::new(new_range.start, new_range.end)
.ok_or(litebox::platform::page_mgmt::RemapError::Unaligned)?;
if old_range.start.max(new_range.start) <= old_range.end.min(new_range.end) {
if old_range.start.max(new_range.start) < old_range.end.min(new_range.end) {
return Err(litebox::platform::page_mgmt::RemapError::Overlapping);
}
unsafe { self.page_table.remap_pages(old_range, new_range) }
Expand Down
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