Re: [PATCH v8 6/7] mm: rust: add VmAreaNew

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Nov 20, 2024 at 02:50:00PM +0000, Alice Ryhl wrote:
> When setting up a new vma in an mmap call, you have a bunch of extra
> permissions that you normally don't have. This introduces a new
> VmAreaNew type that is used for this case.

Hm I'm confused by what you mean here. What permissions do you mean?

Is this to abstract a VMA as passed by f_op->mmap()? I think it would be
better to explicitly say this if so.

>
> To avoid setting invalid flag values, the methods for clearing
> VM_MAYWRITE and similar involve a check of VM_WRITE, and return an error
> if VM_WRITE is set. Trying to use `try_clear_maywrite` without checking
> the return value results in a compilation error because the `Result`
> type is marked #[must_use].

This is nice.

Though note that, it is explicitly not permitted to permit writability for
a VMA that previously had it disallowed, and we explicitly WARN_ON() this
now. Concretely that means a VMA where !(vma->vm_flags & VM_MAYWRITE), you
must not vma->vm_flags |= VM_MAYWRITE.

>
> For now, there's only a method for VM_MIXEDMAP and not VM_PFNMAP. When
> we add a VM_PFNMAP method, we will need some way to prevent you from
> setting both VM_MIXEDMAP and VM_PFNMAP on the same vma.

Yes this would be unwise.

An aside here - really you should _only_ change flags in this hook (perhaps
of course also initialising vma->vm_private_data state), trying to change
anything _core_ here would be deeply dangerous.

We are far too permissive with this right now, and it's something we want
to try ideally to limit in the future.

>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  rust/kernel/mm/virt.rs | 169 ++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 168 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/mm/virt.rs b/rust/kernel/mm/virt.rs
> index de7f2338810a..22aff8e77854 100644
> --- a/rust/kernel/mm/virt.rs
> +++ b/rust/kernel/mm/virt.rs
> @@ -6,7 +6,7 @@
>
>  use crate::{
>      bindings,
> -    error::{to_result, Result},
> +    error::{code::EINVAL, to_result, Result},
>      page::Page,
>      types::Opaque,
>  };
> @@ -148,6 +148,173 @@ pub fn vm_insert_page(&self, address: usize, page: &Page) -> Result {
>      }
>  }
>
> +/// A builder for setting up a vma in an `mmap` call.

Would be better to explicitly reference the struct file_operations->mmap()
hook and to say that we should only be updating flags and vm_private_data
here (though perhaps not worth mentioning _that_ if not explicitly exposed
by your interface).

I'm guessing fields are, unless a setter/builder is established, immutable?

> +///
> +/// # Invariants
> +///
> +/// For the duration of 'a, the referenced vma must be undergoing initialization.
> +pub struct VmAreaNew {
> +    vma: VmAreaRef,
> +}
> +
> +// Make all `VmAreaRef` methods available on `VmAreaNew`.
> +impl Deref for VmAreaNew {
> +    type Target = VmAreaRef;
> +
> +    #[inline]
> +    fn deref(&self) -> &VmAreaRef {
> +        &self.vma
> +    }
> +}
> +
> +impl VmAreaNew {
> +    /// Access a virtual memory area given a raw pointer.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that `vma` is undergoing initial vma setup for the duration of 'a.
> +    #[inline]
> +    pub unsafe fn from_raw<'a>(vma: *const bindings::vm_area_struct) -> &'a Self {
> +        // SAFETY: The caller ensures that the invariants are satisfied for the duration of 'a.
> +        unsafe { &*vma.cast() }
> +    }
> +
> +    /// Internal method for updating the vma flags.
> +    ///
> +    /// # Safety
> +    ///
> +    /// This must not be used to set the flags to an invalid value.
> +    #[inline]
> +    unsafe fn update_flags(&self, set: vm_flags_t, unset: vm_flags_t) {
> +        let mut flags = self.flags();
> +        flags |= set;
> +        flags &= !unset;
> +
> +        // SAFETY: This is not a data race: the vma is undergoing initial setup, so it's not yet
> +        // shared. Additionally, `VmAreaNew` is `!Sync`, so it cannot be used to write in parallel.
> +        // The caller promises that this does not set the flags to an invalid value.
> +        unsafe { (*self.as_ptr()).__bindgen_anon_2.vm_flags = flags };

Hm not sure if this is correct. We explicitly maintain a union in struct vm_area_struct as:

	union {
		const vm_flags_t vm_flags;
		vm_flags_t __private __vm_flags;
	};

Where vma->vm_flags is const, and then use helpers like vm_flags_init() to
set them, which also do things like assert locks (though not in the init
case, of course).

So erally we should at least be updating __vm_flags here, though I'm not
sure how bindgen treats it?

> +    }
> +
> +    /// Set the `VM_MIXEDMAP` flag on this vma.
> +    ///
> +    /// This enables the vma to contain both `struct page` and pure PFN pages. Returns a reference
> +    /// that can be used to call `vm_insert_page` on the vma.
> +    #[inline]
> +    pub fn set_mixedmap(&self) -> &VmAreaMixedMap {
> +        // SAFETY: We don't yet provide a way to set VM_PFNMAP, so this cannot put the flags in an
> +        // invalid state.
> +        unsafe { self.update_flags(flags::MIXEDMAP, 0) };
> +
> +        // SAFETY: We just set `VM_MIXEDMAP` on the vma.
> +        unsafe { VmAreaMixedMap::from_raw(self.vma.as_ptr()) }
> +    }
> +
> +    /// Set the `VM_IO` flag on this vma.
> +    ///
> +    /// This marks the vma as being a memory-mapped I/O region.
> +    #[inline]
> +    pub fn set_io(&self) {
> +        // SAFETY: Setting the VM_IO flag is always okay.
> +        unsafe { self.update_flags(flags::IO, 0) };
> +    }
> +
> +    /// Set the `VM_DONTEXPAND` flag on this vma.
> +    ///
> +    /// This prevents the vma from being expanded with `mremap()`.
> +    #[inline]
> +    pub fn set_dontexpand(&self) {
> +        // SAFETY: Setting the VM_DONTEXPAND flag is always okay.
> +        unsafe { self.update_flags(flags::DONTEXPAND, 0) };
> +    }
> +
> +    /// Set the `VM_DONTCOPY` flag on this vma.
> +    ///
> +    /// This prevents the vma from being copied on fork. This option is only permanent if `VM_IO`
> +    /// is set.
> +    #[inline]
> +    pub fn set_dontcopy(&self) {
> +        // SAFETY: Setting the VM_DONTCOPY flag is always okay.
> +        unsafe { self.update_flags(flags::DONTCOPY, 0) };
> +    }
> +
> +    /// Set the `VM_DONTDUMP` flag on this vma.
> +    ///
> +    /// This prevents the vma from being included in core dumps. This option is only permanent if
> +    /// `VM_IO` is set.
> +    #[inline]
> +    pub fn set_dontdump(&self) {
> +        // SAFETY: Setting the VM_DONTDUMP flag is always okay.
> +        unsafe { self.update_flags(flags::DONTDUMP, 0) };
> +    }
> +
> +    /// Returns whether `VM_READ` is set.
> +    ///
> +    /// This flag indicates whether userspace is mapping this vma as readable.
> +    #[inline]
> +    pub fn get_read(&self) -> bool {
> +        (self.flags() & flags::READ) != 0
> +    }
> +
> +    /// Try to clear the `VM_MAYREAD` flag, failing if `VM_READ` is set.
> +    ///
> +    /// This flag indicates whether userspace is allowed to make this vma readable with
> +    /// `mprotect()`.
> +    #[inline]
> +    pub fn try_clear_mayread(&self) -> Result {
> +        if self.get_read() {
> +            return Err(EINVAL);
> +        }

This is quite nice! Strong(er) typing for the win, again :>)

> +        // SAFETY: Clearing `VM_MAYREAD` is okay when `VM_READ` is not set.
> +        unsafe { self.update_flags(0, flags::MAYREAD) };
> +        Ok(())
> +    }
> +
> +    /// Returns whether `VM_WRITE` is set.
> +    ///
> +    /// This flag indicates whether userspace is mapping this vma as writable.
> +    #[inline]
> +    pub fn get_write(&self) -> bool {
> +        (self.flags() & flags::WRITE) != 0
> +    }
> +
> +    /// Try to clear the `VM_MAYWRITE` flag, failing if `VM_WRITE` is set.
> +    ///
> +    /// This flag indicates whether userspace is allowed to make this vma writable with
> +    /// `mprotect()`.
> +    #[inline]
> +    pub fn try_clear_maywrite(&self) -> Result {
> +        if self.get_write() {
> +            return Err(EINVAL);
> +        }
> +        // SAFETY: Clearing `VM_MAYWRITE` is okay when `VM_WRITE` is not set.
> +        unsafe { self.update_flags(0, flags::MAYWRITE) };
> +        Ok(())
> +    }
> +
> +    /// Returns whether `VM_EXEC` is set.
> +    ///
> +    /// This flag indicates whether userspace is mapping this vma as executable.
> +    #[inline]
> +    pub fn get_exec(&self) -> bool {
> +        (self.flags() & flags::EXEC) != 0
> +    }
> +
> +    /// Try to clear the `VM_MAYEXEC` flag, failing if `VM_EXEC` is set.
> +    ///
> +    /// This flag indicates whether userspace is allowed to make this vma executable with
> +    /// `mprotect()`.
> +    #[inline]
> +    pub fn try_clear_mayexec(&self) -> Result {
> +        if self.get_exec() {
> +            return Err(EINVAL);
> +        }
> +        // SAFETY: Clearing `VM_MAYEXEC` is okay when `VM_EXEC` is not set.
> +        unsafe { self.update_flags(0, flags::MAYEXEC) };
> +        Ok(())
> +    }
> +}
> +
>  /// The integer type used for vma flags.
>  #[doc(inline)]
>  pub use bindings::vm_flags_t;
>
> --
> 2.47.0.371.ga323438b13-goog
>