On Wed, Apr 17, 2024 at 5:27 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote: > > On 17.04.24 16:40, Alice Ryhl wrote: > > On Wed, Apr 17, 2024 at 4:28 PM Gary Guo <gary@xxxxxxxxxxx> wrote: > >> > >> On Mon, 15 Apr 2024 07:13:53 +0000 > >> Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote: > >> > >>> From: Wedson Almeida Filho <wedsonaf@xxxxxxxxx> > >>> > >>> A pointer to an area in userspace memory, which can be either read-only > >>> or read-write. > >>> > >>> All methods on this struct are safe: attempting to read or write on bad > >>> addresses (either out of the bound of the slice or unmapped addresses) > >>> will return `EFAULT`. Concurrent access, *including data races to/from > >>> userspace memory*, is permitted, because fundamentally another userspace > >>> thread/process could always be modifying memory at the same time (in the > >>> same way that userspace Rust's `std::io` permits data races with the > >>> contents of files on disk). In the presence of a race, the exact byte > >>> values read/written are unspecified but the operation is well-defined. > >>> Kernelspace code should validate its copy of data after completing a > >>> read, and not expect that multiple reads of the same address will return > >>> the same value. > >>> > >>> These APIs are designed to make it difficult to accidentally write > >>> TOCTOU bugs. Every time you read from a memory location, the pointer is > >>> advanced by the length so that you cannot use that reader to read the > >>> same memory location twice. Preventing double-fetches avoids TOCTOU > >>> bugs. This is accomplished by taking `self` by value to prevent > >>> obtaining multiple readers on a given `UserSlicePtr`, and the readers > >>> only permitting forward reads. If double-fetching a memory location is > >>> necessary for some reason, then that is done by creating multiple > >>> readers to the same memory location. > >>> > >>> Constructing a `UserSlicePtr` performs no checks on the provided > >>> address and length, it can safely be constructed inside a kernel thread > >>> with no current userspace process. Reads and writes wrap the kernel APIs > >>> `copy_from_user` and `copy_to_user`, which check the memory map of the > >>> current process and enforce that the address range is within the user > >>> range (no additional calls to `access_ok` are needed). > >>> > >>> This code is based on something that was originally written by Wedson on > >>> the old rust branch. It was modified by Alice by removing the > >>> `IoBufferReader` and `IoBufferWriter` traits, and various other changes. > >>> > >>> Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx> > >>> Co-developed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx> > >>> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx> > >>> --- > >>> rust/helpers.c | 14 +++ > >>> rust/kernel/lib.rs | 1 + > >>> rust/kernel/uaccess.rs | 304 +++++++++++++++++++++++++++++++++++++++++++++++++ > >>> 3 files changed, 319 insertions(+) > >>> > >>> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs > >> > >>> +/// [`std::io`]: https://doc.rust-lang.org/std/io/index.html > >>> +/// [`clone_reader`]: UserSliceReader::clone_reader > >>> +pub struct UserSlice { > >>> + ptr: *mut c_void, > >>> + length: usize, > >>> +} > >> > >> How useful is the `c_void` in the struct and new signature? They tend > >> to not be very useful in Rust. Given that provenance doesn't matter > >> for userspace pointers, could this be `usize` simply? > >> > >> I think `*mut u8` or `*mut ()` makes more sense than `*mut c_void` for > >> Rust code even if we don't want to use `usize`. > > > > I don't have a strong opinion here. I suppose a usize could make > > sense. But I also think c_void is fine, and I lean towards not > > changing it. :) > > > >> Some thinking aloud and brainstorming bits about the API. > >> > >> I wonder if it make sense to have `User<[u8]>` instead of `UserSlice`? > >> The `User` type can be defined like this: > >> > >> ```rust > >> struct User<T: ?Sized> { > >> ptr: *mut T, > >> } > >> ``` > >> > >> and this allows arbitrary T as long as it's POD. So we could have > >> `User<[u8]>`, `User<u32>`, `User<PodStruct>`. I imagine the > >> `User<[u8]>` would be the general usage and the latter ones can be > >> especially helpful if you are trying to implement ioctl and need to > >> copy fixed size data structs from userspace. > > > > Hmm, we have to be careful here. Generally, when you get a user slice > > via an ioctl, you should make sure to use the length you get from > > userspace. In binder, it looks like this: > > > > let user_slice = UserSlice::new(arg, _IOC_SIZE(cmd)); > > > > so whichever API we use, we must make sure to get the length as an > > argument in bytes. What should we do if the length is not a multiple > > of size_of(T)? > > We could print a warning and then just floor to the next multiple of > `size_of::<T>()`. I agree that is not perfect, but if one uses the > current API, one also needs to do the length check eventually. Right now, the length check happens when you call `read::<T>` and get EFAULT if the size of T is greater than the length of the user slice. That works pretty well. And there are real-world use-cases for userspace passing in a length longer than what the kernel expects - often adding fields to the end of the struct is how the kernel makes ioctls extensible. So I don't think printing a warning in that case would be good. In Binder, I also have use-cases where I alternate between reading bytes and various different structs. Basically, I read two user slices in lockstep, where the next value in one userslice determines whether I should read some amount of bytes or a specific struct from the other user slice. That's much easier with the current API than this proposal. > > Another issue is that there's no stable way to get the length from a > > `*mut [T]` without creating a reference, which is not okay for a user > > slice. > > Seems like `<* const [T]>::len` (feature `slice_ptr_len`) [1] was just > stabilized 5 days ago [1]. > > [1]: https://doc.rust-lang.org/std/primitive.pointer.html#method.len-1 > [2]: https://github.com/rust-lang/rust/pull/123868 Okay. Alice
