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Clean up SGX user memory copies #101150

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Oct 6, 2023
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Clean up SGX user memory copies #101150

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207 changes: 85 additions & 122 deletions library/std/src/sys/sgx/abi/usercalls/alloc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,8 @@
use crate::arch::asm;
use crate::cell::UnsafeCell;
use crate::cmp;
use crate::convert::TryInto;
use crate::intrinsics;
use crate::mem;
use crate::ops::{CoerceUnsized, Deref, DerefMut, Index, IndexMut};
use crate::ptr::{self, NonNull};
Expand Down Expand Up @@ -306,20 +308,35 @@ where
}
}

// Split a memory region ptr..ptr + len into three parts:
// +--------+
// | small0 | Chunk smaller than 8 bytes
// +--------+
// | big | Chunk 8-byte aligned, and size a multiple of 8 bytes
// +--------+
// | small1 | Chunk smaller than 8 bytes
// +--------+
fn region_as_aligned_chunks(ptr: *const u8, len: usize) -> (usize, usize, usize) {
let small0_size = if ptr.is_aligned_to(8) { 0 } else { 8 - ptr.addr() % 8 };
let small1_size = (len - small0_size) % 8;
let big_size = len - small0_size - small1_size;

(small0_size, big_size, small1_size)
/// Divide the slice `(ptr, len)` into three parts, where the middle part is
/// aligned to `u64`.
///
/// The return values `(prefix_len, mid_len, suffix_len)` add back up to `len`.
/// The return values are such that the memory region `(ptr + prefix_len,
/// mid_len)` is the largest possible region where `ptr + prefix_len` is aligned
/// to `u64` and `mid_len` is a multiple of the byte size of `u64`. This means
/// that `prefix_len` and `suffix_len` are guaranteed to be less than the byte
/// size of `u64`, and that `(ptr, prefix_len)` and `(ptr + prefix_len +
/// mid_len, suffix_len)` don't straddle an alignment boundary.
// Standard Rust functions such as `<[u8]>::align_to::<u64>` and
// `<*const u8>::align_offset` aren't _guaranteed_ to compute the largest
// possible middle region, and as such can't be used.
fn u64_align_to_guaranteed(ptr: *const u8, mut len: usize) -> (usize, usize, usize) {
const QWORD_SIZE: usize = mem::size_of::<u64>();

let offset = ptr as usize % QWORD_SIZE;

let prefix_len = if intrinsics::unlikely(offset > 0) { QWORD_SIZE - offset } else { 0 };
Comment on lines +325 to +329
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It's somewhat challenging to audit this code for correctness if it's handrolling is_aligned_to and the like. Is there a justification for this being written this way? If there is, it would be good to leave a comment here, so some helpful Rustacean doesn't open a PR to "clean it up".

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Thank you.

I feel like this pattern should be a stdlib function, to be honest, given how often I see it, but this is hardly the time or place to start hashing out new public API for core::ptr.


len = match len.checked_sub(prefix_len) {
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Some(remaining_len) => remaining_len,
None => return (len, 0, 0),
};

let suffix_len = len % QWORD_SIZE;
len -= suffix_len;

(prefix_len, len, suffix_len)
}

unsafe fn copy_quadwords(src: *const u8, dst: *mut u8, len: usize) {
Expand Down Expand Up @@ -352,7 +369,13 @@ unsafe fn copy_quadwords(src: *const u8, dst: *mut u8, len: usize) {
/// - https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00615.html
/// - https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/processor-mmio-stale-data-vulnerabilities.html#inpage-nav-3-2-2
pub(crate) unsafe fn copy_to_userspace(src: *const u8, dst: *mut u8, len: usize) {
unsafe fn copy_bytewise_to_userspace(src: *const u8, dst: *mut u8, len: usize) {
/// Like `ptr::copy(src, dst, len)`, except it uses the Intel-recommended
/// instruction sequence for unaligned writes.
unsafe fn write_bytewise_to_userspace(src: *const u8, dst: *mut u8, len: usize) {
if intrinsics::likely(len == 0) {
return;
}

unsafe {
let mut seg_sel: u16 = 0;
for off in 0..len {
Expand Down Expand Up @@ -380,41 +403,15 @@ pub(crate) unsafe fn copy_to_userspace(src: *const u8, dst: *mut u8, len: usize)
assert!(!src.addr().overflowing_add(len).1);
assert!(!dst.addr().overflowing_add(len).1);

if len < 8 {
// Can't align on 8 byte boundary: copy safely byte per byte
unsafe {
copy_bytewise_to_userspace(src, dst, len);
}
} else if len % 8 == 0 && dst.is_aligned_to(8) {
// Copying 8-byte aligned quadwords: copy quad word per quad word
unsafe {
copy_quadwords(src, dst, len);
}
} else {
// Split copies into three parts:
// +--------+
// | small0 | Chunk smaller than 8 bytes
// +--------+
// | big | Chunk 8-byte aligned, and size a multiple of 8 bytes
// +--------+
// | small1 | Chunk smaller than 8 bytes
// +--------+
let (small0_size, big_size, small1_size) = region_as_aligned_chunks(dst, len);
unsafe {
let (len1, len2, len3) = u64_align_to_guaranteed(dst, len);
let (src1, dst1) = (src, dst);
let (src2, dst2) = (src1.add(len1), dst1.add(len1));
let (src3, dst3) = (src2.add(len2), dst2.add(len2));

unsafe {
// Copy small0
copy_bytewise_to_userspace(src, dst, small0_size);

// Copy big
let big_src = src.add(small0_size);
let big_dst = dst.add(small0_size);
copy_quadwords(big_src, big_dst, big_size);

// Copy small1
let small1_src = src.add(big_size + small0_size);
let small1_dst = dst.add(big_size + small0_size);
copy_bytewise_to_userspace(small1_src, small1_dst, small1_size);
}
write_bytewise_to_userspace(src1, dst1, len1);
copy_quadwords(src2, dst2, len2);
write_bytewise_to_userspace(src3, dst3, len3);
}
}

Expand All @@ -434,87 +431,53 @@ pub(crate) unsafe fn copy_to_userspace(src: *const u8, dst: *mut u8, len: usize)
/// - https://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00657.html
/// - https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/advisory-guidance/stale-data-read-from-xapic.html
pub(crate) unsafe fn copy_from_userspace(src: *const u8, dst: *mut u8, len: usize) {
// Copies memory region `src..src + len` to the enclave at `dst`. The source memory region
// is:
// - strictly less than 8 bytes in size and may be
// - located at a misaligned memory location
fn copy_misaligned_chunk_to_enclave(src: *const u8, dst: *mut u8, len: usize) {
let mut tmp_buff = [0u8; 16];
/// Like `ptr::copy(src, dst, len)`, except it uses only u64-aligned reads.
///
/// # Safety
/// The source memory region must not straddle an alignment boundary.
unsafe fn read_misaligned_from_userspace(src: *const u8, dst: *mut u8, len: usize) {
if intrinsics::likely(len == 0) {
return;
}

unsafe {
// Compute an aligned memory region to read from
// +--------+ <-- aligned_src + aligned_len (8B-aligned)
// | pad1 |
// +--------+ <-- src + len (misaligned)
// | |
// | |
// | |
// +--------+ <-- src (misaligned)
// | pad0 |
// +--------+ <-- aligned_src (8B-aligned)
let pad0_size = src as usize % 8;
let aligned_src = src.sub(pad0_size);

let pad1_size = 8 - (src.add(len) as usize % 8);
let aligned_len = pad0_size + len + pad1_size;

debug_assert!(len < 8);
debug_assert_eq!(aligned_src as usize % 8, 0);
debug_assert_eq!(aligned_len % 8, 0);
debug_assert!(aligned_len <= 16);

// Copy the aligned buffer to a temporary buffer
// Note: copying from a slightly different memory location is a bit odd. In this case it
// can't lead to page faults or inadvertent copying from the enclave as we only ensured
// that the `src` pointer is aligned at an 8 byte boundary. As pages are 4096 bytes
// aligned, `aligned_src` must be on the same page as `src`. A similar argument can be made
// for `src + len`
copy_quadwords(aligned_src as _, tmp_buff.as_mut_ptr(), aligned_len);

// Copy the correct parts of the temporary buffer to the destination
ptr::copy(tmp_buff.as_ptr().add(pad0_size), dst, len);
let offset: usize;
let data: u64;
// doing a memory read that's potentially out of bounds for `src`,
// this isn't supported by Rust, so have to use assembly
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asm!("
movl {src:e}, {offset:e}
andl $7, {offset:e}
andq $-8, {src}
movq ({src}), {dst}
",
src = inout(reg) src => _,
offset = out(reg) offset,
dst = out(reg) data,
options(nostack, att_syntax, readonly, pure)
);
let data = data.to_le_bytes();
ptr::copy_nonoverlapping(data.as_ptr().add(offset), dst, len);
}
}

assert!(!src.is_null());
assert!(!dst.is_null());
assert!(is_user_range(src, len));
assert!(is_enclave_range(dst, len));
assert!(!(src as usize).overflowing_add(len + 8).1);
assert!(!(dst as usize).overflowing_add(len + 8).1);
assert!(len < isize::MAX as usize);
assert!(!(src as usize).overflowing_add(len).1);
assert!(!(dst as usize).overflowing_add(len).1);

if len < 8 {
copy_misaligned_chunk_to_enclave(src, dst, len);
} else if len % 8 == 0 && src as usize % 8 == 0 {
// Copying 8-byte aligned quadwords: copy quad word per quad word
unsafe {
copy_quadwords(src, dst, len);
}
} else {
// Split copies into three parts:
// +--------+
// | small0 | Chunk smaller than 8 bytes
// +--------+
// | big | Chunk 8-byte aligned, and size a multiple of 8 bytes
// +--------+
// | small1 | Chunk smaller than 8 bytes
// +--------+
let (small0_size, big_size, small1_size) = region_as_aligned_chunks(dst, len);
unsafe {
let (len1, len2, len3) = u64_align_to_guaranteed(src, len);
let (src1, dst1) = (src, dst);
let (src2, dst2) = (src1.add(len1), dst1.add(len1));
let (src3, dst3) = (src2.add(len2), dst2.add(len2));

unsafe {
// Copy small0
copy_misaligned_chunk_to_enclave(src, dst, small0_size);

// Copy big
let big_src = src.add(small0_size);
let big_dst = dst.add(small0_size);
copy_quadwords(big_src, big_dst, big_size);

// Copy small1
let small1_src = src.add(big_size + small0_size);
let small1_dst = dst.add(big_size + small0_size);
copy_misaligned_chunk_to_enclave(small1_src, small1_dst, small1_size);
}
read_misaligned_from_userspace(src1, dst1, len1);
copy_quadwords(src2, dst2, len2);
read_misaligned_from_userspace(src3, dst3, len3);
}
}

Expand Down Expand Up @@ -609,9 +572,9 @@ where
/// Copies the value from user memory into enclave memory.
pub fn to_enclave(&self) -> T {
unsafe {
let mut data: T = mem::MaybeUninit::uninit().assume_init();
copy_from_userspace(self.0.get() as _, &mut data as *mut T as _, mem::size_of::<T>());
data
let mut data = mem::MaybeUninit::uninit();
copy_from_userspace(self.0.get() as _, data.as_mut_ptr() as _, mem::size_of::<T>());
data.assume_init()
Comment on lines +575 to +577
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This alone is a necessary improvement.

}
}
}
Expand Down
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