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//! RDRAND backend for x86(-64) targets use core::mem::{size_of, MaybeUninit}; use core::num::NonZeroU32; use core::sync::atomic::{AtomicUsize, Ordering}; #[inline(always)] fn slice_as_uninit<T>(slice: &[T]) -> &[MaybeUninit<T>] { let ptr = slice as *const [T] as *const [MaybeUninit<T>]; // SAFETY: `MaybeUninit<T>` is guaranteed to be layout-compatible with `T`. unsafe { &*ptr } } struct LazyUsize(AtomicUsize); impl LazyUsize { // The initialization is not completed. const UNINIT: usize = usize::max_value(); pub const fn new() -> Self { Self(AtomicUsize::new(Self::UNINIT)) } // Runs the init() function at most once, returning the value of some run of // init(). Multiple callers can run their init() functions in parallel. // init() should always return the same value, if it succeeds. pub fn unsync_init(&self, init: impl FnOnce() -> usize) -> usize { // Relaxed ordering is fine, as we only have a single atomic variable. let mut val = self.0.load(Ordering::Relaxed); if val == Self::UNINIT { val = init(); self.0.store(val, Ordering::Relaxed); } val } } struct LazyBool(LazyUsize); impl LazyBool { const fn new() -> Self { Self(LazyUsize::new()) } fn unsync_init(&self, init: impl FnOnce() -> bool) -> bool { self.0.unsync_init(|| usize::from(init())) != 0 } } use core::arch::x86_64 as arch; use arch::_rdrand64_step as rdrand_step; type Word = u64; const fn internal_error(n: u16) -> Error { // SAFETY: code > 0 as INTERNAL_START > 0 and adding n won't overflow a u32. let code = Error::INTERNAL_START + (n as u32); Error(unsafe { NonZeroU32::new_unchecked(code) }) } pub struct Error(NonZeroU32); impl Error { const INTERNAL_START: u32 = 1 << 31; const FAILED_RDRAND: Self = internal_error(5); const NO_RDRAND: Self = internal_error(6); } // Recommendation from "Intel® Digital Random Number Generator (DRNG) Software // Implementation Guide" - Section 5.2.1 and "Intel® 64 and IA-32 Architectures // Software Developer’s Manual" - Volume 1 - Section 7.3.17.1. const RETRY_LIMIT: usize = 10; #[target_feature(enable = "rdrand")] unsafe fn rdrand() -> Option<Word> { for _ in 0..RETRY_LIMIT { let mut val = 0; if rdrand_step(&mut val) == 1 { return Some(val); } } None } // "rdrand" target feature requires "+rdrand" flag, see https://github.com/rust-lang/rust/issues/49653. #[cfg(all(target_env = "sgx", not(target_feature = "rdrand")))] compile_error!( "SGX targets require 'rdrand' target feature. Enable by using -C target-feature=+rdrand." ); // Run a small self-test to make sure we aren't repeating values // Adapted from Linux's test in arch/x86/kernel/cpu/rdrand.c // Fails with probability < 2^(-90) on 32-bit systems #[target_feature(enable = "rdrand")] unsafe fn self_test() -> bool { // On AMD, RDRAND returns 0xFF...FF on failure, count it as a collision. let mut prev = !0; // TODO(MSRV 1.43): Move to usize::MAX let mut fails = 0; for _ in 0..8 { match rdrand() { Some(val) if val == prev => fails += 1, Some(val) => prev = val, None => return false, }; } fails <= 2 } fn is_rdrand_good() -> bool { #[cfg(not(target_feature = "rdrand"))] { // SAFETY: All Rust x86 targets are new enough to have CPUID, and we // check that leaf 1 is supported before using it. let cpuid0 = unsafe { arch::__cpuid(0) }; if cpuid0.eax < 1 { return false; } let cpuid1 = unsafe { arch::__cpuid(1) }; let vendor_id = [ cpuid0.ebx.to_le_bytes(), cpuid0.edx.to_le_bytes(), cpuid0.ecx.to_le_bytes(), ]; if vendor_id == [*b"Auth", *b"enti", *b"cAMD"] { let mut family = (cpuid1.eax >> 8) & 0xF; if family == 0xF { family += (cpuid1.eax >> 20) & 0xFF; } // AMD CPUs families before 17h (Zen) sometimes fail to set CF when // RDRAND fails after suspend. Don't use RDRAND on those families. // See https://bugzilla.redhat.com/show_bug.cgi?id=1150286 if family < 0x17 { return false; } } const RDRAND_FLAG: u32 = 1 << 30; if cpuid1.ecx & RDRAND_FLAG == 0 { return false; } } // SAFETY: We have already checked that rdrand is available. unsafe { self_test() } } pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> { static RDRAND_GOOD: LazyBool = LazyBool::new(); if !RDRAND_GOOD.unsync_init(is_rdrand_good) { return Err(Error::NO_RDRAND); } // SAFETY: After this point, we know rdrand is supported. unsafe { rdrand_exact(dest) }.ok_or(Error::FAILED_RDRAND) } // TODO: make this function safe when we have feature(target_feature_11) #[target_feature(enable = "rdrand")] unsafe fn rdrand_exact(dest: &mut [MaybeUninit<u8>]) -> Option<()> { // We use chunks_exact_mut instead of chunks_mut as it allows almost all // calls to memcpy to be elided by the compiler. let mut chunks = dest.chunks_exact_mut(size_of::<Word>()); for chunk in chunks.by_ref() { let src = rdrand()?.to_ne_bytes(); chunk.copy_from_slice(slice_as_uninit(&src)); } let tail = chunks.into_remainder(); let n = tail.len(); if n > 0 { let src = rdrand()?.to_ne_bytes(); tail.copy_from_slice(slice_as_uninit(&src[..n])); } Some(()) }
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