Ensure at least page size bytes are read from /dev/{u}random and getrandom(2).

This undocumented behavior manifests in Linux and some apps depend on it.
See drivers/char/random.c:get_random_bytes_user().

Fixes #9445
Fixes #4988

PiperOrigin-RevId: 570833447

pkg/rand/rand_linux.go

@@ -54,10 +54,17 @@ type bufferedReader struct {
 
 // Read implements io.Reader.Read.
 func (b *bufferedReader) Read(p []byte) (int, error) {
+	// In Linux, reads of up to page size bytes will always complete fully.
+	// See drivers/char/random.c:get_random_bytes_user().
+	// NOTE(gvisor.dev/issue/9445): Some applications rely on this behavior.
+	const pageSize = 4096
+	min := len(p)
+	if min > pageSize {
+		min = pageSize
+	}
 	b.mu.Lock()
-	n, err := b.r.Read(p)
-	b.mu.Unlock()
-	return n, err
+	defer b.mu.Unlock()
+	return io.ReadAtLeast(b.r, p, min)
 }
 
 // Reader is the default reader.

pkg/sentry/syscalls/linux/sys_random.go

@@ -15,7 +15,6 @@
 package linux
 
 import (
-	"io"
 	"math"
 
 	"gvisor.dev/gvisor/pkg/errors/linuxerr"
@@ -57,37 +56,11 @@ func GetRandom(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintp
 		return 0, nil, linuxerr.EFAULT
 	}
 
-	// "If the urandom source has been initialized, reads of up to 256 bytes
-	// will always return as many bytes as requested and will not be
-	// interrupted by signals. No such guarantees apply for larger buffer
-	// sizes." - getrandom(2)
-	min := int(length)
-	if min > 256 {
-		min = 256
-	}
-	n, err := t.MemoryManager().CopyOutFrom(t, hostarch.AddrRangeSeqOf(ar), safemem.FromIOReader{&randReader{-1, min}}, usermem.IOOpts{
+	n, err := t.MemoryManager().CopyOutFrom(t, hostarch.AddrRangeSeqOf(ar), safemem.FromIOReader{rand.Reader}, usermem.IOOpts{
 		AddressSpaceActive: true,
 	})
-	if n >= int64(min) {
+	if n > 0 {
 		return uintptr(n), nil, nil
 	}
 	return 0, nil, err
 }
-
-// randReader is a io.Reader that handles partial reads from rand.Reader.
-type randReader struct {
-	done int
-	min  int
-}
-
-// Read implements io.Reader.Read.
-func (r *randReader) Read(dst []byte) (int, error) {
-	if r.done >= r.min {
-		return rand.Reader.Read(dst)
-	}
-	min := r.min - r.done
-	if min > len(dst) {
-		min = len(dst)
-	}
-	return io.ReadAtLeast(rand.Reader, dst, min)
-}