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crypto/rand: use blocking getrandom call on Linux when supported
By changing getRandomLinux to immediately use the getrandom() syscall without GRND_NONBLOCK, we now only fall back to reading from /dev/urandom on Linux if the kernel does not support the getrandom() syscall. This means reads for crypto/rand will now block if the kernel has insufficient entropy on Linux kernels after v3.16. Before, if the kernel had insufficient entropy, it would fall back to reading from /dev/urandom. This would potentially return predictable data. Fixes #19274 Change-Id: I1cb081ce2f3096f18ad2820e52ecdbd993dc2afc Reviewed-on: https://go-review.googlesource.com/43852 Reviewed-by: Filippo Valsorda <hi@filippo.io> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org>
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@ -6,34 +6,20 @@ package rand
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import (
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import (
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"internal/syscall/unix"
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"internal/syscall/unix"
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"sync"
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)
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)
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func init() {
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func init() {
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altGetRandom = getRandomLinux
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altGetRandom = getRandomLinux
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}
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}
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var (
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// If the kernel is too old (before 3.17) to support the getrandom syscall(),
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once sync.Once
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// unix.GetRandom will immediately return ENOSYS and we will then fall back to
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useSyscall bool
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// reading from /dev/urandom in rand_unix.go. unix.GetRandom caches the ENOSYS
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)
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// result so we only suffer the syscall overhead once in this case.
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// If the kernel supports the getrandom() syscall, unix.GetRandom will block
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func pickStrategy() {
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// until the kernel has sufficient randomness (as we don't use GRND_NONBLOCK).
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// Test whether we should use the system call or /dev/urandom.
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// In this case, unix.GetRandom will not return an error.
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// We'll fall back to urandom if:
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// - the kernel is too old (before 3.17)
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// - the machine has no entropy available (early boot + no hardware
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// entropy source?) and we want to avoid blocking later.
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var buf [1]byte
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n, err := unix.GetRandom(buf[:], unix.GRND_NONBLOCK)
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useSyscall = n == 1 && err == nil
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}
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func getRandomLinux(p []byte) (ok bool) {
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func getRandomLinux(p []byte) (ok bool) {
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once.Do(pickStrategy)
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if !useSyscall {
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return false
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}
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n, err := unix.GetRandom(p, 0)
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n, err := unix.GetRandom(p, 0)
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return n == len(p) && err == nil
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return n == len(p) && err == nil
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}
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}
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