| Message ID | 20221121152909.3414096-1-Jason@zx2c4.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | implement getrandom() in vDSO | expand |
Hey folks, Exciting development: one of the glibc maintainers, Adhemerval, has written up the beginning of an implementation for this series: https://github.com/zatrazz/glibc/commits/azanella/arc4random-vdso I assume it'll continue to mature while this patch stews on the list here. But so far in my testing, it works well, and the performance boost is there and real. I've patched it into my system's glibc and am daily driving it. Jason
Changes v5->v6: -------------- - Fix various build errors for odd configurations. - Do not leak any secrets onto the stack at all, to account for possibility of fork()ing in a multithreaded scenario, which would ruin forward secrecy. Instead provide a arch-specific implementation that doesn't need stack space. - Prevent page alignment from overflowing variable, and clamp to acceptable limits. - Read/write unaligned bytes using get/put_unaligned. - Add extensive comments to vDSO function explaining subtle aspects. - Account for fork() races when writing generation counter. Changes v4->v5: -------------- - Add example code to vDSO addition commit showing intended use and interaction with allocations. - Reset buffer to beginning when retrying. - Rely on generation counter never being zero for fork detection, rather than adding extra boolean. - Make use of __ARCH_WANT_VGETRANDOM_ALLOC macro around new syscall so that it's condition by archs that actually choose to add this, and don't forget to bump __NR_syscalls. - Separate __cvdso_getrandom() into __cvdso_getrandom() and __cvdso_getrandom_data() so that powerpc can make a more efficient call. Changes v3->v4: -------------- - Split up into small series rather than one big patch. - Use proper ordering in generation counter reads. - Make properly generic, not just a hairball with x86, by moving symbols into correct files. Changes v2->v3: -------------- Big changes: Thomas' previous objection was two-fold: 1) vgetrandom should really have the same function signature as getrandom, in addition to all of the same behavior, and 2) having vgetrandom_alloc be a vDSO function doesn't make sense, because it doesn't actually need anything from the VDSO data page and it doesn't correspond to an existing syscall. After a discussion at Plumbers this last week, we devised the following ways to fix these: 1) we make the opque state argument be the last argument of vgetrandom, rather than the first one, since the real syscall ignores the additional argument, and that way all the registers are the same, and no behavior changes; and 2) we make vgetrandom_alloc a syscall, rather than a vDSO function, which also gives it added flexibility for the future, which is good. Making those changes also reduced the size of this patch a bit. Smaller changes: - Properly add buffer offset position. - Don't EXPORT_SYMBOL for vDSO code. - Account for timens and vvar being in swapped pages. -------------- Two statements: 1) Userspace wants faster cryptographically secure random numbers of arbitrary size, big or small. 2) Userspace is currently unable to safely roll its own RNG with the same security profile as getrandom(). Statement (1) has been debated for years, with arguments ranging from "we need faster cryptographically secure card shuffling!" to "the only things that actually need good randomness are keys, which are few and far between" to "actually, TLS CBC nonces are frequent" and so on. I don't intend to wade into that debate substantially, except to note that recently glibc added arc4random(), whose goal is to return a cryptographically secure uint32_t, and there are real user reports of it being too slow. So here we are. Statement (2) is more interesting. The kernel is the nexus of all entropic inputs that influence the RNG. It is in the best position, and probably the only position, to decide anything at all about the current state of the RNG and of its entropy. One of the things it uniquely knows about is when reseeding is necessary. For example, when a virtual machine is forked, restored, or duplicated, it's imparative that the RNG doesn't generate the same outputs. For this reason, there's a small protocol between hypervisors and the kernel that indicates this has happened, alongside some ID, which the RNG uses to immediately reseed, so as not to return the same numbers. Were userspace to expand a getrandom() seed from time T1 for the next hour, and at some point T2 < hour, the virtual machine forked, userspace would continue to provide the same numbers to two (or more) different virtual machines, resulting in potential cryptographic catastrophe. Something similar happens on resuming from hibernation (or even suspend), with various compromise scenarios there in mind. There's a more general reason why userspace rolling its own RNG from a getrandom() seed is fraught. There's a lot of attention paid to this particular Linuxism we have of the RNG being initialized and thus non-blocking or uninitialized and thus blocking until it is initialized. These are our Two Big States that many hold to be the holy differentiating factor between safe and not safe, between cryptographically secure and garbage. The fact is, however, that the distinction between these two states is a hand-wavy wishy-washy inexact approximation. Outside of a few exceptional cases (e.g. a HW RNG is available), we actually don't really ever know with any rigor at all when the RNG is safe and ready (nor when it's compromised). We do the best we can to "estimate" it, but entropy estimation is fundamentally impossible in the general case. So really, we're just doing guess work, and hoping it's good and conservative enough. Let's then assume that there's always some potential error involved in this differentiator. In fact, under the surface, the RNG is engineered around a different principal, and that is trying to *use* new entropic inputs regularly and at the right specific moments in time. For example, close to boot time, the RNG reseeds itself more often than later. At certain events, like VM fork, the RNG reseeds itself immediately. The various heuristics for when the RNG will use new entropy and how often is really a core aspect of what the RNG has some potential to do decently enough (and something that will probably continue to improve in the future from random.c's present set of algorithms). So in your mind, put away the metal attachment to the Two Big States, which represent an approximation with a potential margin of error. Instead keep in mind that the RNG's primary operating heuristic is how often and exactly when it's going to reseed. So, if userspace takes a seed from getrandom() at point T1, and uses it for the next hour (or N megabytes or some other meaningless metric), during that time, potential errors in the Two Big States approximation are amplified. During that time potential reseeds are being lost, forgotten, not reflected in the output stream. That's not good. The simplest statement you could make is that userspace RNGs that expand a getrandom() seed at some point T1 are nearly always *worse*, in some way, than just calling getrandom() every time a random number is desired. For those reasons, after some discussion on libc-alpha, glibc's arc4random() now just calls getrandom() on each invocation. That's trivially safe, and gives us latitude to then make the safe thing faster without becoming unsafe at our leasure. Card shuffling isn't particularly fast, however. How do we rectify this? By putting a safe implementation of getrandom() in the vDSO, which has access to whatever information a particular iteration of random.c is using to make its decisions. I use that careful language of "particular iteration of random.c", because the set of things that a vDSO getrandom() implementation might need for making decisions as good as the kernel's will likely change over time. This isn't just a matter of exporting certain *data* to userspace. We're not going to commit to a "data API" where the various heuristics used are exposed, locking in how the kernel works for decades to come, and then leave it to various userspaces to roll something on top and shoot themselves in the foot and have all sorts of complexity disasters. Rather, vDSO getrandom() is supposed to be the *same exact algorithm* that runs in the kernel, except it's been hoisted into userspace as much as possible. And so vDSO getrandom() and kernel getrandom() will always mirror each other hermetically. API-wise, the vDSO gains this function: ssize_t vgetrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state); The return value and the first 3 arguments are the same as ordinary getrandom(), while the last argument is a pointer to some state allocated with vgetrandom_alloc(), explained below. Were all four arguments passed to the getrandom syscall, nothing different would happen, and the functions would have the exact same behavior. Then, we introduce a new syscall: void *vgetrandom_alloc([inout] size_t *num, [out] size_t *size_per_each, unsigned int flags); This takes the desired number of opaque states in `num`, and returns a pointer to an array of opaque states, the number actually allocated back in `num`, and the size in bytes of each one in `size_per_each`, enabling a libc to slice up the returned array into a state per each thread. (The `flags` argument is always zero for now.) We very intentionally do *not* leave state allocation up to the caller of vgetrandom, but provide vgetrandom_alloc for that allocation. There are too many weird things that can go wrong, and it's important that vDSO does not provide too generic of a mechanism. It's not going to store its state in just any old memory address. It'll do it only in ones it allocates. Right now this means it's a mlock'd page with WIPEONFORK set. In the future maybe there will be other interesting page flags or anti-heartbleed measures, or other platform-specific kernel-specific things that can be set from the syscall. Again, it's important that the kernel has a say in how this works rather than agreeing to operate on any old address; memory isn't neutral. The syscall currently accomplishes this with a call to vm_mmap() and then a call to do_madvise(). It'd be nice to do this all at once, but I'm not sure that a helper function exists for that now, and it seems a bit premature to add one, at least for now. The interesting meat of the implementation is in lib/vdso/getrandom.c, as generic C code, and it aims to mainly follow random.c's buffered fast key erasure logic. Before the RNG is initialized, it falls back to the syscall. Right now it uses a simple generation counter to make its decisions on reseeding (though this could be made more extensive over time). The actual place that has the most work to do is in all of the other files. Most of the vDSO shared page infrastructure is centered around gettimeofday, and so the main structs are all in arrays for different timestamp types, and attached to time namespaces, and so forth. I've done the best I could to add onto this in an unintrusive way. In my test results, performance is pretty stellar (around 15x for uint32_t generation), and it seems to be working. There's an extended example in the second commit of this series, showing how the syscall and the vDSO function are meant to be used together. Cc: linux-crypto@vger.kernel.org Cc: x86@kernel.org Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org> Cc: Carlos O'Donell <carlos@redhat.com> Jason A. Donenfeld (3): random: add vgetrandom_alloc() syscall random: introduce generic vDSO getrandom() implementation x86: vdso: Wire up getrandom() vDSO implementation MAINTAINERS | 2 + arch/x86/Kconfig | 2 + arch/x86/entry/syscalls/syscall_64.tbl | 1 + arch/x86/entry/vdso/Makefile | 3 +- arch/x86/entry/vdso/vdso.lds.S | 2 + arch/x86/entry/vdso/vgetrandom-chacha.S | 181 ++++++++++++++++++++++++ arch/x86/entry/vdso/vgetrandom.c | 18 +++ arch/x86/include/asm/unistd.h | 1 + arch/x86/include/asm/vdso/getrandom.h | 49 +++++++ arch/x86/include/asm/vdso/vsyscall.h | 2 + arch/x86/include/asm/vvar.h | 16 +++ drivers/char/random.c | 68 +++++++++ include/uapi/asm-generic/unistd.h | 7 +- include/vdso/datapage.h | 6 + kernel/sys_ni.c | 3 + lib/vdso/Kconfig | 5 + lib/vdso/getrandom.c | 113 +++++++++++++++ lib/vdso/getrandom.h | 23 +++ scripts/checksyscalls.sh | 4 + tools/include/uapi/asm-generic/unistd.h | 7 +- 20 files changed, 510 insertions(+), 3 deletions(-) create mode 100644 arch/x86/entry/vdso/vgetrandom-chacha.S create mode 100644 arch/x86/entry/vdso/vgetrandom.c create mode 100644 arch/x86/include/asm/vdso/getrandom.h create mode 100644 lib/vdso/getrandom.c create mode 100644 lib/vdso/getrandom.h