| Message ID | 20230101162910.710293-1-Jason@zx2c4.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | implement getrandom() in vDSO | expand |
On 01-Jan-2023 05:29:03 PM, Jason A. Donenfeld wrote: [...] > 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(). > [...] > 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(unsigned int *num, unsigned int *size_per_each, > unsigned long addr, unsigned int flags); > > This takes a hinted 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` and `addr` arguments, as well as the `*size_per_each` input > value, are reserved for the future and are forced to be zero for now.) > > Libc is expected to allocate a chunk of these on first use, and then > dole them out to threads as they're created, allocating more when > needed. The returned address of the first state may be passed to > munmap(2) with a length of `num * size_per_each`, in order to deallocate > the memory. > > 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. [...] Have you considered extending rseq(2) per-thread "struct rseq" with an additional "prng_seed" pointer field, which would point to a per-thread memory area accessible both from userspace (at address __builtin_thread_pointer() + __rseq_offset) and from kernel's return-to-userspace rseq notification code (which can handle page faults) ? This way, the kernel can update its content when returning to userspace if an update is needed since the last update. Would that be sufficient as prng seed for your security requirements ? Implementation-wise, the semantic of the prng_seed could be entirely internal to a vgetrandom vDSO implementation, but the allocation of the memory holding this seed would be the responsibility of libc. libc could query the size required by the kernel for this prng seed with a new getauxval(3) entry, e.g. AT_RSEQ_PRNG_SIZE. By doing so, libc would only allocate as much memory as needed by the kernel vDSO implementation. This would remove the need for any kind of vgetrandom_alloc system call and all its associated complexity. Thoughts ? Thanks, Mathieu
Changes v13->v14: ---------------- - Fix incorrect comments in chacha assembly, and augment comments in random.c - Get chacha standalone selftest building again after recent upstream changes. - Switch to using `.set` instead of `#define` in asm. - Rebased on e4cf7c25bae5. Changes v12->v13: ----------------- - Use helpers in insn-eval.h instead of open coding the same thing via insn.h. - Due to some enum constants in insn-eval.h clashing with enum constants in an unrelated header that happens to be included by a file this series needs insn-eval.h from, the first patch of this series now renames some enum constants. This is a standalone patch that should probably be applied regardless of the rest of this series. - Rebase atop the latest random.git master branch, which is itself pretty similar to Linus' tree at the moment. Changes v11->v12: ----------------- - In order to avoid mlock()ing pages, and the related rlimit and fork inheritance issues there, Introduce VM_DROPPABLE to prevent swapping while meeting the cache-like requirements of vDSO getrandom(). This has some tenticles in mm/ and arch/x86/ code, so I've marked the two patches for that as still RFC, while the rest of the series is not RFC. - Mandate that opaque state blobs don't straddle page boundaries, so that VM_DROPPABLE can work on page-level granularity rather than allocation-level granularity. - Add compiler barriers to vDSO getrandom() to prevent theoretical reordering potential. - Initialize the trials loop counter in the chacha test. Changes v10->v11: ----------------- - Adhemerval's latest glibc patch currently lives here: https://github.com/zatrazz/glibc/commits/azanella/getrandom-vdso It's still in flux (he hasn't posted it to glibc-alpha yet), but that should still give some idea. - Substantially increase documentation detail throughout. - Numerous fixes to style and clarity. - Add two kselftests - one for the chacha20 assembly implementation, which compares its output to that of libsodium, and one for the actual vDSO function and vgetrandom_alloc() syscall, which also has some built-in quick & dirty benchmarking functionality. - Add reserved `unsigned long addr` argument to syscall, for future use. - Enforce that `flags`, `addr`, and `*size_per_each` are all zero on input, so that they can actually be used in the future. - Use VM_LOCKONFAULT|VM_NORESERVE so that memory isn't wasted until it's used. - Set VM_DONTDUMP to prevent state from being written out to core dumps. - Do mapping and flag setting all in one operation, so that it's not racy, and so that dropping the mm lock between operations isn't required. - Due to the above, there's no longer any purpose in selecting VGETRANDOM_ALLOC_SYSCALL, so remove that. - Don't update *num or *size_per_each until the mapping has succeeded. - Introduce vdso_kernel_ulong type for representing the long type that the kernel uses, even when in 32-bit compat code on a 64-bit kernel. - Don't use 'bool' type in the vDSO page, in case of compiler quirks resulting in a different interpretation of the type in different compilation contexts. -------------- 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(unsigned int *num, unsigned int *size_per_each, unsigned long addr, unsigned int flags); This takes a hinted 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` and `addr` arguments, as well as the `*size_per_each` input value, are reserved for the future and are forced to be zero for now.) Libc is expected to allocate a chunk of these on first use, and then dole them out to threads as they're created, allocating more when needed. The returned address of the first state may be passed to munmap(2) with a length of `num * size_per_each`, in order to deallocate the memory. 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 uses a new mm flag called VM_DROPPABLE, along with VM_WIPEONFORK. 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 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: linux-api@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> Cc: Florian Weimer <fweimer@redhat.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Jann Horn <jannh@google.com> Cc: Christian Brauner <brauner@kernel.org> Jason A. Donenfeld (7): x86: lib: Separate instruction decoder MMIO type from MMIO trace mm: add VM_DROPPABLE for designating always lazily freeable mappings x86: mm: Skip faulting instruction for VM_DROPPABLE faults random: add vgetrandom_alloc() syscall arch: allocate vgetrandom_alloc() syscall number random: introduce generic vDSO getrandom() implementation x86: vdso: Wire up getrandom() vDSO implementation MAINTAINERS | 2 + arch/alpha/kernel/syscalls/syscall.tbl | 1 + arch/arm/tools/syscall.tbl | 1 + arch/arm64/include/asm/unistd.h | 2 +- arch/arm64/include/asm/unistd32.h | 2 + arch/ia64/kernel/syscalls/syscall.tbl | 1 + arch/m68k/kernel/syscalls/syscall.tbl | 1 + arch/microblaze/kernel/syscalls/syscall.tbl | 1 + arch/mips/kernel/syscalls/syscall_n32.tbl | 1 + arch/mips/kernel/syscalls/syscall_n64.tbl | 1 + arch/mips/kernel/syscalls/syscall_o32.tbl | 1 + arch/parisc/kernel/syscalls/syscall.tbl | 1 + arch/powerpc/kernel/syscalls/syscall.tbl | 1 + arch/s390/kernel/syscalls/syscall.tbl | 1 + arch/sh/kernel/syscalls/syscall.tbl | 1 + arch/sparc/kernel/syscalls/syscall.tbl | 1 + arch/x86/Kconfig | 1 + arch/x86/coco/tdx/tdx.c | 26 +- arch/x86/entry/syscalls/syscall_32.tbl | 1 + 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 | 178 +++++++++++ arch/x86/entry/vdso/vgetrandom.c | 17 ++ arch/x86/include/asm/insn-eval.h | 18 +- arch/x86/include/asm/vdso/getrandom.h | 55 ++++ arch/x86/include/asm/vdso/vsyscall.h | 2 + arch/x86/include/asm/vvar.h | 16 + arch/x86/kernel/sev.c | 18 +- arch/x86/lib/insn-eval.c | 20 +- arch/x86/mm/fault.c | 19 ++ arch/xtensa/kernel/syscalls/syscall.tbl | 1 + drivers/char/random.c | 147 +++++++++ fs/proc/task_mmu.c | 3 + include/linux/mm.h | 8 + include/linux/mm_types.h | 5 +- include/linux/syscalls.h | 3 + include/trace/events/mmflags.h | 7 + include/uapi/asm-generic/unistd.h | 5 +- include/vdso/datapage.h | 12 + include/vdso/getrandom.h | 44 +++ include/vdso/types.h | 35 +++ kernel/sys_ni.c | 3 + lib/vdso/Kconfig | 6 + lib/vdso/getrandom.c | 224 ++++++++++++++ mm/Kconfig | 3 + mm/memory.c | 6 + mm/mempolicy.c | 3 + mm/mprotect.c | 2 +- mm/rmap.c | 5 +- tools/include/uapi/asm-generic/unistd.h | 5 +- .../arch/mips/entry/syscalls/syscall_n64.tbl | 1 + .../arch/powerpc/entry/syscalls/syscall.tbl | 1 + .../perf/arch/s390/entry/syscalls/syscall.tbl | 1 + .../arch/x86/entry/syscalls/syscall_64.tbl | 1 + tools/testing/selftests/vDSO/.gitignore | 2 + tools/testing/selftests/vDSO/Makefile | 11 + .../testing/selftests/vDSO/vdso_test_chacha.c | 43 +++ .../selftests/vDSO/vdso_test_getrandom.c | 283 ++++++++++++++++++ 59 files changed, 1217 insertions(+), 49 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 include/vdso/getrandom.h create mode 100644 include/vdso/types.h create mode 100644 lib/vdso/getrandom.c create mode 100644 tools/testing/selftests/vDSO/vdso_test_chacha.c create mode 100644 tools/testing/selftests/vDSO/vdso_test_getrandom.c