Messages in this thread | | | Date | Wed, 13 Jul 2022 08:06:52 +0100 | Subject | Re: [PATCH] um: seed rng using host OS rng | From | Anton Ivanov <> |
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On 13/07/2022 00:27, Jason A. Donenfeld wrote: > UML generally does not provide access to special CPU instructions like > RDRAND, and execution tends to be rather deterministic, with no real > hardware interrupts, making good randomness really very hard, if not > all together impossible. Not only is this a security eyebrow raiser, but > it's also quite annoying when trying to do various pieces of UML-based > automation that takes a long time to boot, if ever. > > Fix this by trivially calling getrandom() in the host and using that > seed as "bootloader randomness", which initializes the rng immediately > at UML boot. > > The old behavior can be restored the same way as on any other arch, by > way of CONFIG_TRUST_BOOTLOADER_RANDOMNESS=n or > random.trust_bootloader=0. So seen from that perspective, this just > makes UML act like other archs, which is positive in its own right. > > Cc: stable@vger.kernel.org > Cc: Johannes Berg <johannes@sipsolutions.net> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> > --- > arch/um/include/shared/os.h | 7 +++++++ > arch/um/kernel/um_arch.c | 8 ++++++++ > arch/um/os-Linux/util.c | 6 ++++++ > 3 files changed, 21 insertions(+) > > diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h > index fafde1d5416e..79644dd88d58 100644 > --- a/arch/um/include/shared/os.h > +++ b/arch/um/include/shared/os.h > @@ -11,6 +11,12 @@ > #include <irq_user.h> > #include <longjmp.h> > #include <mm_id.h> > +/* This is to get size_t */ > +#ifndef __UM_HOST__ > +#include <linux/types.h> > +#else > +#include <stddef.h> > +#endif > > #define CATCH_EINTR(expr) while ((errno = 0, ((expr) < 0)) && (errno == EINTR)) > > @@ -243,6 +249,7 @@ extern void stack_protections(unsigned long address); > extern int raw(int fd); > extern void setup_machinename(char *machine_out); > extern void setup_hostinfo(char *buf, int len); > +extern ssize_t os_getrandom(void *buf, size_t len, unsigned int flags); > extern void os_dump_core(void) __attribute__ ((noreturn)); > extern void um_early_printk(const char *s, unsigned int n); > extern void os_fix_helper_signals(void); > diff --git a/arch/um/kernel/um_arch.c b/arch/um/kernel/um_arch.c > index 0760e24f2eba..74f3efd96bd4 100644 > --- a/arch/um/kernel/um_arch.c > +++ b/arch/um/kernel/um_arch.c > @@ -16,6 +16,7 @@ > #include <linux/sched/task.h> > #include <linux/kmsg_dump.h> > #include <linux/suspend.h> > +#include <linux/random.h> > > #include <asm/processor.h> > #include <asm/cpufeature.h> > @@ -406,6 +407,8 @@ int __init __weak read_initrd(void) > > void __init setup_arch(char **cmdline_p) > { > + u8 rng_seed[32]; > + > stack_protections((unsigned long) &init_thread_info); > setup_physmem(uml_physmem, uml_reserved, physmem_size, highmem); > mem_total_pages(physmem_size, iomem_size, highmem); > @@ -416,6 +419,11 @@ void __init setup_arch(char **cmdline_p) > strlcpy(boot_command_line, command_line, COMMAND_LINE_SIZE); > *cmdline_p = command_line; > setup_hostinfo(host_info, sizeof host_info); > + > + if (os_getrandom(rng_seed, sizeof(rng_seed), 0) == sizeof(rng_seed)) { > + add_bootloader_randomness(rng_seed, sizeof(rng_seed)); > + memzero_explicit(rng_seed, sizeof(rng_seed)); > + } > } > > void __init check_bugs(void) > diff --git a/arch/um/os-Linux/util.c b/arch/um/os-Linux/util.c > index 41297ec404bf..fc0f2a9dee5a 100644 > --- a/arch/um/os-Linux/util.c > +++ b/arch/um/os-Linux/util.c > @@ -14,6 +14,7 @@ > #include <sys/wait.h> > #include <sys/mman.h> > #include <sys/utsname.h> > +#include <sys/random.h> > #include <init.h> > #include <os.h> > > @@ -96,6 +97,11 @@ static inline void __attribute__ ((noreturn)) uml_abort(void) > exit(127); > } > > +ssize_t os_getrandom(void *buf, size_t len, unsigned int flags) > +{ > + return getrandom(buf, len, flags); > +} > + > /* > * UML helper threads must not handle SIGWINCH/INT/TERM > */
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
-- Anton R. Ivanov https://www.kot-begemot.co.uk/
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