linux.git/drivers/char/random.c, branch v6.12.80

Remove WARN_ALL_UNSEEDED_RANDOM kernel config option

2026-03-04T12:21:32+00:00

[ Upstream commit 7dff99b354601dd01829e1511711846e04340a69 ]

This config option goes way back - it used to be an internal debug
option to random.c (at that point called DEBUG_RANDOM_BOOT), then was
renamed and exposed as a config option as CONFIG_WARN_UNSEEDED_RANDOM,
and then further renamed to the current CONFIG_WARN_ALL_UNSEEDED_RANDOM.

It was all done with the best of intentions: the more limited
rate-limited reports were reporting some cases, but if you wanted to see
all the gory details, you'd enable this "ALL" option.

However, it turns out - perhaps not surprisingly - that when people
don't care about and fix the first rate-limited cases, they most
certainly don't care about any others either, and so warning about all
of them isn't actually helping anything.

And the non-ratelimited reporting causes problems, where well-meaning
people enable debug options, but the excessive flood of messages that
nobody cares about will hide actual real information when things go
wrong.

I just got a kernel bug report (which had nothing to do with randomness)
where two thirds of the the truncated dmesg was just variations of

   random: get_random_u32 called from __get_random_u32_below+0x10/0x70 with crng_init=0

and in the process early boot messages had been lost (in addition to
making the messages that _hadn't_ been lost harder to read).

The proper way to find these things for the hypothetical developer that
cares - if such a person exists - is almost certainly with boot time
tracing.  That gives you the option to get call graphs etc too, which is
likely a requirement for fixing any problems anyway.

See Documentation/trace/boottime-trace.rst for that option.

And if we for some reason do want to re-introduce actual printing of
these things, it will need to have some uniqueness filtering rather than
this "just print it all" model.

Fixes: cc1e127bfa95 ("random: remove ratelimiting for in-kernel unseeded randomness")
Acked-by: Jason Donenfeld 
Signed-off-by: Linus Torvalds 
Signed-off-by: Sasha Levin

random: vDSO: add __arch_get_k_vdso_rng_data() helper for data page access

2024-09-13T15:28:35+00:00

_vdso_data is specific to x86 and __arch_get_k_vdso_data() is provided
so that all architectures can provide the requested pointer.

Do the same with _vdso_rng_data, provide __arch_get_k_vdso_rng_data()
and don't use x86 _vdso_rng_data directly.

Until now vdso/vsyscall.h was only included by time/vsyscall.c but now
it will also be included in char/random.c, leading to a duplicate
declaration of _vdso_data and _vdso_rng_data.

To fix this issue, move the declaration in a C file. vma.c looks like
the most appropriate candidate. We don't need to replace the definitions
in vsyscall.h by declarations as declarations are already in asm/vvar.h.

Signed-off-by: Christophe Leroy 
Signed-off-by: Jason A. Donenfeld

random: vDSO: don't use 64-bit atomics on 32-bit architectures

2024-09-13T15:28:35+00:00

Performing SMP atomic operations on u64 fails on powerpc32:

    CC      drivers/char/random.o
  In file included from :
  drivers/char/random.c: In function 'crng_reseed':
  ././include/linux/compiler_types.h:510:45: error: call to '__compiletime_assert_391' declared with attribute error: Need native word sized stores/loads for atomicity.
    510 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
        |                                             ^
  ././include/linux/compiler_types.h:491:25: note: in definition of macro '__compiletime_assert'
    491 |                         prefix ## suffix();                             \
        |                         ^~~~~~
  ././include/linux/compiler_types.h:510:9: note: in expansion of macro '_compiletime_assert'
    510 |         _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
        |         ^~~~~~~~~~~~~~~~~~~
  ././include/linux/compiler_types.h:513:9: note: in expansion of macro 'compiletime_assert'
    513 |         compiletime_assert(__native_word(t),                            \
        |         ^~~~~~~~~~~~~~~~~~
  ./arch/powerpc/include/asm/barrier.h:74:9: note: in expansion of macro 'compiletime_assert_atomic_type'
     74 |         compiletime_assert_atomic_type(*p);                             \
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ./include/asm-generic/barrier.h:172:55: note: in expansion of macro '__smp_store_release'
    172 | #define smp_store_release(p, v) do { kcsan_release(); __smp_store_release(p, v); } while (0)
        |                                                       ^~~~~~~~~~~~~~~~~~~
  drivers/char/random.c:286:9: note: in expansion of macro 'smp_store_release'
    286 |         smp_store_release(&__arch_get_k_vdso_rng_data()->generation, next_gen + 1);
        |         ^~~~~~~~~~~~~~~~~

The kernel-side generation counter in the random driver is handled as an
unsigned long, not as a u64, in base_crng and struct crng.

But on the vDSO side, it needs to be an u64, not just an unsigned long,
in order to support a 32-bit vDSO atop a 64-bit kernel.

On kernel side, however, it is an unsigned long, hence a 32-bit value on
32-bit architectures, so just cast it to unsigned long for the
smp_store_release(). A side effect is that on big endian architectures
the store will be performed in the upper 32 bits. It is not an issue on
its own because the vDSO site doesn't mind the value, as it only checks
differences. Just make sure that the vDSO side checks the full 64 bits.
For that, the local current_generation has to be u64 as well.

Signed-off-by: Christophe Leroy 
Suggested-by: Thomas Gleixner 
Signed-off-by: Jason A. Donenfeld

sysctl: treewide: constify the ctl_table argument of proc_handlers

2024-07-24T18:59:29+00:00

const qualify the struct ctl_table argument in the proc_handler function
signatures. This is a prerequisite to moving the static ctl_table
structs into .rodata data which will ensure that proc_handler function
pointers cannot be modified.

This patch has been generated by the following coccinelle script:

```
  virtual patch

  @r1@
  identifier ctl, write, buffer, lenp, ppos;
  identifier func !~ "appldata_(timer|interval)_handler|sched_(rt|rr)_handler|rds_tcp_skbuf_handler|proc_sctp_do_(hmac_alg|rto_min|rto_max|udp_port|alpha_beta|auth|probe_interval)";
  @@

  int func(
  - struct ctl_table *ctl
  + const struct ctl_table *ctl
    ,int write, void *buffer, size_t *lenp, loff_t *ppos);

  @r2@
  identifier func, ctl, write, buffer, lenp, ppos;
  @@

  int func(
  - struct ctl_table *ctl
  + const struct ctl_table *ctl
    ,int write, void *buffer, size_t *lenp, loff_t *ppos)
  { ... }

  @r3@
  identifier func;
  @@

  int func(
  - struct ctl_table *
  + const struct ctl_table *
    ,int , void *, size_t *, loff_t *);

  @r4@
  identifier func, ctl;
  @@

  int func(
  - struct ctl_table *ctl
  + const struct ctl_table *ctl
    ,int , void *, size_t *, loff_t *);

  @r5@
  identifier func, write, buffer, lenp, ppos;
  @@

  int func(
  - struct ctl_table *
  + const struct ctl_table *
    ,int write, void *buffer, size_t *lenp, loff_t *ppos);

```

* Code formatting was adjusted in xfs_sysctl.c to comply with code
  conventions. The xfs_stats_clear_proc_handler,
  xfs_panic_mask_proc_handler and xfs_deprecated_dointvec_minmax where
  adjusted.

* The ctl_table argument in proc_watchdog_common was const qualified.
  This is called from a proc_handler itself and is calling back into
  another proc_handler, making it necessary to change it as part of the
  proc_handler migration.

Co-developed-by: Thomas Weißschuh 
Signed-off-by: Thomas Weißschuh 
Co-developed-by: Joel Granados 
Signed-off-by: Joel Granados

random: introduce generic vDSO getrandom() implementation

2024-07-19T18:22:12+00:00

Provide a generic C vDSO getrandom() implementation, which operates on
an opaque state returned by vgetrandom_alloc() and produces random bytes
the same way as getrandom(). This has the following API signature:

  ssize_t vgetrandom(void *buffer, size_t len, unsigned int flags,
                     void *opaque_state, size_t opaque_len);

The return value and the first three arguments are the same as ordinary
getrandom(), while the last two arguments are a pointer to the opaque
allocated state and its size. Were all five arguments passed to the
getrandom() syscall, nothing different would happen, and the functions
would have the exact same behavior.

The actual vDSO RNG algorithm implemented is the same one implemented by
drivers/char/random.c, using the same fast-erasure techniques as that.
Should the in-kernel implementation change, so too will the vDSO one.

It requires an implementation of ChaCha20 that does not use any stack,
in order to maintain forward secrecy if a multi-threaded program forks
(though this does not account for a similar issue with SA_SIGINFO
copying registers to the stack), so this is left as an
architecture-specific fill-in. Stack-less ChaCha20 is an easy algorithm
to implement on a variety of architectures, so this shouldn't be too
onerous.

Initially, the state is keyless, and so the first call makes a
getrandom() syscall to generate that key, and then uses it for
subsequent calls. By keeping track of a generation counter, it knows
when its key is invalidated and it should fetch a new one using the
syscall. Later, more than just a generation counter might be used.

Since MADV_WIPEONFORK is set on the opaque state, the key and related
state is wiped during a fork(), so secrets don't roll over into new
processes, and the same state doesn't accidentally generate the same
random stream. The generation counter, as well, is always >0, so that
the 0 counter is a useful indication of a fork() or otherwise
uninitialized state.

If the kernel RNG is not yet initialized, then the vDSO always calls the
syscall, because that behavior cannot be emulated in userspace, but
fortunately that state is short lived and only during early boot. If it
has been initialized, then there is no need to inspect the `flags`
argument, because the behavior does not change post-initialization
regardless of the `flags` value.

Since the opaque state passed to it is mutated, vDSO getrandom() is not
reentrant, when used with the same opaque state, which libc should be
mindful of.

The function works over an opaque per-thread state of a particular size,
which must be marked VM_WIPEONFORK, VM_DONTDUMP, VM_NORESERVE, and
VM_DROPPABLE for proper operation. Over time, the nuances of these
allocations may change or grow or even differ based on architectural
features.

The opaque state passed to vDSO getrandom() must be allocated using the
mmap_flags and mmap_prot parameters provided by the vgetrandom_opaque_params
struct, which also contains the size of each state. That struct can be
obtained with a call to vgetrandom(NULL, 0, 0, ¶ms, ~0UL). Then,
libc can call mmap(2) and slice up the returned array into a state per
each thread, while ensuring that no single state straddles a page
boundary. 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.

vDSO getrandom() provides the ability for userspace to generate random
bytes quickly and safely, and is intended to be integrated into libc's
thread management. As an illustrative example, the introduced code in
the vdso_test_getrandom self test later in this series might be used to
do the same outside of libc. In a libc the various pthread-isms are
expected to be elided into libc internals.

Reviewed-by: Thomas Gleixner 
Signed-off-by: Jason A. Donenfeld

random: handle creditable entropy from atomic process context

2024-04-17T11:53:18+00:00

The entropy accounting changes a static key when the RNG has
initialized, since it only ever initializes once. Static key changes,
however, cannot be made from atomic context, so depending on where the
last creditable entropy comes from, the static key change might need to
be deferred to a worker.

Previously the code used the execute_in_process_context() helper
function, which accounts for whether or not the caller is
in_interrupt(). However, that doesn't account for the case where the
caller is actually in process context but is holding a spinlock.

This turned out to be the case with input_handle_event() in
drivers/input/input.c contributing entropy:

  [] die+0xa8/0x2fc
  [] bug_handler+0x44/0xec
  [] brk_handler+0x90/0x144
  [] do_debug_exception+0xa0/0x148
  [] el1_dbg+0x60/0x7c
  [] el1h_64_sync_handler+0x38/0x90
  [] el1h_64_sync+0x64/0x6c
  [] __might_resched+0x1fc/0x2e8
  [] __might_sleep+0x44/0x7c
  [] cpus_read_lock+0x1c/0xec
  [] static_key_enable+0x14/0x38
  [] crng_set_ready+0x14/0x28
  [] execute_in_process_context+0xb8/0xf8
  [] _credit_init_bits+0x118/0x1dc
  [] add_timer_randomness+0x264/0x270
  [] add_input_randomness+0x38/0x48
  [] input_handle_event+0x2b8/0x490
  [] input_event+0x6c/0x98

According to Guoyong, it's not really possible to refactor the various
drivers to never hold a spinlock there. And in_atomic() isn't reliable.

So, rather than trying to be too fancy, just punt the change in the
static key to a workqueue always. There's basically no drawback of doing
this, as the code already needed to account for the static key not
changing immediately, and given that it's just an optimization, there's
not exactly a hurry to change the static key right away, so deferal is
fine.

Reported-by: Guoyong Wang 
Cc: stable@vger.kernel.org
Fixes: f5bda35fba61 ("random: use static branch for crng_ready()")
Signed-off-by: Jason A. Donenfeld

iov_iter: replace import_single_range() with import_ubuf()

2023-12-05T10:57:37+00:00

With the removal of the 'iov' argument to import_single_range(), the two
functions are now fully identical. Convert the import_single_range()
callers to import_ubuf(), and remove the former fully.

Signed-off-by: Jens Axboe 
Link: https://lore.kernel.org/r/20231204174827.1258875-3-axboe@kernel.dk
Signed-off-by: Christian Brauner

iov_iter: remove unused 'iov' argument from import_single_range()

2023-12-05T10:57:34+00:00

It is entirely unused, just get rid of it.

Signed-off-by: Jens Axboe 
Link: https://lore.kernel.org/r/20231204174827.1258875-2-axboe@kernel.dk
Signed-off-by: Christian Brauner

char-misc: Remove the now superfluous sentinel element from ctl_table array

2023-10-11T19:16:13+00:00

This commit comes at the tail end of a greater effort to remove the
empty elements at the end of the ctl_table arrays (sentinels) which
will reduce the overall build time size of the kernel and run time
memory bloat by ~64 bytes per sentinel (further information Link :
https://lore.kernel.org/all/ZO5Yx5JFogGi%2FcBo@bombadil.infradead.org/)

Remove sentinel from impi_table and random_table

Signed-off-by: Joel Granados 
Signed-off-by: Luis Chamberlain

tty, proc, kernfs, random: Use copy_splice_read()

2023-05-24T14:42:16+00:00

Use copy_splice_read() for tty, procfs, kernfs and random files rather
than going through generic_file_splice_read() as they just copy the file
into the output buffer and don't splice pages.  This avoids the need for
them to have a ->read_folio() to satisfy filemap_splice_read().

Signed-off-by: David Howells 
Acked-by: Greg Kroah-Hartman 
cc: Christoph Hellwig 
cc: Jens Axboe 
cc: Al Viro 
cc: John Hubbard 
cc: David Hildenbrand 
cc: Matthew Wilcox 
cc: Miklos Szeredi 
cc: Arnd Bergmann 
cc: linux-block@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/20230522135018.2742245-13-dhowells@redhat.com
Signed-off-by: Jens Axboe