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author | Jason A. Donenfeld <Jason@zx2c4.com> | 2022-04-06 03:36:16 +0300 |
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committer | Jason A. Donenfeld <Jason@zx2c4.com> | 2022-04-07 02:36:37 +0300 |
commit | e3c1c4fd9e6d14059ed93ebfe15e1c57793b1a05 (patch) | |
tree | a7e553020ae01f78d21f36edb97cae344576090e /drivers/char/random.c | |
parent | 1448769c9cdb69ad65287f4f7ab58bc5f2f5d7ba (diff) | |
download | linux-e3c1c4fd9e6d14059ed93ebfe15e1c57793b1a05.tar.xz |
random: check for signals every PAGE_SIZE chunk of /dev/[u]random
In 1448769c9cdb ("random: check for signal_pending() outside of
need_resched() check"), Jann pointed out that we previously were only
checking the TIF_NOTIFY_SIGNAL and TIF_SIGPENDING flags if the process
had TIF_NEED_RESCHED set, which meant in practice, super long reads to
/dev/[u]random would delay signal handling by a long time. I tried this
using the below program, and indeed I wasn't able to interrupt a
/dev/urandom read until after several megabytes had been read. The bug
he fixed has always been there, and so code that reads from /dev/urandom
without checking the return value of read() has mostly worked for a long
time, for most sizes, not just for <= 256.
Maybe it makes sense to keep that code working. The reason it was so
small prior, ignoring the fact that it didn't work anyway, was likely
because /dev/random used to block, and that could happen for pretty
large lengths of time while entropy was gathered. But now, it's just a
chacha20 call, which is extremely fast and is just operating on pure
data, without having to wait for some external event. In that sense,
/dev/[u]random is a lot more like /dev/zero.
Taking a page out of /dev/zero's read_zero() function, it always returns
at least one chunk, and then checks for signals after each chunk. Chunk
sizes there are of length PAGE_SIZE. Let's just copy the same thing for
/dev/[u]random, and check for signals and cond_resched() for every
PAGE_SIZE amount of data. This makes the behavior more consistent with
expectations, and should mitigate the impact of Jann's fix for the
age-old signal check bug.
---- test program ----
#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <sys/random.h>
static unsigned char x[~0U];
static void handle(int) { }
int main(int argc, char *argv[])
{
pid_t pid = getpid(), child;
signal(SIGUSR1, handle);
if (!(child = fork())) {
for (;;)
kill(pid, SIGUSR1);
}
pause();
printf("interrupted after reading %zd bytes\n", getrandom(x, sizeof(x), 0));
kill(child, SIGTERM);
return 0;
}
Cc: Jann Horn <jannh@google.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Diffstat (limited to 'drivers/char/random.c')
-rw-r--r-- | drivers/char/random.c | 17 |
1 files changed, 7 insertions, 10 deletions
diff --git a/drivers/char/random.c b/drivers/char/random.c index 394cbd814a0b..e15063d61460 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -523,7 +523,6 @@ EXPORT_SYMBOL(get_random_bytes); static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes) { - bool large_request = nbytes > 256; ssize_t ret = 0; size_t len; u32 chacha_state[CHACHA_STATE_WORDS]; @@ -549,15 +548,6 @@ static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes) } do { - if (large_request) { - if (signal_pending(current)) { - if (!ret) - ret = -ERESTARTSYS; - break; - } - cond_resched(); - } - chacha20_block(chacha_state, output); if (unlikely(chacha_state[12] == 0)) ++chacha_state[13]; @@ -571,6 +561,13 @@ static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes) nbytes -= len; buf += len; ret += len; + + BUILD_BUG_ON(PAGE_SIZE % CHACHA_BLOCK_SIZE != 0); + if (!(ret % PAGE_SIZE) && nbytes) { + if (signal_pending(current)) + break; + cond_resched(); + } } while (nbytes); memzero_explicit(output, sizeof(output)); |