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SubjectRe: [PATCH v2] utsname: contribute changes to RNG
On Tue, Sep 27, 2022 at 11:53:54AM +0200, Jason A. Donenfeld wrote:
> On some small machines with little entropy, a quasi-unique hostname is
> sometimes a relevant factor. I've seen, for example, 8 character
> alpha-numeric serial numbers. In addition, the time at which the hostname
> is set is usually a decent measurement of how long early boot took. So,
> call add_device_randomness() on new hostnames, which feeds its arguments
> to the RNG in addition to a fresh cycle counter.
>
> Low cost hooks like this never hurt and can only ever help, and since
> this costs basically nothing for an operation that is never a fast path,
> this is an overall easy win.
>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> kernel/sys.c | 2 ++
> kernel/utsname_sysctl.c | 2 ++
> 2 files changed, 4 insertions(+)
>
> diff --git a/kernel/sys.c b/kernel/sys.c
> index b911fa6d81ab..b4b40ccf0949 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -25,6 +25,7 @@
> #include <linux/times.h>
> #include <linux/posix-timers.h>
> #include <linux/security.h>
> +#include <linux/random.h>
> #include <linux/suspend.h>
> #include <linux/tty.h>
> #include <linux/signal.h>
> @@ -1366,6 +1367,7 @@ SYSCALL_DEFINE2(sethostname, char __user *, name, int, len)
> if (!copy_from_user(tmp, name, len)) {
> struct new_utsname *u;
>
> + add_device_randomness(tmp, len);
> down_write(&uts_sem);
> u = utsname();
> memcpy(u->nodename, tmp, len);
> diff --git a/kernel/utsname_sysctl.c b/kernel/utsname_sysctl.c
> index 4ca61d49885b..de16bcf14b03 100644
> --- a/kernel/utsname_sysctl.c
> +++ b/kernel/utsname_sysctl.c
> @@ -8,6 +8,7 @@
> #include <linux/export.h>
> #include <linux/uts.h>
> #include <linux/utsname.h>
> +#include <linux/random.h>
> #include <linux/sysctl.h>
> #include <linux/wait.h>
> #include <linux/rwsem.h>
> @@ -57,6 +58,7 @@ static int proc_do_uts_string(struct ctl_table *table, int write,
> * theoretically be incorrect if there are two parallel writes
> * at non-zero offsets to the same sysctl.
> */
> + add_device_randomness(tmp_data, sizeof(tmp_data));

Does this matter that we're dumping the same same trailing bytes into
the RNG? (i.e. only min(lenp, table->maxlen) has changed.)

If that's okay (and maybe a comment should be added):

Reviewed-by: Kees Cook <keescook@chromium.org>


> down_write(&uts_sem);
> memcpy(get_uts(table), tmp_data, sizeof(tmp_data));
> up_write(&uts_sem);
> --
> 2.37.3
>

--
Kees Cook

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