[PATCH 1/1] tools: use cryptographically safe RNG

[Mark Kettenis]

> From: Rasmus Villemoes <ravi at prevas.dk>
> Date: Fri, 15 Nov 2024 08:18:17 +0100
> 
> On Fri, Nov 15 2024, Mark Kettenis <mark.kettenis at xs4all.nl> wrote:
> 
> >> Date: Thu, 14 Nov 2024 11:39:27 -0600
> >> From: Tom Rini <trini at konsulko.com>
> >> 
> >> On Thu, Nov 14, 2024 at 06:35:44PM +0100, Heinrich Schuchardt wrote:
> >> > Tom Rini <trini at konsulko.com> schrieb am Do., 14. Nov. 2024, 18:27:
> >> > 
> >> > > On Sat, Nov 02, 2024 at 05:32:59PM +0100, Heinrich Schuchardt wrote:
> >> > >
> >> > > > The PRNG implementing the random() function only has 2^31 states and
> >> > > > therefore is unsafe to use for cryptography. Use arc4random() instead.
> >> > > >
> >> > > > Fixes: cc34f04efd63 ("tools: image-host.c: use random instead of rand")
> >> > > > Addresses-Coverity-ID: 312953 Calling risky function
> >> > > > Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
> >> > > > ---
> >> > > >  tools/image-host.c | 35 +++--------------------------------
> >> > > >  1 file changed, 3 insertions(+), 32 deletions(-)
> >> > >
> >> > > Now I get:
> >> > > /home/uboot/u-boot/u-boot/tools/image-host.c: In function
> >> > > 'fit_image_setup_cipher':
> >> > > /home/uboot/u-boot/u-boot/tools/image-host.c:439:17: warning: implicit
> >> > > declaration of function 'arc4random_buf' [-Wimplicit-function-declaration]
> >> > >   439 |                 arc4random_buf((void *)info->iv,
> >> > > info->cipher->iv_len);
> >> > >       |                 ^~~~~~~~~~~~~~
> >> > > /usr/bin/ld: tools/image-host.o: in function `fit_image_cipher_data':
> >> > > image-host.c:(.text+0xb41): undefined reference to `arc4random_buf'
> >> > > collect2: error: ld returned 1 exit status
> >> > > make[3]: *** [scripts/Makefile.host:104: tools/dumpimage] Error 1
> >> > >
> >> > > in the docker container. I gather this means arc4random_buf is not as
> >> > > widely available as assumed.
> >> > >
> >> > 
> >> > glibc 2.36 is required published 2022-08. Ubuntu Jammy is 22.04.
> >> 
> >> Yeah, that's likely (a) too new and (b) strange because:
> >> https://source.denx.de/u-boot/u-boot/-/jobs/945810 and so is jammy
> >> 22.04.
> >
> > Linking against libbsd might be an alternative on older systems.
> 
> Or use getrandom(), which according to the man page has been exposed via
> glibc since glibc 2.25. Or just read from /dev/urandom which should work
> everywhere.

$ man getrandom
man: No entry for getrandom in the manual.