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From: "adhemerval.zanella at linaro dot org"
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Subject: [Bug libc/29437] arc4random is too slow
Date: Mon, 08 Aug 2022 12:51:24 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D29437

--- Comment #15 from Adhemerval Zanella <adhemerval.zanella at linaro dot o=
rg> ---
(In reply to Florian Weimer from comment #12)
> (In reply to Adhemerval Zanella from comment #9)
> > (In reply to Florian Weimer from comment #7)
> > > (In reply to Adhemerval Zanella from comment #6)
> > > > For example, when a virtual machine is forked, restored, or duplica=
ted, it's
> > > > imperative that the RNG doesn't generate the same outputs. For this=
 reason,
> > > > there's a small protocol between hypervisors and the kernel that in=
dicates
> > > > this has happened, alongside some ID, which the RNG uses to reseed
> > > > immediately, so as not to return the same numbers.
> > >=20
> > > Except that this doesn't work because if the suspend/resume happens a=
 little
> > > bit later, the unprotected application buffer to which the random dat=
a has
> > > been written is restore multiple times. So clearly the actual rules a=
round
> > > this are different and much more complicated.
> >=20
> > I am not following, which buffer are you referring? The kernel one
> > application will be poll with arc4random/getrandom or the possible one
> > maintained by application itself with a previous arc4random call?
>=20
> If suspended right after arc4random () returms, this
>=20
> printf ("%u\n", (unsigned int) arc4random ());
>=20
> will print the same number after each resumption.

So explicit_bzero will not clear all the buffer if the machine is stop/resu=
med
in the middle of the operation.  It is still used an hardening mechanism.

--=20
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