netdev - Re: [PATCH iproute2-next v2] lib/bpf_legacy: Use userspace SHA-1 code instead of AF

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Message-ID: <CAADnVQKTvXWQ72iBaAvCsDumq834t7f_0Vjy+Vz-8zaYtnupwA@mail.gmail.com>
Date: Thu, 2 Oct 2025 10:53:13 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Eric Biggers <ebiggers@...nel.org>
Cc: Network Development <netdev@...r.kernel.org>, Stephen Hemminger <stephen@...workplumber.org>, 
	bpf <bpf@...r.kernel.org>, 
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH iproute2-next v2] lib/bpf_legacy: Use userspace SHA-1 code
 instead of AF_ALG

On Thu, Oct 2, 2025 at 10:37 AM Eric Biggers <ebiggers@...nel.org> wrote:
>
> On Thu, Oct 02, 2025 at 10:12:12AM -0700, Alexei Starovoitov wrote:
> > On Wed, Oct 1, 2025 at 4:33 PM Eric Biggers <ebiggers@...nel.org> wrote:
> > >
> > > On Wed, Oct 01, 2025 at 03:59:31PM -0700, Alexei Starovoitov wrote:
> > > > On Mon, Sep 29, 2025 at 12:48 PM Eric Biggers <ebiggers@...nel.org> wrote:
> > > > >
> > > > > Add a basic SHA-1 implementation to lib/, and make lib/bpf_legacy.c use
> > > > > it to calculate SHA-1 digests instead of the previous AF_ALG-based code.
> > > > >
> > > > > This eliminates the dependency on AF_ALG, specifically the kernel config
> > > > > options CONFIG_CRYPTO_USER_API_HASH and CONFIG_CRYPTO_SHA1.
> > > > >
> > > > > Over the years AF_ALG has been very problematic, and it is also not
> > > > > supported on all kernels.  Escalating to the kernel's privileged
> > > > > execution context merely to calculate software algorithms, which can be
> > > > > done in userspace instead, is not something that should have ever been
> > > > > supported.  Even on kernels that support it, the syscall overhead of
> > > > > AF_ALG means that it is often slower than userspace code.
> > > >
> > > > Help me understand the crusade against AF_ALG.
> > > > Do you want to deprecate AF_ALG altogether or when it's used for
> > > > sha-s like sha1 and sha256 ?
> > >
> > > Altogether, when possible.  AF_ALG has been (and continues to be)
> > > incredibly problematic, for both security and maintainability.
> >
> > Could you provide an example of a security issue with AF_ALG ?
> > Not challenging the statement. Mainly curious what is going
> > to understand it better and pass the message.
>
> It's a gold mine for attackers looking to exploit the kernel.  Here are
> some examples from the CVE list when searching for "AF_ALG":

Ohh. I see. That made it very concrete. Thanks!

Acked-by: Alexei Starovoitov <ast@...nel.org>