Re: [PATCH] crypto: af_alg - Add af_alg_restrict sysctl, defaulting to 1

[Demi Marie Obenour]

On 6/23/26 15:27, Eric Biggers wrote:
> On Tue, Jun 23, 2026 at 12:12:24PM -0700, Andy Lutomirski wrote:
>> On Mon, Jun 22, 2026 at 4:49 PM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>>>
>>> AF_ALG is a frequent source of vulnerabilities and a maintenance
>>> nightmare.  It exposes far more functionality to userspace than ever
>>> should have been exposed, especially to unprivileged processes.  Recent
>>> exploits have targeted kernel internal implementation details like
>>> "authencesn" that have zero use case for userspace access.
>>>
>>> Fortunately, AF_ALG is rarely used in practice, as userspace crypto
>>> libraries exist.  And when it is used, only some functionality is known
>>> to be used, and many users are known to hold capabilities already.
>>> iwd for example requires CAP_NET_ADMIN and has a known algorithm list
>>> (https://lore.kernel.org/linux-crypto/bcbbef00-5881-421b-8892-7be6c04b832d@xxxxxxxxx/).
>>>
>>> Thus, let's restrict the set of allowed algorithms by default, depending
>>> on the capabilities held.
>>>
>>> Add a sysctl /proc/sys/crypto/af_alg_restrict with meaning:
>>>
>>>     0: unrestricted
>>>     1: limited functionality
>>>     2: completely disabled
>>>
>>> Set the default value to 1, which enables an algorithm allowlist for
>>> unprivileged processes and a slightly longer allowlist for privileged
>>> processes.
>>
>> In our brave new world of containers, this is a bit awkward.  The
>> admin is sort of asking two separate questions:
>>
>> 1. Is the actual running distro and its privileged components capable
>> of working without AF_ALG or with only the parts marked as being
>> unprivileged?
>>
>> 2. Is the system running contains that need the unprivileged parts?
>> (Which is maybe just sha1 for ip?  I really don't know.)
>>
>> Should there maybe be two separate options so that all options are
>> available?  Or maybe something between 2 and 3 that means "limited
>> functionality and privileged modes are completely disabled"?
> 
> If we want to offer more settings we could.  I could see this getting
> quite complex pretty quickly once everyone weighs in, though.  There's
> quite a bit of value in keeping things simple, even if the offered
> settings won't be optimal for every case.
> 
> - Eric

What about exposing both allowlists to userspace and making them
configurable?

I'm mostly concerned about systems running code (possibly
closed-source) that uses algorithms that nobody here knows about.
It would be better to allow a single algorithm than to turn off all
restrictions.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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