Re: [PATCH] crypto: af_alg - Remove zero-copy support from AF_ALG

[Eric Biggers <ebiggers@kernel.org>]

On Mon, May 04, 2026 at 02:54:27AM -0400, Demi Marie Obenour wrote:
> On 5/4/26 02:15, Eric Biggers wrote:
> > The zero-copy support is one of the riskiest aspects of AF_ALG.  It
> > allows userspace to request cryptographic operations directly on
> > pagecache pages of files like the 'su' binary.  It also allows userspace
> > to concurrently modify the memory which is being operated on, a huge
> > recipe for TOCTOU vulnerabilities.
> > 
> > While zero-copy support is more valuable in other areas of the kernel
> > like the frequently used networking and file I/O code, it has far less
> > value in AF_ALG, which is a niche UAPI.  AF_ALG primarily just exists
> > for backwards compatibility with a small set of userspace programs such
> > as 'iwd' that haven't yet been fixed to use userspace crypto code.
> > 
> > Originally AF_ALG was intended to be used to access hardware crypto
> > accelerators.  However, it isn't an efficient interface for that anyway,
> > and it turned out to be rarely used in this way in practice.
> > 
> > Thus, the risks of the zero-copy support in AF_ALG vastly outweigh its
> > benefits.  Just remove it.
> > 
> > Note that this isn't a hard break, since the splice syscall is still
> > supported.  The data is just now copied instead.  So it still works,
> > just a bit slower in some cases.
> > 
> > Tested with libkcapi/test.sh.  All its test cases still pass.  I also
> > verified that this would have prevented the copy.fail exploit as well.
> > 
> > Fixes: 8ff590903d5f ("crypto: algif_skcipher - User-space interface for skcipher operations")
> > Fixes: 400c40cf78da ("crypto: algif - add AEAD support")
> > Reported-by: Taeyang Lee <0wn@theori.io>
> > Reported-by: Feng Ning <feng@innora.ai>
[...]
> In light of https://lore.kernel.org/all/afYcc-tZFwvZZo76@ans-MacBook-Pro.local/,
> yes please!
> 
> Should there be a Link: tag referencing that email?

Yes I forgot to put that in, sorry.  It should go after the second
Reported-by:

    Link: https://lore.kernel.org/r/afYcc-tZFwvZZo76@ans-MacBook-Pro.local

- Eric