On 5/4/26 02:56, Eric Biggers wrote:
> 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 <[email protected]>
>>> Reported-by: Feng Ning <[email protected]>
> [...]
>> In light of
>> https://lore.kernel.org/all/[email protected]/,
>> 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/[email protected]
>
> - EricShould this also link to copy.fail or a related email? -- Sincerely, Demi Marie Obenour (she/her/hers)
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