Re: [PATCH] crypto: lib/mpi - Fix integer underflow in mpi_read_raw_from_sgl()

[Eric Biggers <ebiggers@kernel.org>]

On Sun, Apr 12, 2026 at 04:19:47PM +0200, Lukas Wunner wrote:
> Yiming reports an integer underflow in mpi_read_raw_from_sgl() when
> subtracting "lzeros" from the unsigned "nbytes".
> 
> For this to happen, the scatterlist "sgl" needs to occupy more bytes
> than the "nbytes" parameter and the first "nbytes + 1" bytes of the
> scatterlist must be zero.  Under these conditions, the while loop
> iterating over the scatterlist will count more zeroes than "nbytes",
> subtract the number of zeroes from "nbytes" and cause the underflow.
> 
> When commit 2d4d1eea540b ("lib/mpi: Add mpi sgl helpers") originally
> introduced the bug, it couldn't be triggered because all callers of
> mpi_read_raw_from_sgl() passed a scatterlist whose length was equal to
> "nbytes".
> 
> However since commit 63ba4d67594a ("KEYS: asymmetric: Use new crypto
> interface without scatterlists"), the underflow can now actually be
> triggered.  When invoking a KEYCTL_PKEY_ENCRYPT system call with a
> larger "out_len" than "in_len" and filling the "in" buffer with zeroes,
> crypto_akcipher_sync_prep() will create an all-zero scatterlist used for
> both the "src" and "dst" member of struct akcipher_request and thereby
> fulfil the conditions to trigger the bug:
> 
>   sys_keyctl()
>     keyctl_pkey_e_d_s()
>       asymmetric_key_eds_op()
>         software_key_eds_op()
>           crypto_akcipher_sync_encrypt()
>             crypto_akcipher_sync_prep()
>               crypto_akcipher_encrypt()
>                 rsa_enc()
>                   mpi_read_raw_from_sgl()
> 
> To the user this will be visible as a DoS as the kernel spins forever,
> causing soft lockup splats as a side effect.
> 
> Fix it.
> 
> Reported-by: Yiming Qian <yimingqian591@gmail.com> # off-list
> Fixes: 2d4d1eea540b ("lib/mpi: Add mpi sgl helpers")
> Signed-off-by: Lukas Wunner <lukas@wunner.de>
> Cc: stable@vger.kernel.org # v4.4+
> ---

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/log/?h=libcrypto-fixes

This code (which has no tests...) is unnecessarily hard to understand,
though.  I haven't been able to fully understand the logic yet, but it
looks like it still has bugs, including still reading past the given
nbytes.  It should be possible to replace it with something simpler and
less error-prone.

- Eric