FAILED: patch "[PATCH] lib/crypto: mpi: Fix integer underflow in" failed to apply to 6.1-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 8c2f1288250a90a4b5cabed5d888d7e3aeed4035
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026051223-undercoat-reps-6626@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 8c2f1288250a90a4b5cabed5d888d7e3aeed4035 Mon Sep 17 00:00:00 2001
From: Lukas Wunner <lukas@wunner.de>
Date: Sun, 12 Apr 2026 16:19:47 +0200
Subject: [PATCH] lib/crypto: mpi: Fix integer underflow in
 mpi_read_raw_from_sgl()

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+
Reviewed-by: Ignat Korchagin <ignat@linux.win>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/59eca92ff4f87e2081777f1423a0efaaadcfdb39.1776003111.git.lukas@wunner.de
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

diff --git a/lib/crypto/mpi/mpicoder.c b/lib/crypto/mpi/mpicoder.c
index bf716a03c704..9359a58c29ec 100644
--- a/lib/crypto/mpi/mpicoder.c
+++ b/lib/crypto/mpi/mpicoder.c
@@ -347,7 +347,7 @@ MPI mpi_read_raw_from_sgl(struct scatterlist *sgl, unsigned int nbytes)
 	lzeros = 0;
 	len = 0;
 	while (nbytes > 0) {
-		while (len && !*buff) {
+		while (len && !*buff && lzeros < nbytes) {
 			lzeros++;
 			len--;
 			buff++;