struct skcipher_alg {
    int (*setkey)(struct crypto_skcipher *tfm, const u8 *key, unsigned int keylen);
    int (*encrypt)(struct skcipher_request *req);
    int (*decrypt)(struct skcipher_request *req);
    // ...
    struct skcipher_alg_common co; // Contains ivsize, chunksize, etc.
};

static int essiv_aead_crypt(struct aead_request *req, bool enc)
{
	struct crypto_aead *tfm = crypto_aead_reqtfm(req);
	const struct essiv_tfm_ctx *tctx = crypto_aead_ctx(tfm);
	struct essiv_aead_request_ctx *rctx = aead_request_ctx(req);
	struct aead_request *subreq = &rctx->aead_req;
	struct scatterlist *src = req->src;
	int err;

	crypto_cipher_encrypt_one(tctx->essiv_cipher, req->iv, req->iv);

	/*
	 * dm-crypt embeds the sector number and the IV in the AAD region, so
	 * we have to copy the converted IV into the right scatterlist before
	 * we pass it on.
	 */
	rctx->assoc = NULL;
	if (req->src == req->dst || !enc) {
		scatterwalk_map_and_copy(req->iv, req->dst,
					 req->assoclen - crypto_aead_ivsize(tfm),    // <------- bug !
					 crypto_aead_ivsize(tfm), 1);
	} else {
		u8 *iv = (u8 *)aead_request_ctx(req) + tctx->ivoffset;
		int ivsize = crypto_aead_ivsize(tfm);
		int ssize = req->assoclen - ivsize;
		struct scatterlist *sg;
		int nents;
        .
        .
        .

static inline void scatterwalk_map_and_copy(void *buf, struct scatterlist *sg,
					    unsigned int start,
					    unsigned int nbytes, int out)
{
	if (out)
		memcpy_to_sglist(sg, start, buf, nbytes);
	else
		memcpy_from_sglist(buf, sg, start, nbytes);
}

diff --git a/crypto/essiv.c b/crypto/essiv.c
index d003b78fcd855a..a47a3eab693519 100644
--- a/crypto/essiv.c
+++ b/crypto/essiv.c
@@ -186,9 +186,14 @@ static int essiv_aead_crypt(struct aead_request *req, bool enc)
 	const struct essiv_tfm_ctx *tctx = crypto_aead_ctx(tfm);
 	struct essiv_aead_request_ctx *rctx = aead_request_ctx(req);
 	struct aead_request *subreq = &rctx->aead_req;
+	int ivsize = crypto_aead_ivsize(tfm);
+	int ssize = req->assoclen - ivsize;
 	struct scatterlist *src = req->src;
 	int err;
 
+	if (ssize < 0)
+		return -EINVAL;
+
 	crypto_cipher_encrypt_one(tctx->essiv_cipher, req->iv, req->iv);
 
 	/*
@@ -198,19 +203,12 @@ static int essiv_aead_crypt(struct aead_request *req, bool enc)
 	 */
 	rctx->assoc = NULL;
 	if (req->src == req->dst || !enc) {
-		scatterwalk_map_and_copy(req->iv, req->dst,
-					 req->assoclen - crypto_aead_ivsize(tfm),
-					 crypto_aead_ivsize(tfm), 1);
+		scatterwalk_map_and_copy(req->iv, req->dst, ssize, ivsize, 1);
 	} else {
 		u8 *iv = (u8 *)aead_request_ctx(req) + tctx->ivoffset;
-		int ivsize = crypto_aead_ivsize(tfm);
-		int ssize = req->assoclen - ivsize;
 		struct scatterlist *sg;
 		int nents;
 
-		if (ssize < 0)
-			return -EINVAL;
-
 		nents = sg_nents_for_len(req->src, ssize);
 		if (nents < 0)
 			return -EINVAL;

root@syzkaller:/mnt/shared# ls
pwn  pwn.c
root@syzkaller:/mnt/shared# ./pwn
aad_len=8, ivsize=16
[   28.256679] BUG: kernel NULL pointer dereference, address: 000000000000000c
[   28.258348] #PF: supervisor read access in kernel mode
[   28.259377] #PF: error_code(0x0000) - not-present page
[   28.260349] PGD 0 P4D 0
[   28.260904] Oops: Oops: 0000 [#1] SMP PTI
[   28.261605] CPU: 0 UID: 0 PID: 178 Comm: pwn Not tainted 6.17.0-rc1-00082-gc0d36727bf39 #8 PREEMPT(voluntary)
[   28.263236] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.17.0-2-2 04/01/2014
[   28.264805] RIP: 0010:memcpy_to_sglist+0x3a/0x90
[   28.265607] Code: 00 41 54 55 53 48 83 ec 20 65 48 8b 05 27 31 c6 01 48 89 44 24 18 31 c0 48 89 e7 f3 ab 45 85 ed 74 3a 89 f3 49 89 d4 48 89 e5 <41> 8b 40 0c 39 d8 73 0fb
[   28.269176] RSP: 0018:ffffc900001e7c80 EFLAGS: 00010202
[   28.270229] RAX: 0000000000000000 RBX: 00000000ffffffb8 RCX: 0000000000000000
[   28.271392] RDX: ffff8881027a4930 RSI: 00000000fffffff8 RDI: ffff888102b3f820
[   28.272514] RBP: ffffc900001e7c80 R08: 0000000000000000 R09: 0000000000000000
[   28.273681] R10: 0000000000000011 R11: 0000000000000081 R12: ffff8881027a4930
[   28.274997] R13: 0000000000000010 R14: ffff888102b3fa90 R15: ffff888102b3f820
[   28.276423] FS:  00007fb27ef28540(0000) GS:ffff8881b8986000(0000) knlGS:0000000000000000
[   28.277835] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   28.278831] CR2: 000000000000000c CR3: 000000010298e000 CR4: 00000000000006f0
[   28.280253] Call Trace:
[   28.280723]  <TASK>
[   28.281147]  essiv_aead_crypt+0x6d/0x230
[   28.281845]  aead_recvmsg+0x442/0x500
[   28.282630]  sock_recvmsg_nosec+0x57/0x80
[   28.283493]  sock_read_iter+0x7a/0xc0
[   28.284209]  vfs_read+0x14c/0x1e0
[   28.284811]  ksys_read+0x74/0xc0
[   28.285368]  do_syscall_64+0xca/0x1c0
[   28.286120]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[   28.287068] RIP: 0033:0x7fb27ee3e46e
[   28.287675] Code: c0 e9 b6 fe ff ff 50 48 8d 3d ce 07 0b 00 e8 69 01 02 00 66 0f 1f 84 00 00 00 00 00 64 8b 04 25 18 00 00 00 85 c0 75 14 0f 05 <48> 3d 00 f0 ff ff 77 5a8
[   28.290739] RSP: 002b:00007ffcc5ad39a8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[   28.292152] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fb27ee3e46e
[   28.293253] RDX: 0000000000000040 RSI: 00007ffcc5ad3a60 RDI: 0000000000000004
[   28.294356] RBP: 00007ffcc5ad3ba0 R08: 0000000000000000 R09: 00007ffcc5ad3887
[   28.295495] R10: fffffffffffffd8d R11: 0000000000000246 R12: 000055995fbd3150
[   28.296693] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[   28.297842]  </TASK>
[   28.298208] Modules linked in:
[   28.298966] CR2: 000000000000000c
[   28.299630] ---[ end trace 0000000000000000 ]---
[   28.300389] RIP: 0010:memcpy_to_sglist+0x3a/0x90
[   28.301266] Code: 00 41 54 55 53 48 83 ec 20 65 48 8b 05 27 31 c6 01 48 89 44 24 18 31 c0 48 89 e7 f3 ab 45 85 ed 74 3a 89 f3 49 89 d4 48 89 e5 <41> 8b 40 0c 39 d8 73 0fb
[   28.304232] RSP: 0018:ffffc900001e7c80 EFLAGS: 00010202
[   28.305035] RAX: 0000000000000000 RBX: 00000000ffffffb8 RCX: 0000000000000000
[   28.306210] RDX: ffff8881027a4930 RSI: 00000000fffffff8 RDI: ffff888102b3f820
[   28.307396] RBP: ffffc900001e7c80 R08: 0000000000000000 R09: 0000000000000000
[   28.308668] R10: 0000000000000011 R11: 0000000000000081 R12: ffff8881027a4930
[   28.309798] R13: 0000000000000010 R14: ffff888102b3fa90 R15: ffff888102b3f820
[   28.310996] FS:  00007fb27ef28540(0000) GS:ffff8881b8986000(0000) knlGS:0000000000000000
[   28.312321] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   28.313229] CR2: 000000000000000c CR3: 000000010298e000 CR4: 00000000000006f0
Killed