crypto: algif_aead - Revert to operating out-of-place (CVE-2026-31431)

Backport of upstream fix fafe0fa2995a0f7073c1c358d7d3145bcc9aedd8
to 6.12.49. Mostly reverts commit 72548b0 except for the
copying of the associated data.

The 2017 in-place optimization in algif_aead chained tag pages from
splice() by reference into the destination scatterlist, then set
req->src = req->dst. Combined with authencesn's scratch writes at
dst[assoclen + cryptlen], this allowed an unprivileged user to write
to read-only page cache pages (e.g. /usr/bin/su) via AF_ALG sockets,
enabling local privilege escalation.

Fix: remove the in-place enc/dec paths entirely. Pull the full TX SGL
into a per-request SGL (out-of-place), copy only the AAD to the RX
buffer, and pass the TX SGL directly as the crypto source. This
eliminates the sg_chain tag-page chaining that made the write primitive
possible.

6.12-specific: uses existing crypto_aead_copy_sgl() for the AAD copy
instead of the memcpy_sglist() added in later kernels.

Fixes: 72548b0 ("crypto: algif_aead - copy AAD from src to dst")
CVE: CVE-2026-31431

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: shjala

crypto/af_alg.c

@@ -635,15 +635,13 @@ static int af_alg_alloc_tsgl(struct sock *sk)
 /**
  * af_alg_count_tsgl - Count number of TX SG entries
  *
- * The counting starts from the beginning of the SGL to @bytes. If
- * an @offset is provided, the counting of the SG entries starts at the @offset.
+ * The counting starts from the beginning of the SGL to @bytes.
  *
  * @sk: socket of connection to user space
  * @bytes: Count the number of SG entries holding given number of bytes.
- * @offset: Start the counting of SG entries from the given offset.
  * Return: Number of TX SG entries found given the constraints
  */
-unsigned int af_alg_count_tsgl(struct sock *sk, size_t bytes, size_t offset)
+unsigned int af_alg_count_tsgl(struct sock *sk, size_t bytes)
 {
 	const struct alg_sock *ask = alg_sk(sk);
 	const struct af_alg_ctx *ctx = ask->private;
@@ -658,25 +656,11 @@ unsigned int af_alg_count_tsgl(struct sock *sk, size_t bytes, size_t offset)
 		const struct scatterlist *sg = sgl->sg;
 
 		for (i = 0; i < sgl->cur; i++) {
-			size_t bytes_count;
-
-			/* Skip offset */
-			if (offset >= sg[i].length) {
-				offset -= sg[i].length;
-				bytes -= sg[i].length;
-				continue;
-			}
-
-			bytes_count = sg[i].length - offset;
-
-			offset = 0;
 			sgl_count++;
-
-			/* If we have seen requested number of bytes, stop */
-			if (bytes_count >= bytes)
+			if (sg[i].length >= bytes)
 				return sgl_count;
 
-			bytes -= bytes_count;
+			bytes -= sg[i].length;
 		}
 	}
 
@@ -688,19 +672,14 @@ EXPORT_SYMBOL_GPL(af_alg_count_tsgl);
  * af_alg_pull_tsgl - Release the specified buffers from TX SGL
  *
  * If @dst is non-null, reassign the pages to @dst. The caller must release
- * the pages. If @dst_offset is given only reassign the pages to @dst starting
- * at the @dst_offset (byte). The caller must ensure that @dst is large
- * enough (e.g. by using af_alg_count_tsgl with the same offset).
+ * the pages.
  *
  * @sk: socket of connection to user space
  * @used: Number of bytes to pull from TX SGL
  * @dst: If non-NULL, buffer is reassigned to dst SGL instead of releasing. The
  *	 caller must release the buffers in dst.
- * @dst_offset: Reassign the TX SGL from given offset. All buffers before
- *	        reaching the offset is released.
  */
-void af_alg_pull_tsgl(struct sock *sk, size_t used, struct scatterlist *dst,
-		      size_t dst_offset)
+void af_alg_pull_tsgl(struct sock *sk, size_t used, struct scatterlist *dst)
 {
 	struct alg_sock *ask = alg_sk(sk);
 	struct af_alg_ctx *ctx = ask->private;
@@ -725,18 +704,10 @@ void af_alg_pull_tsgl(struct sock *sk, size_t used, struct scatterlist *dst,
 			 * SG entries in dst.
 			 */
 			if (dst) {
-				if (dst_offset >= plen) {
-					/* discard page before offset */
-					dst_offset -= plen;
-				} else {
-					/* reassign page to dst after offset */
-					get_page(page);
-					sg_set_page(dst + j, page,
-						    plen - dst_offset,
-						    sg[i].offset + dst_offset);
-					dst_offset = 0;
-					j++;
-				}
+				/* reassign page to dst after offset */
+				get_page(page);
+				sg_set_page(dst + j, page, plen, sg[i].offset);
+				j++;
 			}
 
 			sg[i].length -= plen;

crypto/algif_aead.c

@@ -26,7 +26,6 @@
 #include <crypto/internal/aead.h>
 #include <crypto/scatterwalk.h>
 #include <crypto/if_alg.h>
-#include <crypto/skcipher.h>
 #include <crypto/null.h>
 #include <linux/init.h>
 #include <linux/list.h>
@@ -96,9 +95,8 @@ static int _aead_recvmsg(struct socket *sock, struct msghdr *msg,
 	struct aead_tfm *aeadc = pask->private;
 	struct crypto_aead *tfm = aeadc->aead;
 	struct crypto_sync_skcipher *null_tfm = aeadc->null_tfm;
-	unsigned int i, as = crypto_aead_authsize(tfm);
+	unsigned int as = crypto_aead_authsize(tfm);
 	struct af_alg_async_req *areq;
-	struct af_alg_tsgl *tsgl, *tmp;
 	struct scatterlist *rsgl_src, *tsgl_src = NULL;
 	int err = 0;
 	size_t used = 0;		/* [in]  TX bufs to be en/decrypted */
@@ -178,23 +176,24 @@ static int _aead_recvmsg(struct socket *sock, struct msghdr *msg,
 		outlen -= less;
 	}
 
+	/*
+	 * Create a per request TX SGL for this request which tracks the
+	 * SG entries from the global TX SGL.
+	 */
 	processed = used + ctx->aead_assoclen;
-	list_for_each_entry_safe(tsgl, tmp, &ctx->tsgl_list, list) {
-		for (i = 0; i < tsgl->cur; i++) {
-			struct scatterlist *process_sg = tsgl->sg + i;
-
-			if (!(process_sg->length) || !sg_page(process_sg))
-				continue;
-			tsgl_src = process_sg;
-			break;
-		}
-		if (tsgl_src)
-			break;
-	}
-	if (processed && !tsgl_src) {
-		err = -EFAULT;
+	areq->tsgl_entries = af_alg_count_tsgl(sk, processed);
+	if (!areq->tsgl_entries)
+		areq->tsgl_entries = 1;
+	areq->tsgl = sock_kmalloc(sk, array_size(sizeof(*areq->tsgl),
+					         areq->tsgl_entries),
+				  GFP_KERNEL);
+	if (!areq->tsgl) {
+		err = -ENOMEM;
 		goto free;
 	}
+	sg_init_table(areq->tsgl, areq->tsgl_entries);
+	af_alg_pull_tsgl(sk, processed, areq->tsgl);
+	tsgl_src = areq->tsgl;
 
 	/*
 	 * Copy of AAD from source to destination
@@ -203,83 +202,18 @@ static int _aead_recvmsg(struct socket *sock, struct msghdr *msg,
 	 * when user space uses an in-place cipher operation, the kernel
 	 * will copy the data as it does not see whether such in-place operation
 	 * is initiated.
-	 *
-	 * To ensure efficiency, the following implementation ensure that the
-	 * ciphers are invoked to perform a crypto operation in-place. This
-	 * is achieved by memory management specified as follows.
 	 */
 
-	/* Use the RX SGL as source (and destination) for crypto op. */
+	/* Use the RX SGL as destination; TX SGL as crypto source. */
 	rsgl_src = areq->first_rsgl.sgl.sgt.sgl;
 
-	if (ctx->enc) {
-		/*
-		 * Encryption operation - The in-place cipher operation is
-		 * achieved by the following operation:
-		 *
-		 * TX SGL: AAD || PT
-		 *	    |	   |
-		 *	    | copy |
-		 *	    v	   v
-		 * RX SGL: AAD || PT || Tag
-		 */
-		err = crypto_aead_copy_sgl(null_tfm, tsgl_src,
-					   areq->first_rsgl.sgl.sgt.sgl,
-					   processed);
-		if (err)
-			goto free;
-		af_alg_pull_tsgl(sk, processed, NULL, 0);
-	} else {
-		/*
-		 * Decryption operation - To achieve an in-place cipher
-		 * operation, the following  SGL structure is used:
-		 *
-		 * TX SGL: AAD || CT || Tag
-		 *	    |	   |	 ^
-		 *	    | copy |	 | Create SGL link.
-		 *	    v	   v	 |
-		 * RX SGL: AAD || CT ----+
-		 */
-
-		 /* Copy AAD || CT to RX SGL buffer for in-place operation. */
-		err = crypto_aead_copy_sgl(null_tfm, tsgl_src,
-					   areq->first_rsgl.sgl.sgt.sgl,
-					   outlen);
-		if (err)
-			goto free;
-
-		/* Create TX SGL for tag and chain it to RX SGL. */
-		areq->tsgl_entries = af_alg_count_tsgl(sk, processed,
-						       processed - as);
-		if (!areq->tsgl_entries)
-			areq->tsgl_entries = 1;
-		areq->tsgl = sock_kmalloc(sk, array_size(sizeof(*areq->tsgl),
-							 areq->tsgl_entries),
-					  GFP_KERNEL);
-		if (!areq->tsgl) {
-			err = -ENOMEM;
-			goto free;
-		}
-		sg_init_table(areq->tsgl, areq->tsgl_entries);
-
-		/* Release TX SGL, except for tag data and reassign tag data. */
-		af_alg_pull_tsgl(sk, processed, areq->tsgl, processed - as);
-
-		/* chain the areq TX SGL holding the tag with RX SGL */
-		if (usedpages) {
-			/* RX SGL present */
-			struct af_alg_sgl *sgl_prev = &areq->last_rsgl->sgl;
-			struct scatterlist *sg = sgl_prev->sgt.sgl;
-
-			sg_unmark_end(sg + sgl_prev->sgt.nents - 1);
-			sg_chain(sg, sgl_prev->sgt.nents + 1, areq->tsgl);
-		} else
-			/* no RX SGL present (e.g. authentication only) */
-			rsgl_src = areq->tsgl;
-	}
+	err = crypto_aead_copy_sgl(null_tfm, tsgl_src, rsgl_src,
+				   ctx->aead_assoclen);
+	if (err)
+		goto free;
 
 	/* Initialize the crypto operation */
-	aead_request_set_crypt(&areq->cra_u.aead_req, rsgl_src,
+	aead_request_set_crypt(&areq->cra_u.aead_req, tsgl_src,
 			       areq->first_rsgl.sgl.sgt.sgl, used, ctx->iv);
 	aead_request_set_ad(&areq->cra_u.aead_req, ctx->aead_assoclen);
 	aead_request_set_tfm(&areq->cra_u.aead_req, tfm);
@@ -514,7 +448,7 @@ static void aead_sock_destruct(struct sock *sk)
 	struct crypto_aead *tfm = aeadc->aead;
 	unsigned int ivlen = crypto_aead_ivsize(tfm);
 
-	af_alg_pull_tsgl(sk, ctx->used, NULL, 0);
+	af_alg_pull_tsgl(sk, ctx->used, NULL);
 	sock_kzfree_s(sk, ctx->iv, ivlen);
 	sock_kfree_s(sk, ctx, ctx->len);
 	af_alg_release_parent(sk);

crypto/algif_skcipher.c

@@ -138,7 +138,7 @@ static int _skcipher_recvmsg(struct socket *sock, struct msghdr *msg,
 	 * Create a per request TX SGL for this request which tracks the
 	 * SG entries from the global TX SGL.
 	 */
-	areq->tsgl_entries = af_alg_count_tsgl(sk, len, 0);
+	areq->tsgl_entries = af_alg_count_tsgl(sk, len);
 	if (!areq->tsgl_entries)
 		areq->tsgl_entries = 1;
 	areq->tsgl = sock_kmalloc(sk, array_size(sizeof(*areq->tsgl),
@@ -149,7 +149,7 @@ static int _skcipher_recvmsg(struct socket *sock, struct msghdr *msg,
 		goto free;
 	}
 	sg_init_table(areq->tsgl, areq->tsgl_entries);
-	af_alg_pull_tsgl(sk, len, areq->tsgl, 0);
+	af_alg_pull_tsgl(sk, len, areq->tsgl);
 
 	/* Initialize the crypto operation */
 	skcipher_request_set_tfm(&areq->cra_u.skcipher_req, tfm);
@@ -363,7 +363,7 @@ static void skcipher_sock_destruct(struct sock *sk)
 	struct alg_sock *pask = alg_sk(psk);
 	struct crypto_skcipher *tfm = pask->private;
 
-	af_alg_pull_tsgl(sk, ctx->used, NULL, 0);
+	af_alg_pull_tsgl(sk, ctx->used, NULL);
 	sock_kzfree_s(sk, ctx->iv, crypto_skcipher_ivsize(tfm));
 	if (ctx->state)
 		sock_kzfree_s(sk, ctx->state, crypto_skcipher_statesize(tfm));

include/crypto/if_alg.h

@@ -230,9 +230,8 @@ static inline bool af_alg_readable(struct sock *sk)
 	return PAGE_SIZE <= af_alg_rcvbuf(sk);
 }
 
-unsigned int af_alg_count_tsgl(struct sock *sk, size_t bytes, size_t offset);
-void af_alg_pull_tsgl(struct sock *sk, size_t used, struct scatterlist *dst,
-		      size_t dst_offset);
+unsigned int af_alg_count_tsgl(struct sock *sk, size_t bytes);
+void af_alg_pull_tsgl(struct sock *sk, size_t used, struct scatterlist *dst);
 void af_alg_wmem_wakeup(struct sock *sk);
 int af_alg_wait_for_data(struct sock *sk, unsigned flags, unsigned min);
 int af_alg_sendmsg(struct socket *sock, struct msghdr *msg, size_t size,