[PATCH 4/5] lib/crypto: poly1305: Take a size_t length in poly1305_update()

[Vladimir Kukharenko via B4 Relay]

From: Vladimir Kukharenko <vladimir at genproof.com>

poly1305_update() adds desc->buflen and nbytes as unsigned int values
to check for a full block.  With 1 to 15 bytes in the buffer and
nbytes >= 2^32 - desc->buflen, the sum wraps.  The tail path then copies
nbytes bytes into the 16-byte desc->buf.

For example, these calls trigger the bug:

    poly1305_update(&desc, p, 1);
    poly1305_update(&desc, p + 1, 0xffffffff);

Commit b646b782e522 ("lib/crypto: poly1305: Consolidate into single
module") introduced this addition.  The previous BLOCK_HASH_UPDATE()
code used size_t for the addition.

The unsigned int nbytes parameter also truncates the size_t lengths
from lib/crypto/chacha20poly1305.c.  For associated data or messages of
4 GiB or more, poly1305_update() authenticates only the first
(length mod 2^32) bytes.  Other hash-like library functions, including
sha256_update() and blake2s_update(), take size_t lengths.

Change nbytes to size_t and compare it with the space left in the
buffer.  poly1305_blocks() takes an unsigned int length, so pass it at
most UINT_MAX bytes per call, rounded down to a whole number of blocks.
On 64-bit MIPS and RISC-V, this relies on the preceding patches, which
make their assembly handle lengths of 2 GiB or more.

No in-tree caller passes inputs this large, so this is an ABI robustness
improvement.  I found the bugs while writing a Coq/VST proof of this
code with LLM agents.  I reproduced the overflow in userspace with
AddressSanitizer, and the truncation by comparing MACs with OpenSSL.

Assisted-by: LLM
Signed-off-by: Vladimir Kukharenko <vladimir at genproof.com>
---
 include/crypto/poly1305.h |  2 +-
 lib/crypto/poly1305.c     | 14 +++++++-------
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/include/crypto/poly1305.h b/include/crypto/poly1305.h
index 190beb427c6d..bf53b1df9aa3 100644
--- a/include/crypto/poly1305.h
+++ b/include/crypto/poly1305.h
@@ -61,7 +61,7 @@ struct poly1305_desc_ctx {
 void poly1305_init(struct poly1305_desc_ctx *desc,
 		   const u8 key[at_least POLY1305_KEY_SIZE]);
 void poly1305_update(struct poly1305_desc_ctx *desc,
-		     const u8 *src, unsigned int nbytes);
+		     const u8 *src, size_t nbytes);
 void poly1305_final(struct poly1305_desc_ctx *desc, u8 *digest);
 
 #endif
diff --git a/lib/crypto/poly1305.c b/lib/crypto/poly1305.c
index f313ccc4b4dd..f04c4cc2f55c 100644
--- a/lib/crypto/poly1305.c
+++ b/lib/crypto/poly1305.c
@@ -35,11 +35,9 @@ void poly1305_init(struct poly1305_desc_ctx *desc,
 EXPORT_SYMBOL(poly1305_init);
 
 void poly1305_update(struct poly1305_desc_ctx *desc,
-		     const u8 *src, unsigned int nbytes)
+		     const u8 *src, size_t nbytes)
 {
-	if (desc->buflen + nbytes >= POLY1305_BLOCK_SIZE) {
-		unsigned int bulk_len;
-
+	if (nbytes >= POLY1305_BLOCK_SIZE - desc->buflen) {
 		if (desc->buflen) {
 			unsigned int l = POLY1305_BLOCK_SIZE - desc->buflen;
 
@@ -52,12 +50,14 @@ void poly1305_update(struct poly1305_desc_ctx *desc,
 			desc->buflen = 0;
 		}
 
-		bulk_len = round_down(nbytes, POLY1305_BLOCK_SIZE);
-		nbytes %= POLY1305_BLOCK_SIZE;
+		while (nbytes >= POLY1305_BLOCK_SIZE) {
+			/* poly1305_blocks() takes an unsigned int length. */
+			unsigned int bulk_len = round_down(min(nbytes, UINT_MAX),
+							   POLY1305_BLOCK_SIZE);
 
-		if (bulk_len) {
 			poly1305_blocks(&desc->state, src, bulk_len, 1);
 			src += bulk_len;
+			nbytes -= bulk_len;
 		}
 	}
 	if (nbytes) {

-- 
2.43.0