[PATCH 3/4] fs: tftp: reject OACK with blocksize of zero

[Sascha Hauer <s.hauer@pengutronix.de>]

tftp_parse_oack() validates that windowsize is non-zero but does not
check blocksize. A malicious TFTP server sending 'blksize\0000\0' in
the OACK sets priv->blocksize to 0, which causes:

  - kfifo_alloc(0 * ...) allocating a zero-sized fifo
  - tftp_write() entering an infinite loop on
    'while (kfifo_len(fifo) >= 0)', spinning on kfifo_get with size 0
  - tftp_put_data() rejecting all data packets as oversized

Add blocksize == 0 to the existing OACK validation check.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 fs/tftp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/tftp.c b/fs/tftp.c
index 03e9d552aa..6dd4829184 100644
--- a/fs/tftp.c
+++ b/fs/tftp.c
@@ -382,7 +382,8 @@ static int tftp_parse_oack(struct file_priv *priv, unsigned char *pkt, int len)
 		s = val + strlen(val) + 1;
 	}
 
-	if (priv->blocksize > TFTP_MTU_SIZE ||
+	if (priv->blocksize == 0 ||
+	    priv->blocksize > TFTP_MTU_SIZE ||
 	    priv->windowsize > TFTP_MAX_WINDOW_SIZE ||
 	    priv->windowsize == 0) {
 		pr_warn("tftp: invalid oack response\n");

-- 
2.47.3