From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001 From: Javier Martinez Canillas Date: Mon, 24 Aug 2020 14:46:27 +0200 Subject: [PATCH] tftp: roll over block counter to prevent timeouts with data packets The block number is a 16-bit counter which only allows to fetch files no bigger than 65535 * blksize. To avoid this limit, the counter is rolled over. This behavior isn't defined in RFC 1350 but is handled by many TFTP servers and it's what GRUB was doing before implicitly due an overflow. Fixing that bug led to TFTP timeouts, since GRUB wasn't acking data packets anymore for files with size bigger than the maximum mentioned above. Restore the old behavior to prevent this issue. Resolves: rhbz#1871034 Suggested-by: Peter Jones Signed-off-by: Javier Martinez Canillas --- grub-core/net/tftp.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff --git a/grub-core/net/tftp.c b/grub-core/net/tftp.c index 2c52f692b25..4f38d7ef1f7 100644 --- a/grub-core/net/tftp.c +++ b/grub-core/net/tftp.c @@ -183,8 +183,20 @@ tftp_receive (grub_net_udp_socket_t sock __attribute__ ((unused)), return GRUB_ERR_NONE; } - /* Ack old/retransmitted block. */ - if (grub_be_to_cpu16 (tftph->u.data.block) < data->block + 1) + /* + * Ack old/retransmitted block. + * + * The block number is a 16-bit counter which only allows to fetch + * files no bigger than 65535 * blksize. To avoid this limit, the + * counter is rolled over. This behavior isn't defined in RFC 1350 + * but is handled by many TFTP servers and it's what GRUB was doing + * before implicitly due an overflow. + * + * Fixing that bug led to TFTP timeouts, since GRUB wasn't acking + * data packets anymore for files with size bigger than the maximum + * mentioned above. Restore the old behavior to prevent this issue. + */ + if (grub_be_to_cpu16 (tftph->u.data.block) < ((data->block + 1) & 0xffffu)) ack (data, grub_be_to_cpu16 (tftph->u.data.block)); /* Ignore unexpected block. */ else if (grub_be_to_cpu16 (tftph->u.data.block) > data->block + 1)