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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Javier Martinez Canillas <javierm@redhat.com>
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Date: Mon, 24 Aug 2020 14:46:27 +0200
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Subject: [PATCH] tftp: roll over block counter to prevent timeouts with data
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packets
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The block number is a 16-bit counter which only allows to fetch
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files no bigger than 65535 * blksize. To avoid this limit, the
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counter is rolled over. This behavior isn't defined in RFC 1350
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but is handled by many TFTP servers and it's what GRUB was doing
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before implicitly due an overflow.
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Fixing that bug led to TFTP timeouts, since GRUB wasn't acking
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data packets anymore for files with size bigger than the maximum
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mentioned above. Restore the old behavior to prevent this issue.
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Resolves: rhbz#1871034
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Suggested-by: Peter Jones <pjones@redhat.com>
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Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
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---
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grub-core/net/tftp.c | 16 ++++++++++++++--
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1 file changed, 14 insertions(+), 2 deletions(-)
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diff --git a/grub-core/net/tftp.c b/grub-core/net/tftp.c
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index 79c16f9b041..b9a4b607a3d 100644
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--- a/grub-core/net/tftp.c
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+++ b/grub-core/net/tftp.c
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@@ -183,8 +183,20 @@ tftp_receive (grub_net_udp_socket_t sock __attribute__ ((unused)),
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return GRUB_ERR_NONE;
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}
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- /* Ack old/retransmitted block. */
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- if (grub_be_to_cpu16 (tftph->u.data.block) < data->block + 1)
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+ /*
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+ * Ack old/retransmitted block.
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+ *
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+ * The block number is a 16-bit counter which only allows to fetch
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+ * files no bigger than 65535 * blksize. To avoid this limit, the
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+ * counter is rolled over. This behavior isn't defined in RFC 1350
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+ * but is handled by many TFTP servers and it's what GRUB was doing
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+ * before implicitly due an overflow.
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+ *
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+ * Fixing that bug led to TFTP timeouts, since GRUB wasn't acking
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+ * data packets anymore for files with size bigger than the maximum
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+ * mentioned above. Restore the old behavior to prevent this issue.
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+ */
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+ if (grub_be_to_cpu16 (tftph->u.data.block) < ((data->block + 1) & 0xffffu))
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ack (data, grub_be_to_cpu16 (tftph->u.data.block));
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/* Ignore unexpected block. */
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else if (grub_be_to_cpu16 (tftph->u.data.block) > data->block + 1)
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