nanocoap: always write at least 1 byte in coap_block2_finish() #20855
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The CoAP block option gets written twice: First a 'dummy' value is written by `coap_opt_add_block2()`, later this gets overwritten by the real option value by coap_block2_finish(). The problem arises when the size of the option changes. If the option ends up smaller than the dummy, we have garbage bytes after the real option value, corrupting the packet. To mitigate this, always write at least one option byte (which will be a 0 byte) to ensure the dummy data is overwritten. fixes RIOT-OS#20686
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maribu
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Sep 12, 2024
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Contribution description
The CoAP block option gets written twice:
First a 'dummy' value is written by
coap_opt_add_block2(), later this gets overwritten by the real option value bycoap_block2_finish().The problem arises when the size of the option changes. If the option ends up smaller than the dummy, we have garbage bytes after the real option value, corrupting the packet.
To mitigate this, always write at least one option byte (which will be a 0 byte) to ensure the dummy data is overwritten.
Testing procedure
run
examples/gcoap_fileserver:create a dummy file
Try to access the file via e.g.
ncgetwith a 16 byte block sizeIssues/PRs references
fixes #20686
alternative to #20688