nanocoap: always write at least 1 byte in coap_block2_finish()#20855
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maribu merged 1 commit intoRIOT-OS:masterfrom Sep 12, 2024
Merged
nanocoap: always write at least 1 byte in coap_block2_finish()#20855maribu merged 1 commit intoRIOT-OS:masterfrom
maribu merged 1 commit intoRIOT-OS:masterfrom
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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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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