A number of HTTP/2 CVEs were documented recently:
https://www.kb.cert.org/vuls/id/605641/
This commit, along with a few changes and additions in Cowlib,
fix or improve protection against all of them.
For CVE-2019-9511, also known as Data Dribble, the new option
stream_window_data_threshold can be used to control how little
the DATA frames that Cowboy sends can get.
For CVE-2019-9516, also known as 0-Length Headers Leak, Cowboy
will now simply reject streams containing 0-length header names.
For CVE-2019-9517, also known as Internal Data Buffering, the
backpressure changes were already pretty good at preventing this
issue, but a new option max_connection_buffer_size was added for
even better control over how much memory we are willing to allocate.
For CVE-2019-9512, also known as Ping Flood; CVE-2019-9515, also
known as Settings Flood; CVE-2019-9518, also known as Empty Frame
Flooding; and similar undocumented scenarios, a frame rate limiting
mechanism was added. By default Cowboy will now allow 1000 frames
every 10 seconds. This can be configured via max_received_frame_rate.
For CVE-2019-9514, also known as Reset Flood, another rate limiting
mechanism was added and can be configured via max_reset_stream_rate.
By default Cowboy will do up to 10 stream resets every 10 seconds.
Finally, nothing was done for CVE-2019-9513, also known as Resource
Loop, because Cowboy does not currently implement the HTTP/2
priority mechanism (in parts because these issues were well known
from the start).
Tests were added for all cases except Internal Data Buffering,
which I'm not sure how to test, and Resource Loop, which is not
currently relevant.
This should limit the amount of memory that Cowboy is using
when a handler is sending data much faster than the network.
The new max_stream_buffer_size is a soft limit and only has
an effect when the cowboy_stream_h handler is used.
For long-running connections it was possible for the connection
window to become larger than allowed by the protocol because the
window increases claimed by stream handlers were never reclaimed
even if no data was consumed.
The new code applies heuristics to fix this and reduce the number
of WINDOW_UPDATE frames that are sent. It includes six new options
to control that behavior: margin, max and threshold for both the
connection and stream windows. The margin is some extra space
added on top of the requested read size. The max is the maximum
window size at any given time. The threshold is a minimum window
size that must be reached before we even consider sending more
WINDOW_UPDATE frames. We also avoid sending WINDOW_UPDATE frames
when there is already enough space in the window, or when the
read size is 0.
Cowlib is set to master until a new tag is done.
It allows disabling the chunked transfer-encoding. It
can also be disabled on a per-request basis, although
it will be ignored for responses that are not streamed.
They allow the server to configure what it is willing to accept
for both the negotiated configuration (takeover and window bits)
and the other zlib options (level, mem_level and strategy).
This can be used to reduce the memory and/or CPU footprint of
the compressed data, which comes with a cost in compression ratio.
It is now possible to stream one or more sendfile tuples.
A simple example of what can now be done would be for
example to build a tar file on the fly using the sendfile
syscall for sending the files, or to support Range requests
with more than one range with the sendfile syscall.
When using cowboy_compress_h unfortunately we have to read
the file in order to send it. More options will be added
at a later time to make sure users don't read too much
into memory. This is a new feature however so existing
code is not affected.
Also rework cowboy_http's data sending to be flatter.
Fix cases where the q-value is 0 and where a wildcard
was sent in the accept-charset header.
Also don't send a charset in the content-type of the
response if the media type is not text.
Thanks to Philip Witty for help figuring this out.
This commit reworks the logging that Cowboy does via
error_logger to make the module that will do the actual
logging configurable.
The logger module interface must be the same as logger
and lager: a separate function per log level with the
same log levels they support.
The default behavior remains to call error_logger,
although some messages were downgraded to warnings
instead of errors. Since error_logger only supports
three different log levels, some messages may get
downgraded/upgraded depending on what the original
log level was to make them compatible with error_logger.
The {log, Level, Format, Args} command was also
added to stream handlers. Stream handlers should
use this command to log messages because it allows
writing a stream handler to intercept some of those
messages and extract information or block them as
necessary.
The logger option only applies to Cowboy itself,
not to the messages Ranch logs, so more work remains
to be done in that area.
