Security fixes #921

Move the checks around to avoid missing cases where we might access memory that is no longer valid. Also avoid touching the underlying stream implicitly (e.g. via the destructor) as it might also no longer be valid. A malicious server could theoretically use this for remote code execution in the client. Issue found by Pavel Cheremushkin from Kaspersky Lab

Don't allow subclasses to just override dimensions or buffer details directly and instead force them to go via methods. This allows us to do sanity checks on the new values and catch bugs and attacks.

We do a lot of calculations based on pixel coordinates and we need to make sure they do not overflow. Restrict the maximum dimensions we support rather than try to switch over all calculations to use 64 bit integers. This prevents attackers from from injecting code by specifying a huge framebuffer size and relying on the values overflowing to access invalid areas of the heap. This primarily affects the client which gets both the screen dimensions and the pixel contents from the remote side. But the server might also be affected as a client can adjust the screen dimensions, as can applications inside the session. Issue found by Pavel Cheremushkin from Kaspersky Lab.

No one should every try to write to this buffer. Enforce that by throwing an exception if any one tries to get a writeable pointer to the data.

We always assumed there would be one pixel per row so a rect with a zero width would result in us writing to unknown memory. This could theoretically be used by a malicious server to inject code in to the viewer process. Issue found by Pavel Cheremushkin from Kaspersky Lab.

Otherwise we might be tricked in to reading and writing things at incorrect offsets for pixels which ultimately could result in an attacker writing things to the stack or heap and executing things they shouldn't. This only affects the server as the client never uses the pixel format suggested by th server. Issue found by Pavel Cheremushkin from Kaspersky Lab.

Provides safety against them accidentally becoming negative because of bugs in the calculations. Also does the same to CharArray and friends as they were strongly connection to the stream objects.

We use a lot of lengths given to us over the network, so be more paranoid about them causing an overflow as otherwise an attacker might trick us in to overwriting other memory. This primarily affects the client which often gets lengths from the server, but there are also some scenarios where the server might theoretically be vulnerable. Issue found by Pavel Cheremushkin from Kaspersky Lab.

Our fast paths assume that each channel fits in to a separate byte. That means the shift needs to be a multiple of 8. Start actually checking this so that a client cannot trip us up and possibly cause incorrect code exection. Issue found by Pavel Cheremushkin from Kaspersky Lab.

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Security fixes #921

Security fixes #921

Commits on Nov 15, 2019