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Fix incorrect globals/sigstack allocation layout #401
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iximeow
previously approved these changes
Jan 24, 2020
Most of this patch is actually to fix up the tests that were erroneously succeeding because of this bug. The core of the fix is here in `mmap.rs`: ``` - let sigstack = globals + host_page_size(); + let sigstack = globals + region.limits.globals_size + host_page_size(); ``` This was causing the signal stack to begin one page after the start of the globals section, regardless of how big the globals are. This mostly wasn't causing problems because the default globals size is a page, and we _were_ still adding in a page for the guard, meaning the segments didn't actually overlap with default `Limits`. However, the guard page for the signal stack was missing, and configurations with larger globals sizes could end up overlapping the signal stack, potentially allowing guests to observe residual signal stack values. The lack of a signal stack guard page was _also_ making it so that tests running in debug mode would succeed, even though the signal handler was overflowing its stack into the globals page. To make those tests pass with the correct memory layout, the signal stack size is now configurable and set higher by default on debug builds (`cfg(debug_assertions)`). On release mode, the stack use is fine as the problematic series of stack allocations get optimized into some straightforward copies, so we continue to use SIGSTKSZ as a default in that configuration. Finally, I discovered some linking errors with the tests on release mode due to overly-aggressive stripping, so I had to refactor a couple tests.
Also adds a test to ensure that a fault in the sigstack guard page is fatal
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iximeow
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Most of this patch is actually to fix up the tests that were erroneously succeeding because of this bug. The core of the fix is here in
mmap.rs:This was causing the signal stack to begin one page after the start of the globals section, regardless of how big the globals are. This mostly wasn't causing problems because the default globals size is a page, and we were still adding in a page for the guard, meaning the segments didn't actually overlap with default
Limits. However, the guard page for the signal stack was missing, and configurations with larger globals sizes could end up overlapping the signal stack, potentially allowing guests to observe residual signal stack values.The lack of a signal stack guard page was also making it so that tests running in debug mode would succeed, even though the signal handler was overflowing its stack into the globals page. To make those tests pass with the correct memory layout, the signal stack size is now configurable and set higher by default on debug builds (
cfg(debug_assertions)). On release mode, the stack use is fine as the problematic series of stack allocations get optimized into some straightforward copies, so we continue to use SIGSTKSZ as a default in that configuration.Finally, I added a test that tries to write to the sigstack guard page, improved the "is this a fatal fault address?" method, and made sure that all tests run correctly in release mode.