Add guard pages to the front of linear memories by alexcrichton · Pull Request #2977 · bytecodealliance/wasmtime

alexcrichton · 2021-06-10T17:58:45Z

This commit implements a safety feature for Wasmtime to place guard
pages before the allocation of all linear memories. Guard pages placed
after linear memories are typically present for performance (at least)
because it can help elide bounds checks. Guard pages before a linear
memory, however, are never strictly needed for performance or features.
The intention of a preceding guard page is to help insulate against bugs
in Cranelift or other code generators, such as CVE-2021-32629.

This commit adds a Config::guard_before_linear_memory configuration
option, defaulting to true, which indicates whether guard pages should
be present both before linear memories as well as afterwards. Guard
regions continue to be controlled by
{static,dynamic}_memory_guard_size methods.

The implementation here affects both on-demand allocated memories as
well as the pooling allocator for memories. For on-demand memories this
adjusts the size of the allocation as well as adjusts the calculations
for the base pointer of the wasm memory. For the pooling allocator this
will place a singular extra guard region at the very start of the
allocation for memories. Since linear memories in the pooling allocator
are contiguous every memory already had a preceding guard region in
memory, it was just the previous memory's guard region afterwards. Only
the first memory needed this extra guard.

I've attempted to write some tests to help test all this, but this is
all somewhat tricky to test because the settings are pretty far away
from the actual behavior. I think, though, that the tests added here
should help cover various use cases and help us have confidence in
tweaking the various Config settings beyond their defaults.

Note that this also contains a semantic change where
InstanceLimits::memory_reservation_size has been removed. Instead this
field is now inferred from the static_memory_maximum_size and guard
size settings. This should hopefully remove some duplication in these
settings, canonicalizing on the guard-size/static-size settings as the
way to control memory sizes and virtual reservations.

github-actions · 2021-06-10T18:11:20Z

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peterhuene

This looks great!

Just the one question about what to do regarding 32-bit default values for static memory sizes and a few nits.

This commit implements a safety feature for Wasmtime to place guard pages before the allocation of all linear memories. Guard pages placed after linear memories are typically present for performance (at least) because it can help elide bounds checks. Guard pages before a linear memory, however, are never strictly needed for performance or features. The intention of a preceding guard page is to help insulate against bugs in Cranelift or other code generators, such as CVE-2021-32629. This commit adds a `Config::guard_before_linear_memory` configuration option, defaulting to `true`, which indicates whether guard pages should be present both before linear memories as well as afterwards. Guard regions continue to be controlled by `{static,dynamic}_memory_guard_size` methods. The implementation here affects both on-demand allocated memories as well as the pooling allocator for memories. For on-demand memories this adjusts the size of the allocation as well as adjusts the calculations for the base pointer of the wasm memory. For the pooling allocator this will place a singular extra guard region at the very start of the allocation for memories. Since linear memories in the pooling allocator are contiguous every memory already had a preceding guard region in memory, it was just the previous memory's guard region afterwards. Only the first memory needed this extra guard. I've attempted to write some tests to help test all this, but this is all somewhat tricky to test because the settings are pretty far away from the actual behavior. I think, though, that the tests added here should help cover various use cases and help us have confidence in tweaking the various `Config` settings beyond their defaults. Note that this also contains a semantic change where `InstanceLimits::memory_reservation_size` has been removed. Instead this field is now inferred from the `static_memory_maximum_size` and guard size settings. This should hopefully remove some duplication in these settings, canonicalizing on the guard-size/static-size settings as the way to control memory sizes and virtual reservations.

Makes the pooling allocator a bit more reasonable by default on 32-bit with these settings.

uweigand · 2021-06-21T17:27:13Z

+            (2, &["i32.load16_s"]),
+            (4, &["i32.load" /*, "f32.load"*/]),
+            (8, &["i64.load" /*, "f64.load"*/]),
+            (16, &["v128.load"]),


This unconditionally adds a SIMD operation, which we don't support on s390x yet, causing the test to fail.

Can we make this part architecture-dependent for now?

Oh sure yeah, it's also fine to just comment this out since it's not really the most critical part of the test anyway

uweigand · 2021-06-22T12:02:53Z

Huh, it turns out this test case also exposes multiple pre-existing bugs in trap handling, both in the s390x back-end and in common code. All in all, I need the following four patches to make the test case pass:
#3012
#3013
#3014
#3015

github-actions Bot added fuzzing Issues related to our fuzzing infrastructure wasmtime:api Related to the API of the `wasmtime` crate itself labels Jun 10, 2021

alexcrichton force-pushed the pre-guard branch from a000746 to 88128ed Compare June 10, 2021 20:54

alexcrichton requested a review from peterhuene June 17, 2021 14:59