Project Fortify, an experimental security project, has identified the following potential security issue. If you’re a feature owner CC-ed on this bug, please do your best to review these reports. Please see https://chromium.googlesource.com/chromium/src/+/main/docs/security/ai-generated-security-bugs-faq.md for more information.

Overview: The GLES2 passthrough command decoder on macOS fails to block the GL_TEXTURE_RECTANGLE_ANGLE capability in its Enable command handler. A compromised renderer can potentially re-enable this internal-only capability to use restricted rectangle textures in WebGL shaders, bypassing a defense-in-depth measure.

Affected files:

• gpu/command_buffer/service/gles2_cmd_decoder_passthrough.cc
• gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc

Estimated timestamp from git blame: 2020-01-30

Description

On macOS, Chromium uses the GL_TEXTURE_RECTANGLE_ANGLE capability internally to support IOSurface-backed textures. To prevent WebGL user shaders from accessing these textures, Chromium implements a hardening measure by explicitly disabling this capability for WebGL contexts during initialization. This isolation is enforced by the ANGLE shader translator, which blocks the ARB_texture_rectangle extension if the capability is disabled.

However, the passthrough command decoder’s DoEnable handler does not sufficiently filter this capability. The handler relies on IsIgnoredCap to block restricted enums, but IsIgnoredCap lacks an entry for GL_TEXTURE_RECTANGLE_ANGLE. Since ANGLE allows this capability to be toggled for WebGL contexts, a compromised renderer can re-enable it via a standard glEnable command.

Technical Details

In gpu/command_buffer/service/gles2_cmd_decoder_passthrough.cc, the capability is disabled at initialization for WebGL contexts:

#if BUILDFLAG(IS_MAC)
  if (feature_info_->IsWebGLContext())
    api()->glDisableFn(GL_TEXTURE_RECTANGLE_ANGLE);
#endif

The DoEnable implementation in gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc validates capabilities using IsIgnoredCap:

error::Error GLES2DecoderPassthroughImpl::DoEnable(GLenum cap) {
  if (IsIgnoredCap(cap)) {
    InsertError(GL_INVALID_ENUM, "Invalid cap.");
    return error::kNoError;
  }
  api()->glEnableFn(cap);
  return error::kNoError;
}

The IsIgnoredCap function in gpu/command_buffer/service/gles2_cmd_decoder_passthrough.cc fails to include GL_TEXTURE_RECTANGLE_ANGLE (0x84F5), allowing it to fall through to the default return false case. This allows the glEnable call to reach ANGLE.

Impact

By re-enabling GL_TEXTURE_RECTANGLE_ANGLE, the ANGLE shader translator will no longer set the disableARBTextureRectangle compiler option. This allows a compromised renderer to compile and execute shaders that use #extension GL_ARB_texture_rectangle : enable and sampler2DRect types. This represents a bypass of a hardening layer designed to isolate WebGL from internal macOS texture types.

Potential Steps to Reproduce

These steps are based on code analysis and represent a potential attack vector:

1. From a compromised renderer process on macOS (using the ANGLE OpenGL backend), send a gles2::cmds::Enable command with cap = 0x84F5 (GL_TEXTURE_RECTANGLE_ANGLE).
2. Send a ShaderSource command containing GLSL code that attempts to use the GL_ARB_texture_rectangle extension.
3. Send a CompileShader command and observe that the shader compiles successfully.
4. In a debug build, subsequent internal usage of ScopedEnableTextureRectangleInShaderCompiler may trigger a DCHECK in ui/gl/gl_utils.cc because the capability is unexpectedly already enabled.

Suggested Fix

Modify GLES2DecoderPassthroughImpl::IsIgnoredCap in gpu/command_buffer/service/gles2_cmd_decoder_passthrough.cc to include GL_TEXTURE_RECTANGLE_ANGLE, ensuring it returns true (or at least feature_info_->IsWebGLContext()) to block the renderer from toggling this state.

Evaluated with Chrome root at commit: b3153093eb3c78c3e88ccf562bcbc20437a04b0e

Results so far have been promising, but there can be wrong deductions. Feel free to adjust as follows:

• If you are familiar with the severity guidelines, you may adjust the severity.
• If this is a false positive, and there’s no work to be done, please close as WAI.
• If there is work to do here but not a vulnerability, please change the issue type to Task/Bug/FR.

Data from false positives will be used to improve accuracy over time. And please feel free to reach out to me directly if you have concerns or feedback on the project.