[Mesa-dev] [PATCH v2] glsl: fix heap-use-after-free in ast_declarator_list::hir()

[Ian Romanick]

NAK.

The way the code is currently architected, if earlier is non-NULL, var
should not be used.  There are quite a few places where the callees do
things like 'const glsl_type *const t = (earlier == NULL) ? var->type :
earlier->type;'.  This is a bit confusing, but this change muddles that
and makes it worse.

I think the actual use-after-free is in setting implicitly_sized at line
5265.  I think doing something like

        const enum ir_variable_mode *mode = earlier == NULL
            ? &var->data.mode : &earlier->data.mode;
         const bool implicitly_sized =
            (mode == ir_var_shader_in &&
             state->stage >= MESA_SHADER_TESS_CTRL &&
             state->stage <= MESA_SHADER_GEOMETRY) ||
            (mode == ir_var_shader_out &&
             state->stage == MESA_SHADER_TESS_CTRL);

should also fix the problem.  This should be tagged for stable.

After that fix, we may want to look at rearchitecting this code to be
less... confusing.  The fundamental problem is
get_variable_being_redeclared is really trying to return two pieces of
information, but it does this through a single variable.

1. Is this actually a redeclaration?

2. What is the variable that needs to be modified?

When get_variable_being_redeclared returns NULL, it's not a
redeclaration, and the variable being modified is the ir_variable passed
in.  When it returns non-NULL, it is a redeclaration, and the variable
being modified is the one returned.

Maybe the function should always return non-NULL (returning either
'earlier' or 'var') and have an extra parameter 'bool *redeclaration'.
I think that would result in almost no changes to the second caller and
several simplifications to the first caller.  Thoughts?

On 02/07/2017 01:47 PM, Samuel Iglesias Gonsálvez wrote:
> The get_variable_being_redeclared() function can free 'var' because
> a re-declaration of an unsized array variable can establish the size, so
> we set the array type to the 'earlier' declaration and free 'var' as it is
> not needed anymore.
> 
> However, the same 'var' is referenced later in ast_declarator_list::hir().
> 
> This patch fixes it by assign the pointer 'var' to the pointer 'earlier'.
> 
> This error was detected by Address Sanitizer.
> 
> v2:
> 
> * Pointer-to-pointer assignment (Bartosz Tomczyk)
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99677
> Signed-off-by: Samuel Iglesias Gonsálvez <siglesias at igalia.com>
> ---
> 
> Another possibility is to use reference-to-pointer but it is a C++
> thing. IIRC, we agreed on avoiding C++-specific features to make it
> easy for C developers, but I have no strong opinion for either option.
> 
>  src/compiler/glsl/ast_to_hir.cpp | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/src/compiler/glsl/ast_to_hir.cpp b/src/compiler/glsl/ast_to_hir.cpp
> index b31b61d1ed6..93ba1d510fa 100644
> --- a/src/compiler/glsl/ast_to_hir.cpp
> +++ b/src/compiler/glsl/ast_to_hir.cpp
> @@ -3958,10 +3958,12 @@ apply_type_qualifier_to_variable(const struct ast_type_qualifier *qual,
>   * is a redeclaration, \c NULL otherwise.
>   */
>  static ir_variable *
> -get_variable_being_redeclared(ir_variable *var, YYLTYPE loc,
> +get_variable_being_redeclared(ir_variable **var_pointer, YYLTYPE loc,
>                                struct _mesa_glsl_parse_state *state,
>                                bool allow_all_redeclarations)
>  {
> +   ir_variable *var = *var_pointer;
> +
>     /* Check if this declaration is actually a re-declaration, either to
>      * resize an array or add qualifiers to an existing variable.
>      *
> @@ -3999,7 +4001,7 @@ get_variable_being_redeclared(ir_variable *var, YYLTYPE loc,
>  
>        earlier->type = var->type;
>        delete var;
> -      var = NULL;
> +      *var_pointer = earlier;
>     } else if ((state->ARB_fragment_coord_conventions_enable ||
>                state->is_version(150, 0))
>                && strcmp(var->name, "gl_FragCoord") == 0
> @@ -5207,7 +5209,7 @@ ast_declarator_list::hir(exec_list *instructions,
>        bool var_is_gl_id = is_gl_identifier(var->name);
>  
>        ir_variable *earlier =
> -         get_variable_being_redeclared(var, decl->get_location(), state,
> +         get_variable_being_redeclared(&var, decl->get_location(), state,
>                                         false /* allow_all_redeclarations */);
>        if (earlier != NULL) {
>           if (var_is_gl_id &&
> @@ -7873,7 +7875,7 @@ ast_interface_block::hir(exec_list *instructions,
>  
>           if (redeclaring_per_vertex) {
>              ir_variable *earlier =
> -               get_variable_being_redeclared(var, loc, state,
> +               get_variable_being_redeclared(&var, loc, state,
>                                               true /* allow_all_redeclarations */);
>              if (!var_is_gl_id || earlier == NULL) {
>                 _mesa_glsl_error(&loc, state,
>