cpu_context_barrier and cpu_speculation_barrier Subroutines
Purpose
Provides protection against speculative execution side-channel attacks.
Library
Standard C Library (libc.a)
Syntax
#include <sys/processor.h>void cpu_context_barrier (int value)
void cpu_speculation_barrier (void)
Description
The cpu_context_barrier and cpu_speculation_barrier subroutines
provide applications with processor-model-dependent mitigation against known speculative-execution
vulnerabilities. These subroutines can be used by both 32-bit and 64-bit applications to protect
applications against data-dependent storage access and to provide isolation between the trusted and
untrusted segments of an application.
Note: Application performance might reduce when the
cpu_context_barrier or cpu_speculation_barrier subroutine is used.
The cpu_context_barrier subroutine must be called from within the trusted domain and must be executed at each transition between the trusted domain and the untrusted domain. This subroutine accepts a single parameter that specifies the method in which the subroutine is used. Alternatively, a comprehensive variation of the barrier kernel subroutine can be used for scenarios where it is difficult to distinguish the method in which the subroutine must be used.
The cpu_speculation_barrier subroutine must be called from within the trusted domain before storage is accessed by using addresses that are computed from an untrusted source.
Parameters
| Item | Description |
|---|---|
| value | Specifies the method in which the barrier subroutine is being invoked. |
CPU context barrier values
| Item | Description |
|---|---|
| CCB_ENTRY | Specify this value when transitioning into a trusted context domain. |
| CCB_EXIT | Specify this value when transitioning out of a trusted context domain. |
| CCB_ALL | Specify this value when transitioning into a trusted context domain or transitioning out of a trusted context domain. |
Example
The following example shows how the trusted domain of an application calls an untrusted
domain:
int index;
char val,
udata[];
extern int max_tdata_index;
extern char tdata[];
/* Fetch index from untrusted user */
cpu_context_barrier(CCB_EXIT);
index = get_index_from_user(...);
cpu_context_barrier(CCB_ENTRY);
/* Select trusted data from user input */
if (index < max_tdata_index) {
cpu_speculation_barrier();
val = tdata[index];
udata[val]++;
}