selftests/x86/fsgsbase: Test ptracer-induced GSBASE write

The test validates that the selector is not changed when a ptracer writes
the ptracee's GSBASE.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: https://lkml.kernel.org/r/1557309753-24073-3-git-send-email-chang.seok.bae@intel.com

diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
index af85bd4..b02ddce 100644 (file)
--- a/tools/testing/selftests/x86/fsgsbase.c
+++ b/tools/testing/selftests/x86/fsgsbase.c
@@ -23,6 +23,9 @@
 #include <pthread.h>
 #include <asm/ldt.h>
 #include <sys/mman.h>
+#include <stddef.h>
+#include <sys/ptrace.h>
+#include <sys/wait.h>
 
 #ifndef __x86_64__
 # error This test is 64-bit only
@@ -367,6 +370,71 @@ static void test_unexpected_base(void)
        }
 }
 
+#define USER_REGS_OFFSET(r) offsetof(struct user_regs_struct, r)
+
+static void test_ptrace_write_gsbase(void)
+{
+       int status;
+       pid_t child = fork();
+
+       if (child < 0)
+               err(1, "fork");
+
+       if (child == 0) {
+               printf("[RUN]\tPTRACE_POKE(), write GSBASE from ptracer\n");
+
+               /*
+                * Use the LDT setup and fetch the GSBASE from the LDT
+                * by switching to the (nonzero) selector (again)
+                */
+               do_unexpected_base();
+               asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0x7));
+
+               if (ptrace(PTRACE_TRACEME, 0, NULL, NULL) != 0)
+                       err(1, "PTRACE_TRACEME");
+
+               raise(SIGTRAP);
+               _exit(0);
+       }
+
+       wait(&status);
+
+       if (WSTOPSIG(status) == SIGTRAP) {
+               unsigned long gs;
+               unsigned long gs_offset = USER_REGS_OFFSET(gs);
+               unsigned long base_offset = USER_REGS_OFFSET(gs_base);
+
+               gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);
+
+               if (gs != 0x7) {
+                       nerrs++;
+                       printf("[FAIL]\tGS is not prepared with nonzero\n");
+                       goto END;
+               }
+
+               if (ptrace(PTRACE_POKEUSER, child, base_offset, 0xFF) != 0)
+                       err(1, "PTRACE_POKEUSER");
+
+               gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);
+
+               /*
+                * In a non-FSGSBASE system, the nonzero selector will load
+                * GSBASE (again). But what is tested here is whether the
+                * selector value is changed or not by the GSBASE write in
+                * a ptracer.
+                */
+               if (gs != 0x7) {
+                       nerrs++;
+                       printf("[FAIL]\tGS changed to %lx\n", gs);
+               } else {
+                       printf("[OK]\tGS remained 0x7\n");
+               }
+       }
+
+END:
+       ptrace(PTRACE_CONT, child, NULL, NULL);
+}
+
 int main()
 {
        pthread_t thread;
@@ -423,5 +491,7 @@ int main()
        if (pthread_join(thread, NULL) != 0)
                err(1, "pthread_join");
 
+       test_ptrace_write_gsbase();
+
        return nerrs == 0 ? 0 : 1;
 }