selftests/x86/fsgsbase: Test PTRACE_PEEKUSER for GSBASE with invalid LDT GS

This tests commit:

  8ab49526b53d ("x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task")

Unpatched kernels will OOPS.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/c618ae86d1f757e01b1a8e79869f553cb88acf9a.1598461151.git.luto@kernel.org

diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
index 0056e2597f53a08937926bf75b48ba56b4c645d2..7161cfc2e60b4e92c85fe0e97e95f7f3e668c1c1 100644 (file)
--- a/tools/testing/selftests/x86/fsgsbase.c
+++ b/tools/testing/selftests/x86/fsgsbase.c
@@ -443,6 +443,68 @@ static void test_unexpected_base(void)
 
 #define USER_REGS_OFFSET(r) offsetof(struct user_regs_struct, r)
 
+static void test_ptrace_write_gs_read_base(void)
+{
+       int status;
+       pid_t child = fork();
+
+       if (child < 0)
+               err(1, "fork");
+
+       if (child == 0) {
+               printf("[RUN]\tPTRACE_POKE GS, read GSBASE back\n");
+
+               printf("[RUN]\tARCH_SET_GS to 1\n");
+               if (syscall(SYS_arch_prctl, ARCH_SET_GS, 1) != 0)
+                       err(1, "ARCH_SET_GS");
+
+               if (ptrace(PTRACE_TRACEME, 0, NULL, NULL) != 0)
+                       err(1, "PTRACE_TRACEME");
+
+               raise(SIGTRAP);
+               _exit(0);
+       }
+
+       wait(&status);
+
+       if (WSTOPSIG(status) == SIGTRAP) {
+               unsigned long base;
+               unsigned long gs_offset = USER_REGS_OFFSET(gs);
+               unsigned long base_offset = USER_REGS_OFFSET(gs_base);
+
+               /* Read the initial base.  It should be 1. */
+               base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);
+               if (base == 1) {
+                       printf("[OK]\tGSBASE started at 1\n");
+               } else {
+                       nerrs++;
+                       printf("[FAIL]\tGSBASE started at 0x%lx\n", base);
+               }
+
+               printf("[RUN]\tSet GS = 0x7, read GSBASE\n");
+
+               /* Poke an LDT selector into GS. */
+               if (ptrace(PTRACE_POKEUSER, child, gs_offset, 0x7) != 0)
+                       err(1, "PTRACE_POKEUSER");
+
+               /* And read the base. */
+               base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);
+
+               if (base == 0 || base == 1) {
+                       printf("[OK]\tGSBASE reads as 0x%lx with invalid GS\n", base);
+               } else {
+                       nerrs++;
+                       printf("[FAIL]\tGSBASE=0x%lx (should be 0 or 1)\n", base);
+               }
+       }
+
+       ptrace(PTRACE_CONT, child, NULL, NULL);
+
+       wait(&status);
+       if (!WIFEXITED(status))
+               printf("[WARN]\tChild didn't exit cleanly.\n");
+}
+
 static void test_ptrace_write_gsbase(void)
 {
        int status;
@@ -529,6 +591,9 @@ int main()
        shared_scratch = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
                              MAP_ANONYMOUS | MAP_SHARED, -1, 0);
 
+       /* Do these tests before we have an LDT. */
+       test_ptrace_write_gs_read_base();
+
        /* Probe FSGSBASE */
        sethandler(SIGILL, sigill, 0);
        if (sigsetjmp(jmpbuf, 1) == 0) {