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Subject[tip:x86/cpu] selftests/x86/fsgsbase: Fix some test case bugs
Commit-ID:  697096b14444f458fb81212d1c82d7846e932455
Gitweb: https://git.kernel.org/tip/697096b14444f458fb81212d1c82d7846e932455
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Tue, 2 Jul 2019 20:43:04 -0700
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 3 Jul 2019 16:24:56 +0200

selftests/x86/fsgsbase: Fix some test case bugs

This refactors do_unexpected_base() to clean up some code. It also
fixes the following bugs in test_ptrace_write_gsbase():

- Incorrect printf() format string caused crashes.

- Hardcoded 0x7 for the gs selector was not reliably correct.

It also documents the fact that the test is expected to fail on old
kernels.

Fixes: a87730cc3acc ("selftests/x86/fsgsbase: Test ptracer-induced GSBASE write with FSGSBASE")
Fixes: 1b6858d5a2eb ("selftests/x86/fsgsbase: Test ptracer-induced GSBASE write")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "BaeChang Seok" <chang.seok.bae@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: "BaeChang Seok" <chang.seok.bae@intel.com>
Link: https://lkml.kernel.org/r/bab29c84f2475e2c30ddb00f1b877fcd7f4f96a8.1562125333.git.luto@kernel.org


---
tools/testing/selftests/x86/fsgsbase.c | 74 ++++++++++++++++++----------------
1 file changed, 40 insertions(+), 34 deletions(-)

diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
index 21fd4f94b5b0..5ab4c60c100e 100644
--- a/tools/testing/selftests/x86/fsgsbase.c
+++ b/tools/testing/selftests/x86/fsgsbase.c
@@ -35,6 +35,8 @@
static volatile sig_atomic_t want_segv;
static volatile unsigned long segv_addr;

+static unsigned short *shared_scratch;
+
static int nerrs;

static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
@@ -242,16 +244,11 @@ static void do_remote_base()

static __thread int set_thread_area_entry_number = -1;

-static void do_unexpected_base(void)
+static unsigned short load_gs(void)
{
/*
- * The goal here is to try to arrange for GS == 0, GSBASE !=
- * 0, and for the the kernel the think that GSBASE == 0.
- *
- * To make the test as reliable as possible, this uses
- * explicit descriptors. (This is not the only way. This
- * could use ARCH_SET_GS with a low, nonzero base, but the
- * relevant side effect of ARCH_SET_GS could change.)
+ * Sets GS != 0 and GSBASE != 0 but arranges for the kernel to think
+ * that GSBASE == 0 (i.e. thread.gsbase == 0).
*/

/* Step 1: tell the kernel that we have GSBASE == 0. */
@@ -271,8 +268,9 @@ static void do_unexpected_base(void)
.useable = 0
};
if (syscall(SYS_modify_ldt, 1, &desc, sizeof(desc)) == 0) {
- printf("\tother thread: using LDT slot 0\n");
+ printf("\tusing LDT slot 0\n");
asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0x7));
+ return 0x7;
} else {
/* No modify_ldt for us (configured out, perhaps) */

@@ -294,20 +292,15 @@ static void do_unexpected_base(void)

if (ret != 0) {
printf("[NOTE]\tcould not create a segment -- test won't do anything\n");
- return;
+ return 0;
}
- printf("\tother thread: using GDT slot %d\n", desc.entry_number);
+ printf("\tusing GDT slot %d\n", desc.entry_number);
set_thread_area_entry_number = desc.entry_number;

- asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)((desc.entry_number << 3) | 0x3)));
+ unsigned short gs = (unsigned short)((desc.entry_number << 3) | 0x3);
+ asm volatile ("mov %0, %%gs" : : "rm" (gs));
+ return gs;
}
-
- /*
- * Step 3: set the selector back to zero. On AMD chips, this will
- * preserve GSBASE.
- */
-
- asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
}

void test_wrbase(unsigned short index, unsigned long base)
@@ -346,12 +339,19 @@ static void *threadproc(void *ctx)
if (ftx == 3)
return NULL;

- if (ftx == 1)
+ if (ftx == 1) {
do_remote_base();
- else if (ftx == 2)
- do_unexpected_base();
- else
+ } else if (ftx == 2) {
+ /*
+ * On AMD chips, this causes GSBASE != 0, GS == 0, and
+ * thread.gsbase == 0.
+ */
+
+ load_gs();
+ asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
+ } else {
errx(1, "helper thread got bad command");
+ }

ftx = 0;
syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
@@ -453,12 +453,7 @@ static void test_ptrace_write_gsbase(void)
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));
+ *shared_scratch = load_gs();

if (ptrace(PTRACE_TRACEME, 0, NULL, NULL) != 0)
err(1, "PTRACE_TRACEME");
@@ -476,7 +471,7 @@ static void test_ptrace_write_gsbase(void)

gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);

- if (gs != 0x7) {
+ if (gs != *shared_scratch) {
nerrs++;
printf("[FAIL]\tGS is not prepared with nonzero\n");
goto END;
@@ -494,16 +489,24 @@ static void test_ptrace_write_gsbase(void)
* selector value is changed or not by the GSBASE write in
* a ptracer.
*/
- if (gs != 0x7) {
+ if (gs != *shared_scratch) {
nerrs++;
printf("[FAIL]\tGS changed to %lx\n", gs);
+
+ /*
+ * On older kernels, poking a nonzero value into the
+ * base would zero the selector. On newer kernels,
+ * this behavior has changed -- poking the base
+ * changes only the base and, if FSGSBASE is not
+ * available, this may have no effect.
+ */
+ if (gs == 0)
+ printf("\tNote: this is expected behavior on older kernels.\n");
} else if (have_fsgsbase && (base != 0xFF)) {
nerrs++;
printf("[FAIL]\tGSBASE changed to %lx\n", base);
} else {
- printf("[OK]\tGS remained 0x7 %s");
- if (have_fsgsbase)
- printf("and GSBASE changed to 0xFF");
+ printf("[OK]\tGS remained 0x%hx%s", *shared_scratch, have_fsgsbase ? " and GSBASE changed to 0xFF" : "");
printf("\n");
}
}
@@ -516,6 +519,9 @@ int main()
{
pthread_t thread;

+ shared_scratch = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
+ MAP_ANONYMOUS | MAP_SHARED, -1, 0);
+
/* Probe FSGSBASE */
sethandler(SIGILL, sigill, 0);
if (sigsetjmp(jmpbuf, 1) == 0) {
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