linux-kernel - [tip:x86/cpu] selftests/x86/fsgsbase: Test ptracer-induced GSBASE write

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Message-ID: <tip-1b6858d5a2eb2485761f06bd48055ed5bed08464@git.kernel.org>
Date:   Sat, 22 Jun 2019 03:04:47 -0700
From:   "tip-bot for Chang S. Bae" <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     ak@...ux.intel.com, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, luto@...nel.org, hpa@...or.com,
        chang.seok.bae@...el.com, ravi.v.shankar@...el.com,
        mingo@...nel.org
Subject: [tip:x86/cpu] selftests/x86/fsgsbase: Test ptracer-induced GSBASE
 write

Commit-ID:  1b6858d5a2eb2485761f06bd48055ed5bed08464
Gitweb:     https://git.kernel.org/tip/1b6858d5a2eb2485761f06bd48055ed5bed08464
Author:     Chang S. Bae <chang.seok.bae@...el.com>
AuthorDate: Wed, 8 May 2019 03:02:17 -0700
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Sat, 22 Jun 2019 11:38:51 +0200

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

---
 tools/testing/selftests/x86/fsgsbase.c | 70 ++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)

diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
index af85bd4752a5..b02ddce49bbb 100644
--- 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;
 }