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Subject[tip:x86/asm] x86, selftests: Add a test for the "sysret_ss_attrs " bug
Commit-ID:  e22438f8e997ac1c8911d8808b6a4c492cd8bc6e
Gitweb: http://git.kernel.org/tip/e22438f8e997ac1c8911d8808b6a4c492cd8bc6e
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Fri, 24 Apr 2015 15:09:19 -0700
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 8 May 2015 13:33:59 +0200

x86, selftests: Add a test for the "sysret_ss_attrs" bug

On AMD CPUs, SYSRET can return with a valid SS descriptor with
with the hidden attributes set to an unusable state. Make sure
the kernel doesn't let this happen. This detects an
as-yet-unfixed regression.

Note that the 64-bit version of this test fails on AMD CPUs on
all kernel versions, although the issue in the 64-bit case is
much less severe than in the 32-bit case.

Reported-by: Brian Gerst <brgerst@gmail.com>
Tested-by: Denys Vlasenko <dvlasenk@redhat.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Tests: e7d6eefaaa44 ("x86/vdso32/syscall.S: Do not load __USER32_DS to %ss")
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/resend_4d740841bac383742949e2fefb03982736595087.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
tools/testing/selftests/x86/Makefile | 5 +-
tools/testing/selftests/x86/run_x86_tests.sh | 2 +
tools/testing/selftests/x86/sysret_ss_attrs.c | 112 ++++++++++++++++++++++++++
tools/testing/selftests/x86/thunks.S | 67 +++++++++++++++
4 files changed, 185 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index ddf6356..9309097 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -1,6 +1,6 @@
.PHONY: all all_32 all_64 check_build32 clean run_tests

-TARGETS_C_BOTHBITS := sigreturn single_step_syscall
+TARGETS_C_BOTHBITS := sigreturn single_step_syscall sysret_ss_attrs

BINARIES_32 := $(TARGETS_C_BOTHBITS:%=%_32)
BINARIES_64 := $(TARGETS_C_BOTHBITS:%=%_64)
@@ -46,3 +46,6 @@ check_build32:
echo " yum install glibc-devel.*i686"; \
exit 1; \
fi
+
+# Some tests have additional dependencies.
+sysret_ss_attrs_64: thunks.S
diff --git a/tools/testing/selftests/x86/run_x86_tests.sh b/tools/testing/selftests/x86/run_x86_tests.sh
index 3fc19b3..d250342 100644
--- a/tools/testing/selftests/x86/run_x86_tests.sh
+++ b/tools/testing/selftests/x86/run_x86_tests.sh
@@ -4,10 +4,12 @@
# script here.
./sigreturn_32 || exit 1
./single_step_syscall_32 || exit 1
+./sysret_ss_attrs_32 || exit 1

if [[ "$uname -p" -eq "x86_64" ]]; then
./sigreturn_64 || exit 1
./single_step_syscall_64 || exit 1
+ ./sysret_ss_attrs_64 || exit 1
fi

exit 0
diff --git a/tools/testing/selftests/x86/sysret_ss_attrs.c b/tools/testing/selftests/x86/sysret_ss_attrs.c
new file mode 100644
index 0000000..ce42d5a
--- /dev/null
+++ b/tools/testing/selftests/x86/sysret_ss_attrs.c
@@ -0,0 +1,112 @@
+/*
+ * sysret_ss_attrs.c - test that syscalls return valid hidden SS attributes
+ * Copyright (c) 2015 Andrew Lutomirski
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * On AMD CPUs, SYSRET can return with a valid SS descriptor with with
+ * the hidden attributes set to an unusable state. Make sure the kernel
+ * doesn't let this happen.
+ */
+
+#define _GNU_SOURCE
+
+#include <stdlib.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <err.h>
+#include <stddef.h>
+#include <stdbool.h>
+#include <pthread.h>
+
+static void *threadproc(void *ctx)
+{
+ /*
+ * Do our best to cause sleeps on this CPU to exit the kernel and
+ * re-enter with SS = 0.
+ */
+ while (true)
+ ;
+
+ return NULL;
+}
+
+#ifdef __x86_64__
+extern unsigned long call32_from_64(void *stack, void (*function)(void));
+
+asm (".pushsection .text\n\t"
+ ".code32\n\t"
+ "test_ss:\n\t"
+ "pushl $0\n\t"
+ "popl %eax\n\t"
+ "ret\n\t"
+ ".code64");
+extern void test_ss(void);
+#endif
+
+int main()
+{
+ /*
+ * Start a busy-looping thread on the same CPU we're on.
+ * For simplicity, just stick everything to CPU 0. This will
+ * fail in some containers, but that's probably okay.
+ */
+ cpu_set_t cpuset;
+ CPU_ZERO(&cpuset);
+ CPU_SET(0, &cpuset);
+ if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0)
+ printf("[WARN]\tsched_setaffinity failed\n");
+
+ pthread_t thread;
+ if (pthread_create(&thread, 0, threadproc, 0) != 0)
+ err(1, "pthread_create");
+
+#ifdef __x86_64__
+ unsigned char *stack32 = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
+ MAP_32BIT | MAP_ANONYMOUS | MAP_PRIVATE,
+ -1, 0);
+ if (stack32 == MAP_FAILED)
+ err(1, "mmap");
+#endif
+
+ printf("[RUN]\tSyscalls followed by SS validation\n");
+
+ for (int i = 0; i < 1000; i++) {
+ /*
+ * Go to sleep and return using sysret (if we're 64-bit
+ * or we're 32-bit on AMD on a 64-bit kernel). On AMD CPUs,
+ * SYSRET doesn't fix up the cached SS descriptor, so the
+ * kernel needs some kind of workaround to make sure that we
+ * end the system call with a valid stack segment. This
+ * can be a confusing failure because the SS *selector*
+ * is the same regardless.
+ */
+ usleep(2);
+
+#ifdef __x86_64__
+ /*
+ * On 32-bit, just doing a syscall through glibc is enough
+ * to cause a crash if our cached SS descriptor is invalid.
+ * On 64-bit, it's not, so try extra hard.
+ */
+ call32_from_64(stack32 + 4088, test_ss);
+#endif
+ }
+
+ printf("[OK]\tWe survived\n");
+
+#ifdef __x86_64__
+ munmap(stack32, 4096);
+#endif
+
+ return 0;
+}
diff --git a/tools/testing/selftests/x86/thunks.S b/tools/testing/selftests/x86/thunks.S
new file mode 100644
index 0000000..ce8a995
--- /dev/null
+++ b/tools/testing/selftests/x86/thunks.S
@@ -0,0 +1,67 @@
+/*
+ * thunks.S - assembly helpers for mixed-bitness code
+ * Copyright (c) 2015 Andrew Lutomirski
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * These are little helpers that make it easier to switch bitness on
+ * the fly.
+ */
+
+ .text
+
+ .global call32_from_64
+ .type call32_from_64, @function
+call32_from_64:
+ // rdi: stack to use
+ // esi: function to call
+
+ // Save registers
+ pushq %rbx
+ pushq %rbp
+ pushq %r12
+ pushq %r13
+ pushq %r14
+ pushq %r15
+ pushfq
+
+ // Switch stacks
+ mov %rsp,(%rdi)
+ mov %rdi,%rsp
+
+ // Switch to compatibility mode
+ pushq $0x23 /* USER32_CS */
+ pushq $1f
+ lretq
+
+1:
+ .code32
+ // Call the function
+ call *%esi
+ // Switch back to long mode
+ jmp $0x33,$1f
+ .code64
+
+1:
+ // Restore the stack
+ mov (%rsp),%rsp
+
+ // Restore registers
+ popfq
+ popq %r15
+ popq %r14
+ popq %r13
+ popq %r12
+ popq %rbp
+ popq %rbx
+
+ ret
+
+.size call32_from_64, .-call32_from_64

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