[RFC 4/4] selftests/x86: Add tests for UC_SIGCONTEXT_SS and UC_STRICT_RESTORE_SS

Andy Lutomirski
2015-10-13T01:04:24Z

This tests the two ABI-preserving cases that DOSEMU cares about, and
it also explicitly tests the new UC_SIGCONTEXT_SS and
UC_STRICT_RESTORE_SS flags.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
tools/testing/selftests/x86/sigreturn.c | 240 ++++++++++++++++++++++++++++----
1 file changed, 212 insertions(+), 28 deletions(-)

diff --git a/tools/testing/selftests/x86/sigreturn.c b/tools/testing/selftests/x86/sigreturn.c
index b5aa1bab7416..43e840470e32 100644
--- a/tools/testing/selftests/x86/sigreturn.c
+++ b/tools/testing/selftests/x86/sigreturn.c
@@ -55,6 +55,47 @@
#include <sys/user.h>

/*
+ * Copied from asm/ucontext.h, as asm/ucontext.h conflicts badly with the glibc
+ * headers.
+ */
+#ifdef __x86_64__
+/*
+ * UC_SAVED_SS will be set when delivering 64-bit or x32 signals on
+ * kernels that save SS in the sigcontext. Kernels that set UC_SAVED_SS
+ * allow signal handlers to set UC_RESTORE_SS; if UC_RESTORE_SS is set,
+ * then sigreturn will restore SS.
+ *
+ * For compatibility with old programs, the kernel will *not* set
+ * UC_RESTORE_SS when delivering signals.
+ */
+#define UC_SIGCONTEXT_SS 0x2
+#define UC_STRICT_RESTORE_SS 0x4
+#endif
+
+/* Access rights as returned by LAR */
+#define AR_TYPE_RODATA (0 * (1 << 9))
+#define AR_TYPE_RWDATA (1 * (1 << 9))
+#define AR_TYPE_RODATA_EXPDOWN (2 * (1 << 9))
+#define AR_TYPE_RWDATA_EXPDOWN (3 * (1 << 9))
+#define AR_TYPE_XOCODE (4 * (1 << 9))
+#define AR_TYPE_XRCODE (5 * (1 << 9))
+#define AR_TYPE_XOCODE_CONF (6 * (1 << 9))
+#define AR_TYPE_XRCODE_CONF (7 * (1 << 9))
+#define AR_TYPE_MASK (7 * (1 << 9))
+
+#define AR_DPL0 (0 * (1 << 13))
+#define AR_DPL3 (3 * (1 << 13))
+#define AR_DPL_MASK (3 * (1 << 13))
+
+#define AR_A (1 << 8) /* A means "accessed" */
+#define AR_S (1 << 12) /* S means "not system" */
+#define AR_P (1 << 15) /* P means "present" */
+#define AR_AVL (1 << 20) /* AVL does nothing */
+#define AR_L (1 << 21) /* L means "long mode" */
+#define AR_DB (1 << 22) /* D or B, depending on type */
+#define AR_G (1 << 23) /* G means "limit in pages" */
+
+/*
* In principle, this test can run on Linux emulation layers (e.g.
* Illumos "LX branded zones"). Solaris-based kernels reserve LDT
* entries 0-5 for their own internal purposes, so start our LDT
@@ -267,6 +308,9 @@ static gregset_t initial_regs, requested_regs, resulting_regs;
/* Instructions for the SIGUSR1 handler. */
static volatile unsigned short sig_cs, sig_ss;
static volatile sig_atomic_t sig_trapped, sig_err, sig_trapno;
+#ifdef __x86_64__
+static volatile sig_atomic_t sig_corrupt_final_ss;
+#endif

/* Abstractions for some 32-bit vs 64-bit differences. */
#ifdef __x86_64__
@@ -305,9 +349,105 @@ static greg_t *csptr(ucontext_t *ctx)
}
#endif

+/*
+ * Checks a given selector for its code bitness or returns -1 if it's not
+ * a usable code segment selector.
+ */
+int cs_bitness(unsigned short cs)
+{
+ uint32_t valid = 0, ar;
+ asm ("lar %[cs], %[ar]\n\t"
+ "jnz 1f\n\t"
+ "mov $1, %[valid]\n\t"
+ "1:"
+ : [ar] "=r" (ar), [valid] "+rm" (valid)
+ : [cs] "r" (cs));
+
+ if (!valid)
+ return -1;
+
+ bool db = (ar & (1 << 22));
+ bool l = (ar & (1 << 21));
+
+ if (!(ar & (1<<11)))
+ return -1; /* Not code. */
+
+ if (l && !db)
+ return 64;
+ else if (!l && db)
+ return 32;
+ else if (!l && !db)
+ return 16;
+ else
+ return -1; /* Unknown bitness. */
+}
+
+/*
+ * Checks a given selector for its code bitness or returns -1 if it's not
+ * a usable code segment selector.
+ */
+bool is_valid_ss(unsigned short cs)
+{
+ uint32_t valid = 0, ar;
+ asm ("lar %[cs], %[ar]\n\t"
+ "jnz 1f\n\t"
+ "mov $1, %[valid]\n\t"
+ "1:"
+ : [ar] "=r" (ar), [valid] "+rm" (valid)
+ : [cs] "r" (cs));
+
+ if (!valid)
+ return false;
+
+ if ((ar & AR_TYPE_MASK) != AR_TYPE_RWDATA &&
+ (ar & AR_TYPE_MASK) != AR_TYPE_RWDATA_EXPDOWN)
+ return false;
+
+ return (ar & AR_P);
+}
+
/* Number of errors in the current test case. */
static volatile sig_atomic_t nerrs;

+static void validate_signal_ss(int sig, ucontext_t *ctx)
+{
+#ifdef __x86_64__
+ bool was_64bit = (cs_bitness(*csptr(ctx)) == 64);
+
+ if (!(ctx->uc_flags & UC_SIGCONTEXT_SS)) {
+ printf("[FAIL]\tUC_SIGCONTEXT_SS was not set\n");
+ nerrs++;
+
+ /*
+ * This happens on Linux 4.1. The rest will fail, too, so
+ * return now to reduce the noise.
+ */
+ return;
+ }
+
+ /* UC_STRICT_RESTORE_SS is set iff we came from 64-bit mode. */
+ if (!!(ctx->uc_flags & UC_STRICT_RESTORE_SS) != was_64bit) {
+ printf("[FAIL]\tUC_STRICT_RESTORE_SS was wrong in signal %d\n",
+ sig);
+ nerrs++;
+ }
+
+ if (is_valid_ss(*ssptr(ctx))) {
+ /*
+ * DOSEMU was written before 64-bit sigcontext had SS, and
+ * it tries to figure out the signal source SS by looking at
+ * the physical register. Make sure that keeps working.
+ */
+ unsigned short hw_ss;
+ asm ("mov %%ss, %0" : "=rm" (hw_ss));
+ if (hw_ss != *ssptr(ctx)) {
+ printf("[FAIL]\tHW SS didn't match saved SS\n");
+ nerrs++;
+ }
+ }
+#endif
+}
+
/*
* SIGUSR1 handler. Sets CS and SS as requested and points IP to the
* int3 trampoline. Sets SP to a large known value so that we can see
@@ -317,6 +457,8 @@ static void sigusr1(int sig, siginfo_t *info, void *ctx_void)
{
ucontext_t *ctx = (ucontext_t*)ctx_void;

+ validate_signal_ss(sig, ctx);
+
memcpy(&initial_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));

*csptr(ctx) = sig_cs;
@@ -334,13 +476,16 @@ static void sigusr1(int sig, siginfo_t *info, void *ctx_void)
}

/*
- * Called after a successful sigreturn. Restores our state so that
- * the original raise(SIGUSR1) returns.
+ * Called after a successful sigreturn (via int3) or from a failed
+ * sigreturn (directly by kernel). Restores our state so that the
+ * original raise(SIGUSR1) returns.
*/
static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
{
ucontext_t *ctx = (ucontext_t*)ctx_void;

+ validate_signal_ss(sig, ctx);
+
sig_err = ctx->uc_mcontext.gregs[REG_ERR];
sig_trapno = ctx->uc_mcontext.gregs[REG_TRAPNO];

@@ -358,41 +503,62 @@ static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
memcpy(&resulting_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));
memcpy(&ctx->uc_mcontext.gregs, &initial_regs, sizeof(gregset_t));

+#ifdef __x86_64__
+ if (sig_corrupt_final_ss) {
+ if (ctx->uc_flags & UC_STRICT_RESTORE_SS) {
+ printf("[FAIL]\tUC_STRICT_RESTORE_SS was set inappropriately\n");
+ nerrs++;
+ } else {
+ /*
+ * DOSEMU transitions from 32-bit to 64-bit mode by
+ * adjusting sigcontext, and it requires that this work
+ * even if the saved SS is bogus.
+ */
+ printf("\tCorrupting SS on return to 64-bit mode\n");
+ *ssptr(ctx) = 0;
+ }
+ }
+#endif
+
sig_trapped = sig;
}

-/*
- * Checks a given selector for its code bitness or returns -1 if it's not
- * a usable code segment selector.
- */
-int cs_bitness(unsigned short cs)
+#ifdef __x86_64__
+/* Tests recovery if !UC_STRICT_RESTORE_SS */
+static void sigusr2(int sig, siginfo_t *info, void *ctx_void)
{
- uint32_t valid = 0, ar;
- asm ("lar %[cs], %[ar]\n\t"
- "jnz 1f\n\t"
- "mov $1, %[valid]\n\t"
- "1:"
- : [ar] "=r" (ar), [valid] "+rm" (valid)
- : [cs] "r" (cs));
+ ucontext_t *ctx = (ucontext_t*)ctx_void;

- if (!valid)
- return -1;
+ if (!(ctx->uc_flags & UC_STRICT_RESTORE_SS)) {
+ printf("[FAIL]\traise(2) didn't set UC_STRICT_RESTORE_SS\n");
+ nerrs++;
+ return; /* We can't do the rest. */
+ }

- bool db = (ar & (1 << 22));
- bool l = (ar & (1 << 21));
+ ctx->uc_flags &= ~UC_STRICT_RESTORE_SS;
+ *ssptr(ctx) = 0;

- if (!(ar & (1<<11)))
- return -1; /* Not code. */
+ /* Return. The kernel should recover without sending another signal. */
+}

- if (l && !db)
- return 64;
- else if (!l && db)
- return 32;
- else if (!l && !db)
- return 16;
- else
- return -1; /* Unknown bitness. */
+static int test_nonstrict_ss(void)
+{
+ clearhandler(SIGUSR1);
+ clearhandler(SIGTRAP);
+ clearhandler(SIGSEGV);
+ clearhandler(SIGILL);
+ sethandler(SIGUSR2, sigusr2, 0);
+
+ nerrs = 0;
+
+ printf("[RUN]\tClear UC_STRICT_RESTORE_SS and corrupt SS\n");
+ raise(SIGUSR2);
+ if (!nerrs)
+ printf("[OK]\tIt worked\n");
+
+ return nerrs;
}
+#endif

/* Finds a usable code segment of the requested bitness. */
int find_cs(int bitness)
@@ -576,6 +742,12 @@ static int test_bad_iret(int cs_bits, unsigned short ss, int force_cs)
errdesc, strsignal(sig_trapped));
return 0;
} else {
+ /*
+ * This also implicitly tests UC_STRICT_RESTORE_SS:
+ * We check that these signals set UC_STRICT_RESTORE_SS and,
+ * if UC_STRICT_RESTORE_SS doesn't cause strict behavior,
+ * then we won't get SIGSEGV.
+ */
printf("[FAIL]\tDid not get SIGSEGV\n");
return 1;
}
@@ -632,6 +804,14 @@ int main()
GDT3(gdt_data16_idx));
}

+#ifdef __x86_64__
+ /* Nasty ABI case: check SS corruption handling. */
+ sig_corrupt_final_ss = 1;
+ total_nerrs += test_valid_sigreturn(32, false, -1);
+ total_nerrs += test_valid_sigreturn(32, true, -1);
+ sig_corrupt_final_ss = 0;
+#endif
+
/*
* We're done testing valid sigreturn cases. Now we test states
* for which sigreturn itself will succeed but the subsequent
@@ -680,5 +860,9 @@ int main()
if (gdt_npdata32_idx)
test_bad_iret(32, GDT3(gdt_npdata32_idx), -1);

+#ifdef __x86_64__
+ total_nerrs += test_nonstrict_ss();
+#endif
+
return total_nerrs ? 1 : 0;
}
--
2.4.3