[PATCH v9 09/12] x86, mpx: decode MPX instruction to get bound violation information

Qiaowei Ren
2014-10-12T04:41:52Z

This patch sets bound violation fields of siginfo struct in #BR
exception handler by decoding the user instruction and constructing
the faulting pointer.

This patch does't use the generic decoder, and implements a limited
special-purpose decoder to decode MPX instructions, simply because the
generic decoder is very heavyweight not just in terms of performance
but in terms of interface -- because it has to.

Signed-off-by: Qiaowei Ren <qiaowei.ren@intel.com>
---
arch/x86/include/asm/mpx.h | 23 ++++
arch/x86/kernel/mpx.c | 299 ++++++++++++++++++++++++++++++++++++++++++++
arch/x86/kernel/traps.c | 6 +
3 files changed, 328 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/mpx.h b/arch/x86/include/asm/mpx.h
index b7598ac..780af63 100644
--- a/arch/x86/include/asm/mpx.h
+++ b/arch/x86/include/asm/mpx.h
@@ -3,6 +3,7 @@

#include <linux/types.h>
#include <asm/ptrace.h>
+#include <asm/insn.h>

#ifdef CONFIG_X86_64

@@ -44,15 +45,37 @@
#define MPX_BNDSTA_ERROR_CODE 0x3
#define MPX_BD_ENTRY_VALID_FLAG 0x1

+struct mpx_insn {
+ struct insn_field rex_prefix; /* REX prefix */
+ struct insn_field modrm;
+ struct insn_field sib;
+ struct insn_field displacement;
+
+ unsigned char addr_bytes; /* effective address size */
+ unsigned char limit;
+ unsigned char x86_64;
+
+ const unsigned char *kaddr; /* kernel address of insn to analyze */
+ const unsigned char *next_byte;
+};
+
+#define MAX_MPX_INSN_SIZE 15
+
unsigned long mpx_mmap(unsigned long len);

#ifdef CONFIG_X86_INTEL_MPX
int do_mpx_bt_fault(struct xsave_struct *xsave_buf);
+void do_mpx_bounds(struct pt_regs *regs, siginfo_t *info,
+ struct xsave_struct *xsave_buf);
#else
static inline int do_mpx_bt_fault(struct xsave_struct *xsave_buf)
{
return -EINVAL;
}
+static inline void do_mpx_bounds(struct pt_regs *regs, siginfo_t *info,
+ struct xsave_struct *xsave_buf)
+{
+}
#endif /* CONFIG_X86_INTEL_MPX */

#endif /* _ASM_X86_MPX_H */
diff --git a/arch/x86/kernel/mpx.c b/arch/x86/kernel/mpx.c
index 2103b5e..b7e4c0e 100644
--- a/arch/x86/kernel/mpx.c
+++ b/arch/x86/kernel/mpx.c
@@ -10,6 +10,275 @@
#include <linux/syscalls.h>
#include <asm/mpx.h>

+enum reg_type {
+ REG_TYPE_RM = 0,
+ REG_TYPE_INDEX,
+ REG_TYPE_BASE,
+};
+
+static unsigned long get_reg(struct mpx_insn *insn, struct pt_regs *regs,
+ enum reg_type type)
+{
+ int regno = 0;
+ unsigned char modrm = (unsigned char)insn->modrm.value;
+ unsigned char sib = (unsigned char)insn->sib.value;
+
+ static const int regoff[] = {
+ offsetof(struct pt_regs, ax),
+ offsetof(struct pt_regs, cx),
+ offsetof(struct pt_regs, dx),
+ offsetof(struct pt_regs, bx),
+ offsetof(struct pt_regs, sp),
+ offsetof(struct pt_regs, bp),
+ offsetof(struct pt_regs, si),
+ offsetof(struct pt_regs, di),
+#ifdef CONFIG_X86_64
+ offsetof(struct pt_regs, r8),
+ offsetof(struct pt_regs, r9),
+ offsetof(struct pt_regs, r10),
+ offsetof(struct pt_regs, r11),
+ offsetof(struct pt_regs, r12),
+ offsetof(struct pt_regs, r13),
+ offsetof(struct pt_regs, r14),
+ offsetof(struct pt_regs, r15),
+#endif
+ };
+
+ switch (type) {
+ case REG_TYPE_RM:
+ regno = X86_MODRM_RM(modrm);
+ if (X86_REX_B(insn->rex_prefix.value) = 1)
+ regno += 8;
+ break;
+
+ case REG_TYPE_INDEX:
+ regno = X86_SIB_INDEX(sib);
+ if (X86_REX_X(insn->rex_prefix.value) = 1)
+ regno += 8;
+ break;
+
+ case REG_TYPE_BASE:
+ regno = X86_SIB_BASE(sib);
+ if (X86_REX_B(insn->rex_prefix.value) = 1)
+ regno += 8;
+ break;
+
+ default:
+ break;
+ }
+
+ return regs_get_register(regs, regoff[regno]);
+}
+
+/*
+ * return the address being referenced be instruction
+ * for rm=3 returning the content of the rm reg
+ * for rm!=3 calculates the address using SIB and Disp
+ */
+static unsigned long get_addr_ref(struct mpx_insn *insn, struct pt_regs *regs)
+{
+ unsigned long addr;
+ unsigned long base;
+ unsigned long indx;
+ unsigned char modrm = (unsigned char)insn->modrm.value;
+ unsigned char sib = (unsigned char)insn->sib.value;
+
+ if (X86_MODRM_MOD(modrm) = 3) {
+ addr = get_reg(insn, regs, REG_TYPE_RM);
+ } else {
+ if (insn->sib.nbytes) {
+ base = get_reg(insn, regs, REG_TYPE_BASE);
+ indx = get_reg(insn, regs, REG_TYPE_INDEX);
+ addr = base + indx * (1 << X86_SIB_SCALE(sib));
+ } else {
+ addr = get_reg(insn, regs, REG_TYPE_RM);
+ }
+ addr += insn->displacement.value;
+ }
+
+ return addr;
+}
+
+/* Verify next sizeof(t) bytes can be on the same instruction */
+#define validate_next(t, insn, n) \
+ ((insn)->next_byte + sizeof(t) + n - (insn)->kaddr <= (insn)->limit)
+
+#define __get_next(t, insn) \
+({ \
+ t r = *(t *)insn->next_byte; \
+ insn->next_byte += sizeof(t); \
+ r; \
+})
+
+#define __peek_next(t, insn) \
+({ \
+ t r = *(t *)insn->next_byte; \
+ r; \
+})
+
+#define get_next(t, insn) \
+({ \
+ if (unlikely(!validate_next(t, insn, 0))) \
+ goto err_out; \
+ __get_next(t, insn); \
+})
+
+#define peek_next(t, insn) \
+({ \
+ if (unlikely(!validate_next(t, insn, 0))) \
+ goto err_out; \
+ __peek_next(t, insn); \
+})
+
+static void mpx_insn_get_prefixes(struct mpx_insn *insn)
+{
+ unsigned char b;
+
+ /* Decode legacy prefix and REX prefix */
+ b = peek_next(unsigned char, insn);
+ while (b != 0x0f) {
+ /*
+ * look for a rex prefix
+ * a REX prefix cannot be followed by a legacy prefix.
+ */
+ if (insn->x86_64 && ((b&0xf0) = 0x40)) {
+ insn->rex_prefix.value = b;
+ insn->rex_prefix.nbytes = 1;
+ insn->next_byte++;
+ break;
+ }
+
+ /* check the other legacy prefixes */
+ switch (b) {
+ case 0xf2:
+ case 0xf3:
+ case 0xf0:
+ case 0x64:
+ case 0x65:
+ case 0x2e:
+ case 0x3e:
+ case 0x26:
+ case 0x36:
+ case 0x66:
+ case 0x67:
+ insn->next_byte++;
+ break;
+ default: /* everything else is garbage */
+ goto err_out;
+ }
+ b = peek_next(unsigned char, insn);
+ }
+
+err_out:
+ return;
+}
+
+static void mpx_insn_get_modrm(struct mpx_insn *insn)
+{
+ insn->modrm.value = get_next(unsigned char, insn);
+ insn->modrm.nbytes = 1;
+
+err_out:
+ return;
+}
+
+static void mpx_insn_get_sib(struct mpx_insn *insn)
+{
+ unsigned char modrm = (unsigned char)insn->modrm.value;
+
+ if (X86_MODRM_MOD(modrm) != 3 && X86_MODRM_RM(modrm) = 4) {
+ insn->sib.value = get_next(unsigned char, insn);
+ insn->sib.nbytes = 1;
+ }
+
+err_out:
+ return;
+}
+
+static void mpx_insn_get_displacement(struct mpx_insn *insn)
+{
+ unsigned char mod, rm, base;
+
+ /*
+ * Interpreting the modrm byte:
+ * mod = 00 - no displacement fields (exceptions below)
+ * mod = 01 - 1-byte displacement field
+ * mod = 10 - displacement field is 4 bytes
+ * mod = 11 - no memory operand
+ *
+ * mod != 11, r/m = 100 - SIB byte exists
+ * mod = 00, SIB base = 101 - displacement field is 4 bytes
+ * mod = 00, r/m = 101 - rip-relative addressing, displacement
+ * field is 4 bytes
+ */
+ mod = X86_MODRM_MOD(insn->modrm.value);
+ rm = X86_MODRM_RM(insn->modrm.value);
+ base = X86_SIB_BASE(insn->sib.value);
+ if (mod = 3)
+ return;
+ if (mod = 1) {
+ insn->displacement.value = get_next(unsigned char, insn);
+ insn->displacement.nbytes = 1;
+ } else if ((mod = 0 && rm = 5) || mod = 2 ||
+ (mod = 0 && base = 5)) {
+ insn->displacement.value = get_next(int, insn);
+ insn->displacement.nbytes = 4;
+ }
+
+err_out:
+ return;
+}
+
+static void mpx_insn_init(struct mpx_insn *insn, struct pt_regs *regs)
+{
+ unsigned char buf[MAX_MPX_INSN_SIZE];
+ int bytes;
+
+ memset(insn, 0, sizeof(*insn));
+
+ bytes = copy_from_user(buf, (void __user *)regs->ip, MAX_MPX_INSN_SIZE);
+ insn->limit = MAX_MPX_INSN_SIZE - bytes;
+ insn->kaddr = buf;
+ insn->next_byte = buf;
+
+ /*
+ * In 64-bit Mode, all Intel MPX instructions use 64-bit
+ * operands for bounds and 64 bit addressing, i.e. REX.W &
+ * 67H have no effect on data or address size.
+ *
+ * In compatibility and legacy modes (including 16-bit code
+ * segments, real and virtual 8086 modes) all Intel MPX
+ * instructions use 32-bit operands for bounds and 32 bit
+ * addressing.
+ */
+#ifdef CONFIG_X86_64
+ insn->x86_64 = 1;
+ insn->addr_bytes = 8;
+#else
+ insn->x86_64 = 0;
+ insn->addr_bytes = 4;
+#endif
+}
+
+static unsigned long mpx_insn_decode(struct mpx_insn *insn,
+ struct pt_regs *regs)
+{
+ mpx_insn_init(insn, regs);
+
+ /*
+ * In this case, we only need decode bndcl/bndcn/bndcu,
+ * so we can use private diassembly interfaces to get
+ * prefixes, modrm, sib, displacement, etc..
+ */
+ mpx_insn_get_prefixes(insn);
+ insn->next_byte += 2; /* ignore opcode */
+ mpx_insn_get_modrm(insn);
+ mpx_insn_get_sib(insn);
+ mpx_insn_get_displacement(insn);
+
+ return get_addr_ref(insn, regs);
+}
+
/*
* With 32-bit mode, MPX_BT_SIZE_BYTES is 4MB, and the size of each
* bounds table is 16KB. With 64-bit mode, MPX_BT_SIZE_BYTES is 2GB,
@@ -99,3 +368,33 @@ int do_mpx_bt_fault(struct xsave_struct *xsave_buf)

return allocate_bt((long __user *)bd_entry);
}
+
+/*
+ * If a bounds overflow occurs then a #BR is generated. The fault
+ * handler will decode MPX instructions to get violation address
+ * and set this address into extended struct siginfo.
+ */
+void do_mpx_bounds(struct pt_regs *regs, siginfo_t *info,
+ struct xsave_struct *xsave_buf)
+{
+ struct mpx_insn insn;
+ uint8_t bndregno;
+ unsigned long addr_vio;
+
+ addr_vio = mpx_insn_decode(&insn, regs);
+
+ bndregno = X86_MODRM_REG(insn.modrm.value);
+ if (bndregno > 3)
+ return;
+
+ /* Note: the upper 32 bits are ignored in 32-bit mode. */
+ info->si_lower = (void __user *)(unsigned long)
+ (xsave_buf->bndregs.bndregs[2*bndregno]);
+ info->si_upper = (void __user *)(unsigned long)
+ (~xsave_buf->bndregs.bndregs[2*bndregno+1]);
+ info->si_addr_lsb = 0;
+ info->si_signo = SIGSEGV;
+ info->si_errno = 0;
+ info->si_code = SEGV_BNDERR;
+ info->si_addr = (void __user *)addr_vio;
+}
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 611b6ec..b2a916b 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -284,6 +284,7 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
unsigned long status;
struct xsave_struct *xsave_buf;
struct task_struct *tsk = current;
+ siginfo_t info;

prev_state = exception_enter();
if (notify_die(DIE_TRAP, "bounds", regs, error_code,
@@ -316,6 +317,11 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
break;

case 1: /* Bound violation. */
+ do_mpx_bounds(regs, &info, xsave_buf);
+ do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs,
+ error_code, &info);
+ break;
+
case 0: /* No exception caused by Intel MPX operations. */
do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, NULL);
break;
--
1.7.1