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Subject[PATCH v1 2/4] [RFC] x86/trampfd: Provide support for the trampoline file descriptor
Date
From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

Implement 32-bit and 64-bit X86 support for the trampoline file descriptor.

- Define architecture specific register names
- Handle the trampoline invocation page fault
- Setup the user register context on trampoline invocation
- Setup the user stack context on trampoline invocation

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
arch/x86/include/uapi/asm/ptrace.h | 38 +++
arch/x86/kernel/Makefile | 2 +
arch/x86/kernel/trampfd.c | 313 +++++++++++++++++++++++++
arch/x86/mm/fault.c | 11 +
6 files changed, 366 insertions(+)
create mode 100644 arch/x86/kernel/trampfd.c

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index d8f8a1a69ed1..77eb50414591 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -443,3 +443,4 @@
437 i386 openat2 sys_openat2
438 i386 pidfd_getfd sys_pidfd_getfd
439 i386 faccessat2 sys_faccessat2
+440 i386 trampfd_create sys_trampfd_create
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 78847b32e137..9d962de1d21f 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -360,6 +360,7 @@
437 common openat2 sys_openat2
438 common pidfd_getfd sys_pidfd_getfd
439 common faccessat2 sys_faccessat2
+440 common trampfd_create sys_trampfd_create

#
# x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/arch/x86/include/uapi/asm/ptrace.h b/arch/x86/include/uapi/asm/ptrace.h
index 85165c0edafc..b031598f857e 100644
--- a/arch/x86/include/uapi/asm/ptrace.h
+++ b/arch/x86/include/uapi/asm/ptrace.h
@@ -9,6 +9,44 @@

#ifndef __ASSEMBLY__

+/*
+ * These register names are to be used by 32-bit applications.
+ */
+enum reg_32_name {
+ x32_eax,
+ x32_ebx,
+ x32_ecx,
+ x32_edx,
+ x32_esi,
+ x32_edi,
+ x32_ebp,
+ x32_eip,
+ x32_max,
+};
+
+/*
+ * These register names are to be used by 64-bit applications.
+ */
+enum reg_64_name {
+ x64_rax = x32_max,
+ x64_rbx,
+ x64_rcx,
+ x64_rdx,
+ x64_rsi,
+ x64_rdi,
+ x64_rbp,
+ x64_r8,
+ x64_r9,
+ x64_r10,
+ x64_r11,
+ x64_r12,
+ x64_r13,
+ x64_r14,
+ x64_r15,
+ x64_rip,
+ x64_max,
+};
+
#ifdef __i386__
/* this struct defines the way the registers are stored on the
stack during a system call. */
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index e77261db2391..5d968ac4c7d9 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -157,3 +157,5 @@ ifeq ($(CONFIG_X86_64),y)
endif

obj-$(CONFIG_IMA_SECURE_AND_OR_TRUSTED_BOOT) += ima_arch.o
+
+obj-$(CONFIG_TRAMPFD) += trampfd.o
diff --git a/arch/x86/kernel/trampfd.c b/arch/x86/kernel/trampfd.c
new file mode 100644
index 000000000000..f6b5507134d2
--- /dev/null
+++ b/arch/x86/kernel/trampfd.c
@@ -0,0 +1,313 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Trampoline File Descriptor - X86 support.
+ *
+ * Author: Madhavan T. Venkataraman (madvenka@linux.microsoft.com)
+ *
+ * Copyright (c) 2020, Microsoft Corporation.
+ */
+
+#include <linux/thread_info.h>
+#include <linux/mm_types.h>
+#include <linux/trampfd.h>
+#include <linux/uaccess.h>
+
+/* ---------------------------- Register Context ---------------------------- */
+
+static inline bool is_compat(void)
+{
+ return (IS_ENABLED(CONFIG_X86_32) ||
+ (IS_ENABLED(CONFIG_COMPAT) && test_thread_flag(TIF_ADDR32)));
+}
+
+static void set_reg_32(struct pt_regs *pt_regs, u32 name, u64 value)
+{
+ switch (name) {
+ case x32_eax:
+ pt_regs->ax = (unsigned long)value;
+ break;
+ case x32_ebx:
+ pt_regs->bx = (unsigned long)value;
+ break;
+ case x32_ecx:
+ pt_regs->cx = (unsigned long)value;
+ break;
+ case x32_edx:
+ pt_regs->dx = (unsigned long)value;
+ break;
+ case x32_esi:
+ pt_regs->si = (unsigned long)value;
+ break;
+ case x32_edi:
+ pt_regs->di = (unsigned long)value;
+ break;
+ case x32_ebp:
+ pt_regs->bp = (unsigned long)value;
+ break;
+ case x32_eip:
+ pt_regs->ip = (unsigned long)value;
+ break;
+ default:
+ WARN(1, "%s: Illegal register name %d\n", __func__, name);
+ break;
+ }
+}
+
+#ifdef __i386__
+
+static void set_reg_64(struct pt_regs *pt_regs, u32 name, u64 value)
+{
+}
+
+#else
+
+static void set_reg_64(struct pt_regs *pt_regs, u32 name, u64 value)
+{
+ switch (name) {
+ case x64_rax:
+ pt_regs->ax = (unsigned long)value;
+ break;
+ case x64_rbx:
+ pt_regs->bx = (unsigned long)value;
+ break;
+ case x64_rcx:
+ pt_regs->cx = (unsigned long)value;
+ break;
+ case x64_rdx:
+ pt_regs->dx = (unsigned long)value;
+ break;
+ case x64_rsi:
+ pt_regs->si = (unsigned long)value;
+ break;
+ case x64_rdi:
+ pt_regs->di = (unsigned long)value;
+ break;
+ case x64_rbp:
+ pt_regs->bp = (unsigned long)value;
+ break;
+ case x64_r8:
+ pt_regs->r8 = (unsigned long)value;
+ break;
+ case x64_r9:
+ pt_regs->r9 = (unsigned long)value;
+ break;
+ case x64_r10:
+ pt_regs->r10 = (unsigned long)value;
+ break;
+ case x64_r11:
+ pt_regs->r11 = (unsigned long)value;
+ break;
+ case x64_r12:
+ pt_regs->r12 = (unsigned long)value;
+ break;
+ case x64_r13:
+ pt_regs->r13 = (unsigned long)value;
+ break;
+ case x64_r14:
+ pt_regs->r14 = (unsigned long)value;
+ break;
+ case x64_r15:
+ pt_regs->r15 = (unsigned long)value;
+ break;
+ case x64_rip:
+ pt_regs->ip = (unsigned long)value;
+ break;
+ default:
+ WARN(1, "%s: Illegal register name %d\n", __func__, name);
+ break;
+ }
+}
+
+#endif /* __i386__ */
+
+static void set_regs(struct pt_regs *pt_regs, struct trampfd_regs *tregs)
+{
+ struct trampfd_reg *reg = tregs->regs;
+ struct trampfd_reg *reg_end = reg + tregs->nregs;
+ bool compat = is_compat();
+
+ for (; reg < reg_end; reg++) {
+ if (compat)
+ set_reg_32(pt_regs, reg->name, reg->value);
+ else
+ set_reg_64(pt_regs, reg->name, reg->value);
+ }
+}
+
+/*
+ * Check if the register names are valid. Check if the user PC has been set.
+ */
+bool trampfd_valid_regs(struct trampfd_regs *tregs)
+{
+ struct trampfd_reg *reg = tregs->regs;
+ struct trampfd_reg *reg_end = reg + tregs->nregs;
+ int min, max, pc_name;
+ bool pc_set = false;
+
+ if (is_compat()) {
+ min = 0;
+ pc_name = x32_eip;
+ max = x32_max;
+ } else {
+ min = x32_max;
+ pc_name = x64_rip;
+ max = x64_max;
+ }
+
+ for (; reg < reg_end; reg++) {
+ if (reg->name < min || reg->name >= max || reg->reserved)
+ return false;
+ if (reg->name == pc_name && reg->value)
+ pc_set = true;
+ }
+ return pc_set;
+}
+EXPORT_SYMBOL_GPL(trampfd_valid_regs);
+
+/*
+ * Check if the PC specified in a register context is allowed.
+ */
+bool trampfd_allowed_pc(struct trampfd *trampfd, struct trampfd_regs *tregs)
+{
+ struct trampfd_reg *reg = tregs->regs;
+ struct trampfd_reg *reg_end = reg + tregs->nregs;
+ struct trampfd_values *allowed_pcs = trampfd->allowed_pcs;
+ u64 *allowed_values, pc_value = 0;
+ u32 nvalues, pc_name;
+ int i;
+
+ if (!allowed_pcs)
+ return true;
+
+ pc_name = is_compat() ? x32_eip : x64_rip;
+
+ /*
+ * Find the PC register and its value. If the PC register has been
+ * specified multiple times, only the last one counts.
+ */
+ for (; reg < reg_end; reg++) {
+ if (reg->name == pc_name)
+ pc_value = reg->value;
+ }
+
+ allowed_values = allowed_pcs->values;
+ nvalues = allowed_pcs->nvalues;
+
+ for (i = 0; i < nvalues; i++) {
+ if (pc_value == allowed_values[i])
+ return true;
+ }
+ return false;
+}
+EXPORT_SYMBOL_GPL(trampfd_allowed_pc);
+
+/* ---------------------------- Stack Context ---------------------------- */
+
+static int push_data(struct pt_regs *pt_regs, struct trampfd_stack *tstack)
+{
+ unsigned long sp;
+
+ sp = user_stack_pointer(pt_regs) - tstack->size - tstack->offset;
+ if (tstack->flags & TRAMPFD_SET_SP) {
+ if (is_compat())
+ sp = ((sp + 4) & -16ul) - 4;
+ else
+ sp = round_down(sp, 16) - 8;
+ }
+
+ if (!access_ok(sp, user_stack_pointer(pt_regs) - sp))
+ return -EFAULT;
+
+ if (copy_to_user(USERPTR(sp), tstack->data, tstack->size))
+ return -EFAULT;
+
+ if (tstack->flags & TRAMPFD_SET_SP)
+ user_stack_pointer_set(pt_regs, sp);
+
+ return 0;
+}
+
+/* ---------------------------- Fault Handlers ---------------------------- */
+
+static int trampfd_user_fault(struct trampfd *trampfd,
+ struct vm_area_struct *vma,
+ struct pt_regs *pt_regs)
+{
+ char buf[TRAMPFD_MAX_STACK_SIZE];
+ struct trampfd_regs *tregs;
+ struct trampfd_stack *tstack = NULL;
+ unsigned long addr;
+ size_t size;
+ int rc = 0;
+
+ mutex_lock(&trampfd->lock);
+
+ /*
+ * Execution of the trampoline must start at the offset specfied by
+ * the kernel.
+ */
+ addr = vma->vm_start + trampfd->map.ioffset;
+ if (addr != pt_regs->ip) {
+ rc = -EINVAL;
+ goto unlock;
+ }
+
+ /*
+ * At a minimum, the user PC register must be specified for a
+ * user trampoline.
+ */
+ tregs = trampfd->regs;
+ if (!tregs) {
+ rc = -EINVAL;
+ goto unlock;
+ }
+
+ /*
+ * Set the register context for the trampoline.
+ */
+ set_regs(pt_regs, tregs);
+
+ if (trampfd->stack) {
+ /*
+ * Copy the stack context into a local buffer and push stack
+ * data after dropping the lock.
+ */
+ size = sizeof(*trampfd->stack) + trampfd->stack->size;
+ tstack = (struct trampfd_stack *) buf;
+ memcpy(tstack, trampfd->stack, size);
+ }
+unlock:
+ mutex_unlock(&trampfd->lock);
+
+ if (!rc && tstack) {
+ mmap_read_unlock(vma->vm_mm);
+ rc = push_data(pt_regs, tstack);
+ mmap_read_lock(vma->vm_mm);
+ }
+ return rc;
+}
+
+/*
+ * Handle it if it is a trampoline fault.
+ */
+bool trampfd_fault(struct vm_area_struct *vma, struct pt_regs *pt_regs)
+{
+ struct trampfd *trampfd;
+
+ if (!is_trampfd_vma(vma))
+ return false;
+ trampfd = vma->vm_private_data;
+
+ if (trampfd->type == TRAMPFD_USER)
+ return !trampfd_user_fault(trampfd, vma, pt_regs);
+ return false;
+}
+EXPORT_SYMBOL_GPL(trampfd_fault);
+
+/* ------------------------- Arch Initialization ------------------------- */
+
+int trampfd_check_arch(struct trampfd *trampfd)
+{
+ return 0;
+}
+EXPORT_SYMBOL_GPL(trampfd_check_arch);
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 1ead568c0101..a1432ee2a1a2 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -18,6 +18,7 @@
#include <linux/uaccess.h> /* faulthandler_disabled() */
#include <linux/efi.h> /* efi_recover_from_page_fault()*/
#include <linux/mm_types.h>
+#include <linux/trampfd.h> /* trampoline invocation */

#include <asm/cpufeature.h> /* boot_cpu_has, ... */
#include <asm/traps.h> /* dotraplinkage, ... */
@@ -1142,6 +1143,7 @@ void do_user_addr_fault(struct pt_regs *regs,
struct mm_struct *mm;
vm_fault_t fault, major = 0;
unsigned int flags = FAULT_FLAG_DEFAULT;
+ unsigned long tflags = X86_PF_INSTR | X86_PF_USER;

tsk = current;
mm = tsk->mm;
@@ -1275,6 +1277,15 @@ void do_user_addr_fault(struct pt_regs *regs,
*/
good_area:
if (unlikely(access_error(hw_error_code, vma))) {
+ /*
+ * If it is a user execute fault, it could be a trampoline
+ * invocation.
+ */
+ if ((hw_error_code & tflags) == tflags &&
+ trampfd_fault(vma, regs)) {
+ mmap_read_unlock(mm);
+ return;
+ }
bad_area_access_error(regs, hw_error_code, address, vma);
return;
}
--
2.17.1
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