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

Implement 32-bit ARM 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/arm/include/uapi/asm/ptrace.h | 20 +++
arch/arm/kernel/Makefile | 1 +
arch/arm/kernel/trampfd.c | 214 +++++++++++++++++++++++++++++
arch/arm/mm/fault.c | 12 +-
arch/arm/tools/syscall.tbl | 1 +
5 files changed, 246 insertions(+), 2 deletions(-)
create mode 100644 arch/arm/kernel/trampfd.c

diff --git a/arch/arm/include/uapi/asm/ptrace.h b/arch/arm/include/uapi/asm/ptrace.h
index e61c65b4018d..47b1c5e2f32c 100644
--- a/arch/arm/include/uapi/asm/ptrace.h
+++ b/arch/arm/include/uapi/asm/ptrace.h
@@ -151,6 +151,26 @@ struct pt_regs {
#define ARM_r0 uregs[0]
#define ARM_ORIG_r0 uregs[17]

+/*
+ * These register names are to be used by 32-bit applications.
+ */
+enum reg_32_name {
+ arm_r0,
+ arm_r1,
+ arm_r2,
+ arm_r3,
+ arm_r4,
+ arm_r5,
+ arm_r6,
+ arm_r7,
+ arm_r8,
+ arm_r9,
+ arm_r10,
+ arm_ip,
+ arm_pc,
+ arm_max,
+};
+
/*
* The size of the user-visible VFP state as seen by PTRACE_GET/SETVFPREGS
* and core dumps.
diff --git a/arch/arm/kernel/Makefile b/arch/arm/kernel/Makefile
index 89e5d864e923..652c54c2f19a 100644
--- a/arch/arm/kernel/Makefile
+++ b/arch/arm/kernel/Makefile
@@ -105,5 +105,6 @@ obj-$(CONFIG_SMP) += psci_smp.o
endif

obj-$(CONFIG_HAVE_ARM_SMCCC) += smccc-call.o
+obj-$(CONFIG_TRAMPFD) += trampfd.o

extra-y := $(head-y) vmlinux.lds
diff --git a/arch/arm/kernel/trampfd.c b/arch/arm/kernel/trampfd.c
new file mode 100644
index 000000000000..50fc5706e85b
--- /dev/null
+++ b/arch/arm/kernel/trampfd.c
@@ -0,0 +1,214 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Trampoline File Descriptor - ARM support.
+ *
+ * Author: Madhavan T. Venkataraman (madvenka@linux.microsoft.com)
+ *
+ * Copyright (c) 2020, Microsoft Corporation.
+ */
+
+#include <linux/trampfd.h>
+#include <linux/mm_types.h>
+#include <linux/uaccess.h>
+
+/* ---------------------------- Register Context ---------------------------- */
+
+static void set_reg(long *uregs, u32 name, u64 value)
+{
+ switch (name) {
+ case arm_r0:
+ case arm_r1:
+ case arm_r2:
+ case arm_r3:
+ case arm_r4:
+ case arm_r5:
+ case arm_r6:
+ case arm_r7:
+ case arm_r8:
+ case arm_r9:
+ case arm_r10:
+ uregs[name] = (__u64)value;
+ break;
+ case arm_ip:
+ ARM_ip = (__u64)value;
+ break;
+ case arm_pc:
+ ARM_pc = (__u64)value;
+ break;
+ default:
+ WARN(1, "%s: Illegal register name %d\n", __func__, name);
+ break;
+ }
+}
+
+static void set_regs(long *uregs, struct trampfd_regs *tregs)
+{
+ struct trampfd_reg *reg = tregs->regs;
+ struct trampfd_reg *reg_end = reg + tregs->nregs;
+
+ for (; reg < reg_end; reg++)
+ set_reg(uregs, 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;
+ bool pc_set = false;
+
+ for (; reg < reg_end; reg++) {
+ if (reg->name >= arm_max || reg->reserved)
+ return false;
+ if (reg->name == arm_pc && 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 = arm_pc;
+
+ /*
+ * 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(long *uregs, struct trampfd_stack *tstack)
+{
+ unsigned long sp;
+
+ sp = ARM_sp - tstack->size - tstack->offset;
+ if (tstack->flags & TRAMPFD_SET_SP)
+ sp &= ~7;
+
+ if (!access_ok(sp, ARM_sp - sp))
+ return -EFAULT;
+
+ if (copy_to_user(USERPTR(sp), tstack->data, tstack->size))
+ return -EFAULT;
+
+ if (tstack->flags & TRAMPFD_SET_SP)
+ ARM_sp = sp;
+ return 0;
+}
+
+/* ---------------------------- Fault Handlers ---------------------------- */
+
+static int trampfd_user_fault(struct trampfd *trampfd,
+ struct vm_area_struct *vma,
+ long *uregs)
+{
+ char buf[TRAMPFD_MAX_STACK_SIZE];
+ struct trampfd_regs *tregs;
+ struct trampfd_stack *tstack = NULL;
+ unsigned long addr;
+ size_t size;
+ int rc;
+
+ 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 != ARM_pc) {
+ 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(uregs, 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(uregs, 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;
+ unsigned long *uregs = pt_regs->uregs;
+
+ if (!is_trampfd_vma(vma))
+ return false;
+ trampfd = vma->vm_private_data;
+
+ if (trampfd->type == TRAMPFD_USER)
+ return !trampfd_user_fault(trampfd, vma, uregs);
+ return false;
+}
+EXPORT_SYMBOL_GPL(trampfd_fault);
+
+/* ---------------------------- Miscellaneous ---------------------------- */
+
+int trampfd_check_arch(struct trampfd *trampfd)
+{
+ return 0;
+}
+EXPORT_SYMBOL_GPL(trampfd_check_arch);
diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index c6550eddfce1..21a81d19336b 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -17,6 +17,7 @@
#include <linux/sched/debug.h>
#include <linux/highmem.h>
#include <linux/perf_event.h>
+#include <linux/trampfd.h>

#include <asm/system_misc.h>
#include <asm/system_info.h>
@@ -202,7 +203,8 @@ static inline bool access_error(unsigned int fsr, struct vm_area_struct *vma)

static vm_fault_t __kprobes
__do_page_fault(struct mm_struct *mm, unsigned long addr, unsigned int fsr,
- unsigned int flags, struct task_struct *tsk)
+ unsigned int flags, struct task_struct *tsk,
+ struct pt_regs *regs)
{
struct vm_area_struct *vma;
vm_fault_t fault;
@@ -220,6 +222,12 @@ __do_page_fault(struct mm_struct *mm, unsigned long addr, unsigned int fsr,
*/
good_area:
if (access_error(fsr, vma)) {
+ /*
+ * If it is an execute fault, it could be a trampoline
+ * invocation.
+ */
+ if ((fsr & FSR_LNX_PF) && trampfd_fault(vma, regs))
+ return 0;
fault = VM_FAULT_BADACCESS;
goto out;
}
@@ -290,7 +298,7 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
#endif
}

- fault = __do_page_fault(mm, addr, fsr, flags, tsk);
+ fault = __do_page_fault(mm, addr, fsr, flags, tsk, regs);

/* If we need to retry but a fatal signal is pending, handle the
* signal first. We do not need to release the mmap_lock because
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index d5cae5ffede0..88cf4c45069a 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -452,3 +452,4 @@
437 common openat2 sys_openat2
438 common pidfd_getfd sys_pidfd_getfd
439 common faccessat2 sys_faccessat2
+440 common trampfd_create sys_trampfd_create
--
2.17.1
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