Re: [PATCH v3 2/6] uaccess-buffer: add core code

Add the core code to support uaccess logging. Subsequent patches will
hook this up to the arch-specific kernel entry and exit code for
certain architectures.

Link: https://linux-review.googlesource.com/id/I6581765646501a5631b281d670903945ebadc57d
Signed-off-by: Peter Collingbourne <redacted>
---
v3:
- performance optimizations for entry/exit code
- don't use kcur == NULL to mean overflow
- fix potential double free in clone()
- don't allocate a new kernel-side uaccess buffer for each syscall
- fix uaccess buffer leak on exit
- fix some sparse warnings

v2:
- New interface that avoids multiple syscalls per real syscall and
  is arch-generic
- Avoid logging uaccesses done by BPF programs
- Add documentation
- Split up into multiple patches
- Various code moves, renames etc as requested by Marco

 fs/exec.c                            |   3 +
 include/linux/instrumented-uaccess.h |   6 +-
 include/linux/sched.h                |   5 ++
 include/linux/uaccess-buffer-info.h  |  46 ++++++++++
 include/linux/uaccess-buffer.h       | 112 +++++++++++++++++++++++
 include/uapi/linux/prctl.h           |   3 +
 include/uapi/linux/uaccess-buffer.h  |  27 ++++++
 kernel/Makefile                      |   1 +
 kernel/bpf/helpers.c                 |   7 +-
 kernel/fork.c                        |   4 +
 kernel/signal.c                      |   4 +-
 kernel/sys.c                         |   6 ++
 kernel/uaccess-buffer.c              | 129 +++++++++++++++++++++++++++
 13 files changed, 350 insertions(+), 3 deletions(-)
 create mode 100644 include/linux/uaccess-buffer-info.h
 create mode 100644 include/linux/uaccess-buffer.h
 create mode 100644 include/uapi/linux/uaccess-buffer.h
 create mode 100644 kernel/uaccess-buffer.c

diff --git a/fs/exec.c b/fs/exec.c
index 537d92c41105..c9975e790f30 100644
--- a/fs/exec.c
+++ b/fs/exec.c

@@ -65,6 +65,7 @@
 #include <linux/vmalloc.h>
 #include <linux/io_uring.h>
 #include <linux/syscall_user_dispatch.h>
+#include <linux/uaccess-buffer.h>
 
 #include <linux/uaccess.h>
 #include <asm/mmu_context.h>

@@ -1313,6 +1314,8 @@ int begin_new_exec(struct linux_binprm * bprm)
 	me->personality &= ~bprm->per_clear;
 
 	clear_syscall_work_syscall_user_dispatch(me);
+	uaccess_buffer_set_descriptor_addr_addr(0);
+	uaccess_buffer_free(current);
 
 	/*
 	 * We have to apply CLOEXEC before we change whether the process is

diff --git a/include/linux/instrumented-uaccess.h b/include/linux/instrumented-uaccess.h
index ece549088e50..b967f4436d15 100644
--- a/include/linux/instrumented-uaccess.h
+++ b/include/linux/instrumented-uaccess.h

@@ -2,7 +2,8 @@
 
 /*
  * This header provides generic wrappers for memory access instrumentation for
- * uaccess routines that the compiler cannot emit for: KASAN, KCSAN.
+ * uaccess routines that the compiler cannot emit for: KASAN, KCSAN,
+ * uaccess buffers.
  */
 #ifndef _LINUX_INSTRUMENTED_UACCESS_H
 #define _LINUX_INSTRUMENTED_UACCESS_H

@@ -11,6 +12,7 @@
 #include <linux/kasan-checks.h>
 #include <linux/kcsan-checks.h>
 #include <linux/types.h>
+#include <linux/uaccess-buffer.h>
 
 /**
  * instrument_copy_to_user - instrument reads of copy_to_user

@@ -27,6 +29,7 @@ instrument_copy_to_user(void __user *to, const void *from, unsigned long n)
 {
 	kasan_check_read(from, n);
 	kcsan_check_read(from, n);
+	uaccess_buffer_log_write(to, n);
 }
 
 /**

@@ -44,6 +47,7 @@ instrument_copy_from_user(const void *to, const void __user *from, unsigned long
 {
 	kasan_check_write(to, n);
 	kcsan_check_write(to, n);
+	uaccess_buffer_log_read(from, n);
 }
 
 #endif /* _LINUX_INSTRUMENTED_UACCESS_H */

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 78c351e35fec..7c5278d7b57d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h

@@ -34,6 +34,7 @@
 #include <linux/rseq.h>
 #include <linux/seqlock.h>
 #include <linux/kcsan.h>
+#include <linux/uaccess-buffer-info.h>
 #include <asm/kmap_size.h>
 
 /* task_struct member predeclarations (sorted alphabetically): */

@@ -1484,6 +1485,10 @@ struct task_struct {
 	struct callback_head		l1d_flush_kill;
 #endif
 
+#ifdef CONFIG_HAVE_ARCH_UACCESS_BUFFER
+	struct uaccess_buffer_info	uaccess_buffer;
+#endif
+
 	/*
 	 * New fields for task_struct should be added above here, so that
 	 * they are included in the randomized portion of task_struct.

diff --git a/include/linux/uaccess-buffer-info.h b/include/linux/uaccess-buffer-info.h
new file mode 100644
index 000000000000..15e2d8f7c074
--- /dev/null
+++ b/include/linux/uaccess-buffer-info.h

@@ -0,0 +1,46 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_UACCESS_BUFFER_INFO_H
+#define _LINUX_UACCESS_BUFFER_INFO_H
+
+#ifdef CONFIG_HAVE_ARCH_UACCESS_BUFFER
+
+struct uaccess_buffer_info {
+	/*
+	 * The pointer to pointer to struct uaccess_descriptor. This is the
+	 * value controlled by prctl(PR_SET_UACCESS_DESCRIPTOR_ADDR_ADDR).
+	 */
+	struct uaccess_descriptor __user *__user *desc_ptr_ptr;
+
+	/*
+	 * The pointer to struct uaccess_descriptor read at syscall entry time.
+	 */
+	struct uaccess_descriptor __user *desc_ptr;
+
+	/*
+	 * A pointer to the kernel's temporary copy of the uaccess log for the
+	 * current syscall. We log to a kernel buffer in order to avoid leaking
+	 * timing information to userspace.
+	 */
+	struct uaccess_buffer_entry *kbegin;
+
+	/*
+	 * The position of the next uaccess buffer entry for the current
+	 * syscall, or NULL if we are not logging the current syscall.
+	 */
+	struct uaccess_buffer_entry *kcur;
+
+	/*
+	 * A pointer to the end of the kernel's uaccess log.
+	 */
+	struct uaccess_buffer_entry *kend;
+
+	/*
+	 * The pointer to the userspace uaccess log, as read from the
+	 * struct uaccess_descriptor.
+	 */
+	struct uaccess_buffer_entry __user *ubegin;
+};
+
+#endif
+
+#endif  /* _LINUX_UACCESS_BUFFER_INFO_H */

diff --git a/include/linux/uaccess-buffer.h b/include/linux/uaccess-buffer.h
new file mode 100644
index 000000000000..f2f46db274f3
--- /dev/null
+++ b/include/linux/uaccess-buffer.h

@@ -0,0 +1,112 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_UACCESS_BUFFER_H
+#define _LINUX_UACCESS_BUFFER_H
+
+#include <linux/sched.h>
+#include <uapi/linux/uaccess-buffer.h>
+
+#include <asm-generic/errno-base.h>
+
+#ifdef CONFIG_HAVE_ARCH_UACCESS_BUFFER

+
+static inline void uaccess_buffer_log_read(const void __user *from,
+					   unsigned long n)
+{
+}
+static inline void uaccess_buffer_log_write(void __user *to, unsigned long n)
+{
+}
+
+#endif
+
+#endif  /* _LINUX_UACCESS_BUFFER_H */

diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index bb73e9a0b24f..74b37469c7b3 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h

@@ -272,4 +272,7 @@ struct prctl_mm_map {
 # define PR_SCHED_CORE_SCOPE_THREAD_GROUP	1
 # define PR_SCHED_CORE_SCOPE_PROCESS_GROUP	2
 
+/* Configure uaccess logging feature */
+#define PR_SET_UACCESS_DESCRIPTOR_ADDR_ADDR	63
+
 #endif /* _LINUX_PRCTL_H */

diff --git a/include/uapi/linux/uaccess-buffer.h b/include/uapi/linux/uaccess-buffer.h
new file mode 100644
index 000000000000..bf10f7c78857
--- /dev/null
+++ b/include/uapi/linux/uaccess-buffer.h

@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_LINUX_UACCESS_BUFFER_H
+#define _UAPI_LINUX_UACCESS_BUFFER_H
+
+#include <linux/types.h>
+
+/* Location of the uaccess log. */
+struct uaccess_descriptor {
+	/* Address of the uaccess_buffer_entry array. */
+	__u64 addr;
+	/* Size of the uaccess_buffer_entry array in number of elements. */
+	__u64 size;
+};
+
+/* Format of the entries in the uaccess log. */
+struct uaccess_buffer_entry {
+	/* Address being accessed. */
+	__u64 addr;
+	/* Number of bytes that were accessed. */
+	__u64 size;
+	/* UACCESS_BUFFER_* flags. */
+	__u64 flags;
+};
+
+#define UACCESS_BUFFER_FLAG_WRITE	1 /* access was a write */
+
+#endif /* _UAPI_LINUX_UACCESS_BUFFER_H */

diff --git a/kernel/Makefile b/kernel/Makefile
index 186c49582f45..d4d9be5146c3 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile

@@ -114,6 +114,7 @@ obj-$(CONFIG_KCSAN) += kcsan/
 obj-$(CONFIG_SHADOW_CALL_STACK) += scs.o
 obj-$(CONFIG_HAVE_STATIC_CALL_INLINE) += static_call.o
 obj-$(CONFIG_CFI_CLANG) += cfi.o
+obj-$(CONFIG_HAVE_ARCH_UACCESS_BUFFER) += uaccess-buffer.o
 
 obj-$(CONFIG_PERF_EVENTS) += events/
 

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 649f07623df6..ab6520a633ef 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c

@@ -15,6 +15,7 @@
 #include <linux/pid_namespace.h>
 #include <linux/proc_ns.h>
 #include <linux/security.h>
+#include <linux/uaccess-buffer.h>
 
 #include "../../lib/kstrtox.h"
 

@@ -637,7 +638,11 @@ const struct bpf_func_proto bpf_event_output_data_proto =  {
 BPF_CALL_3(bpf_copy_from_user, void *, dst, u32, size,
 	   const void __user *, user_ptr)
 {
-	int ret = copy_from_user(dst, user_ptr, size);
+	/*
+	 * Avoid logging uaccesses here as the BPF program may not be following
+	 * the uaccess log rules.
+	 */
+	int ret = copy_from_user_nolog(dst, user_ptr, size);
 
 	if (unlikely(ret)) {
 		memset(dst, 0, size);

diff --git a/kernel/fork.c b/kernel/fork.c
index 3244cc56b697..8be2ca528a65 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c

@@ -96,6 +96,7 @@
 #include <linux/scs.h>
 #include <linux/io_uring.h>
 #include <linux/bpf.h>
+#include <linux/uaccess-buffer.h>
 
 #include <asm/pgalloc.h>
 #include <linux/uaccess.h>

@@ -754,6 +755,7 @@ void __put_task_struct(struct task_struct *tsk)
 	delayacct_tsk_free(tsk);
 	put_signal_struct(tsk->signal);
 	sched_core_free(tsk);
+	uaccess_buffer_free(tsk);
 
 	if (!profile_handoff_task(tsk))
 		free_task(tsk);

@@ -890,6 +892,8 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 	if (memcg_charge_kernel_stack(tsk))
 		goto free_stack;
 
+	uaccess_buffer_free(orig);
+
 	stack_vm_area = task_stack_vm_area(tsk);
 
 	err = arch_dup_task_struct(tsk, orig);

diff --git a/kernel/signal.c b/kernel/signal.c
index a629b11bf3e0..69bf21518bd0 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c

@@ -45,6 +45,7 @@
 #include <linux/posix-timers.h>
 #include <linux/cgroup.h>
 #include <linux/audit.h>
+#include <linux/uaccess-buffer.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/signal.h>

@@ -1031,7 +1032,8 @@ static void complete_signal(int sig, struct task_struct *p, enum pid_type type)
 	if (sig_fatal(p, sig) &&
 	    !(signal->flags & SIGNAL_GROUP_EXIT) &&
 	    !sigismember(&t->real_blocked, sig) &&
-	    (sig == SIGKILL || !p->ptrace)) {
+	    (sig == SIGKILL ||
+	     !(p->ptrace || uaccess_buffer_maybe_blocked(p)))) {
 		/*
 		 * This signal will be fatal to the whole group.
 		 */

diff --git a/kernel/sys.c b/kernel/sys.c
index 8fdac0d90504..c71a9a9c0f68 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c

@@ -42,6 +42,7 @@
 #include <linux/version.h>
 #include <linux/ctype.h>
 #include <linux/syscall_user_dispatch.h>
+#include <linux/uaccess-buffer.h>
 
 #include <linux/compat.h>
 #include <linux/syscalls.h>

@@ -2530,6 +2531,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		error = sched_core_share_pid(arg2, arg3, arg4, arg5);
 		break;
 #endif
+	case PR_SET_UACCESS_DESCRIPTOR_ADDR_ADDR:
+		if (arg3 || arg4 || arg5)
+			return -EINVAL;
+		error = uaccess_buffer_set_descriptor_addr_addr(arg2);
+		break;
 	default:
 		error = -EINVAL;
 		break;

diff --git a/kernel/uaccess-buffer.c b/kernel/uaccess-buffer.c
new file mode 100644
index 000000000000..088e43f7611c
--- /dev/null
+++ b/kernel/uaccess-buffer.c

@@ -0,0 +1,129 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Support for uaccess logging via uaccess buffers.
+ *
+ * Copyright (C) 2021, Google LLC.
+ */
+
+#include <linux/compat.h>
+#include <linux/mm.h>
+#include <linux/prctl.h>
+#include <linux/ptrace.h>
+#include <linux/sched.h>
+#include <linux/signal.h>
+#include <linux/slab.h>
+#include <linux/uaccess.h>
+#include <linux/uaccess-buffer.h>
+
+static void uaccess_buffer_log(unsigned long addr, unsigned long size,
+			      unsigned long flags)
+{
+	struct uaccess_buffer_info *buf = &current->uaccess_buffer;
+	struct uaccess_buffer_entry *entry = buf->kcur;
+
+	if (entry == buf->kend || unlikely(uaccess_kernel()))
+		return;
+	entry->addr = addr;
+	entry->size = size;
+	entry->flags = flags;
+
+	++buf->kcur;
+}
+
+void __uaccess_buffer_log_read(const void __user *from, unsigned long n)
+{
+	uaccess_buffer_log((unsigned long)from, n, 0);
+}
+EXPORT_SYMBOL(__uaccess_buffer_log_read);
+
+void __uaccess_buffer_log_write(void __user *to, unsigned long n)
+{
+	uaccess_buffer_log((unsigned long)to, n, UACCESS_BUFFER_FLAG_WRITE);
+}
+EXPORT_SYMBOL(__uaccess_buffer_log_write);
+
+bool __uaccess_buffer_pre_exit_loop(void)
+{
+	struct uaccess_buffer_info *buf = &current->uaccess_buffer;
+	struct uaccess_descriptor __user *desc_ptr;
+	sigset_t tmp_mask;
+
+	if (get_user(desc_ptr, buf->desc_ptr_ptr) || !desc_ptr)
+		return false;
+
+	current->real_blocked = current->blocked;
+	sigfillset(&tmp_mask);
+	set_current_blocked(&tmp_mask);
+	return true;
+}
+
+void __uaccess_buffer_post_exit_loop(void)
+{
+	spin_lock_irq(&current->sighand->siglock);
+	current->blocked = current->real_blocked;
+	recalc_sigpending();
+	spin_unlock_irq(&current->sighand->siglock);
+}
+
+void uaccess_buffer_free(struct task_struct *tsk)
+{
+	struct uaccess_buffer_info *buf = &tsk->uaccess_buffer;
+
+	kfree(buf->kbegin);
+	clear_syscall_work(UACCESS_BUFFER_EXIT);
+	buf->kbegin = buf->kcur = buf->kend = NULL;
+}
+
+void __uaccess_buffer_syscall_entry(void)
+{
+	struct uaccess_buffer_info *buf = &current->uaccess_buffer;
+	struct uaccess_descriptor desc;
+
+	if (get_user(buf->desc_ptr, buf->desc_ptr_ptr) || !buf->desc_ptr ||
+	    put_user(0, buf->desc_ptr_ptr) ||
+	    copy_from_user(&desc, buf->desc_ptr, sizeof(desc)))
+		return;
+
+	if (desc.size > 1024)
+		desc.size = 1024;
+
+	if (buf->kend - buf->kbegin != desc.size)
+		buf->kbegin =
+			krealloc_array(buf->kbegin, desc.size,
+				       sizeof(struct uaccess_buffer_entry),
+				       GFP_KERNEL);
+	if (!buf->kbegin)
+		return;
+
+	set_syscall_work(UACCESS_BUFFER_EXIT);
+	buf->kcur = buf->kbegin;
+	buf->kend = buf->kbegin + desc.size;
+	buf->ubegin =
+		(struct uaccess_buffer_entry __user *)(unsigned long)desc.addr;
+}
+
+void __uaccess_buffer_syscall_exit(void)
+{
+	struct uaccess_buffer_info *buf = &current->uaccess_buffer;
+	u64 num_entries = buf->kcur - buf->kbegin;
+	struct uaccess_descriptor desc;
+
+	clear_syscall_work(UACCESS_BUFFER_EXIT);
+	desc.addr = (u64)(unsigned long)(buf->ubegin + num_entries);
+	desc.size = buf->kend - buf->kcur;
+	buf->kcur = NULL;
+	if (copy_to_user(buf->ubegin, buf->kbegin,
+			 num_entries * sizeof(struct uaccess_buffer_entry)) == 0)
+		(void)copy_to_user(buf->desc_ptr, &desc, sizeof(desc));
+}
+
+size_t copy_from_user_nolog(void *to, const void __user *from, size_t len)
+{
+	size_t retval;
+
+	clear_syscall_work(UACCESS_BUFFER_EXIT);
+	retval = copy_from_user(to, from, len);
+	if (current->uaccess_buffer.kcur)
+		set_syscall_work(UACCESS_BUFFER_EXIT);
+	return retval;
+}

-- 
2.34.1.173.g76aa8bc2d0-goog