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

From: Peter Collingbourne
Date: Tue Dec 07 2021 - 23:48:29 EST

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 <pcc@xxxxxxxxxx>
---
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 bool uaccess_buffer_maybe_blocked(struct task_struct *tsk)
+{
+ return test_task_syscall_work(tsk, UACCESS_BUFFER_ENTRY);
+}
+
+void __uaccess_buffer_syscall_entry(void);
+static inline void uaccess_buffer_syscall_entry(void)
+{
+ __uaccess_buffer_syscall_entry();
+}
+
+void __uaccess_buffer_syscall_exit(void);
+static inline void uaccess_buffer_syscall_exit(void)
+{
+ __uaccess_buffer_syscall_exit();
+}
+
+bool __uaccess_buffer_pre_exit_loop(void);
+static inline bool uaccess_buffer_pre_exit_loop(void)
+{
+ if (!test_syscall_work(UACCESS_BUFFER_ENTRY))
+ return false;
+ return __uaccess_buffer_pre_exit_loop();
+}
+
+void __uaccess_buffer_post_exit_loop(void);
+static inline void uaccess_buffer_post_exit_loop(bool pending)
+{
+ if (pending)
+ __uaccess_buffer_post_exit_loop();
+}
+
+static inline int uaccess_buffer_set_descriptor_addr_addr(unsigned long addr)
+{
+ current->uaccess_buffer.desc_ptr_ptr =
+ (struct uaccess_descriptor __user * __user *)addr;
+ if (addr)
+ set_syscall_work(UACCESS_BUFFER_ENTRY);
+ else
+ clear_syscall_work(UACCESS_BUFFER_ENTRY);
+ return 0;
+}
+
+size_t copy_from_user_nolog(void *to, const void __user *from, size_t len);
+
+void uaccess_buffer_free(struct task_struct *tsk);
+
+void __uaccess_buffer_log_read(const void __user *from, unsigned long n);
+static inline void uaccess_buffer_log_read(const void __user *from, unsigned long n)
+{
+ if (unlikely(test_syscall_work(UACCESS_BUFFER_EXIT)))
+ __uaccess_buffer_log_read(from, n);
+}
+
+void __uaccess_buffer_log_write(void __user *to, unsigned long n);
+static inline void uaccess_buffer_log_write(void __user *to, unsigned long n)
+{
+ if (unlikely(test_syscall_work(UACCESS_BUFFER_EXIT)))
+ __uaccess_buffer_log_write(to, n);
+}
+
+#else
+
+static inline bool uaccess_buffer_maybe_blocked(struct task_struct *tsk)
+{
+ return false;
+}
+static inline void uaccess_buffer_syscall_entry(void)
+{
+}
+static inline void uaccess_buffer_syscall_exit(void)
+{
+}
+static inline bool uaccess_buffer_pre_exit_loop(void)
+{
+ return false;
+}
+static inline void uaccess_buffer_post_exit_loop(bool pending)
+{
+}
+static inline int uaccess_buffer_set_descriptor_addr_addr(unsigned long addr)
+{
+ return -EINVAL;
+}
+static inline void uaccess_buffer_free(struct task_struct *tsk)
+{
+}
+
+#define copy_from_user_nolog(to, from, len) copy_from_user(to, from, len)
+
+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