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 <CACT4Y+aRcD_bUnyqOp-PM3YOguFwSZxpurZnC2YHo4kFFLbiTw@mail.gmail.com>
In-Reply-To: <CACT4Y+aRcD_bUnyqOp-PM3YOguFwSZxpurZnC2YHo4kFFLbiTw@mail.gmail.com>
From:   Peter Collingbourne <pcc@google.com>
Date:   Thu, 9 Dec 2021 14:13:06 -0800
Message-ID: <CAMn1gO7jXPdSf60d5pd4uEvTEVmcK=cBf0ceW4F1F44wbFg9SQ@mail.gmail.com>
Subject: Re: [PATCH v3 2/6] uaccess-buffer: add core code
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        YiFei Zhu <yifeifz2@illinois.edu>,
        Mark Rutland <mark.rutland@arm.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Gabriel Krisman Bertazi <krisman@collabora.com>,
        Chris Hyser <chris.hyser@oracle.com>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        Arnd Bergmann <arnd@arndb.de>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Alexey Gladkov <legion@kernel.org>,
        Ran Xiaokai <ran.xiaokai@zte.com.cn>,
        David Hildenbrand <david@redhat.com>,
        Xiaofeng Cao <caoxiaofeng@yulong.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Thomas Cedeno <thomascedeno@google.com>,
        Marco Elver <elver@google.com>,
        Alexander Potapenko <glider@google.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Evgenii Stepanov <eugenis@google.com>
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Precedence: bulk

On Wed, Dec 8, 2021 at 2:21 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Wed, 8 Dec 2021 at 05:48, Peter Collingbourne <pcc@google.com> wrote:
> >
> > 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@google.com>
> > ---
> > 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)
>
> I would move the implementation to .c file. It's a rare path.

Done.

> > +{
> > +       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_EXIT is only defined in future patches, so this won't compile.

Right, but there's no way for CONFIG_UACCESS_BUFFER to be defined at this point,
so this won't compile anyway. We define the constants for this
(TIF_UACCESS_BUFFER_EXIT for arm64, SYSCALL_WORK_UACCESS_BUFFER_EXIT for
GENERIC_ENTRY) at the same time as we enable the respective architecture
support.

>
> > +               __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);
>
> Does this miss uaccess_buffer_free() for the 0 case?
> It seems that when we set it to 0 we always want to free as well (e.g.
> in exec). I wonder if freeing should be done by
> uaccess_buffer_set_descriptor_addr_addr() itself.
> Both uaccess_buffer_set_descriptor_addr_addr() and
> uaccess_buffer_free() reset task work, which is fine but is somewhat
> suboptimal logically.
> Then task exit could do uaccess_buffer_set_descriptor_addr_addr(0).

We don't need to free in that case because we can just log to the original
location. I originally had uaccess_buffer_set_descriptor_addr_addr() do the
freeing, but you asked me to change it to avoid the free [1]. I guess I don't
have a strong opinion but slightly prefer having the two functions do orthogonal
things.

[1] https://lore.kernel.org/all/CACT4Y+aoiT+z+3CMBNmO0SwXBXpfDCsHY7pPLf54S8V=c-a8ag@mail.gmail.com/#:~:text=Is%20this%20necessary

>
> > +               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);
>
> This and __uaccess_buffer_post_exit_loop() runs only when we have a
> signal/timer interrupt between setting the descriptor address in
> userspace and entering the next syscall, right?
> Just want to make sure this code is not executed for normal uaccess
> tracing for performance reasons.

They only need to run if something in _TIF_WORK_MASK (arm64) or
EXIT_TO_USER_MODE_WORK (GENERIC_ENTRY) is set, i.e. signals pending
or some kind of tracing enabled. That's how it works on the arm64
side (check is in the caller of do_notify_resume) but I had neglected
to move the calls into the if statement on the GENERIC_ENTRY side;
done in v4.

> > +       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)
>
> I think we also need to set at least buf->kend to NULL here.
> I am not sure what can go wrong now, but it's a strange state. On next
> iteration we will do "buf->kend - buf->kbegin", where kend is a
> dangling pointer and kbegin is NULL.

Done.

Peter