bpf: Add bpf_probe_write_user BPF helper to be called in tracers

This allows user memory to be written to during the course of a kprobe.
It shouldn't be used to implement any kind of security mechanism
because of TOC-TOU attacks, but rather to debug, divert, and
manipulate execution of semi-cooperative processes.

Although it uses probe_kernel_write, we limit the address space
the probe can write into by checking the space with access_ok.
We do this as opposed to calling copy_to_user directly, in order
to avoid sleeping. In addition we ensure the threads's current fs
/ segment is USER_DS and the thread isn't exiting nor a kernel thread.

Given this feature is meant for experiments, and it has a risk of
crashing the system, and running programs, we print a warning on
when a proglet that attempts to use this helper is installed,
along with the pid and process name.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 57 insertions, 0 deletions

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 2b7076f5b5ad..da218fec6056 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -365,6 +365,16 @@ enum bpf_func_id {
          */
         BPF_FUNC_get_current_task,
 
+        /**
+         * bpf_probe_write_user(void *dst, void *src, int len)
+         * safely attempt to write to a location
+         * @dst: destination address in userspace
+         * @src: source address on stack
+         * @len: number of bytes to copy
+         * Return: 0 on success or negative error
+         */
+        BPF_FUNC_probe_write_user,
+
         __BPF_FUNC_MAX_ID,
 };
 
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index a12bbd32c0a6..b20438fdb029 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -81,6 +81,49 @@ static const struct bpf_func_proto bpf_probe_read_proto = {
         .arg3_type      = ARG_ANYTHING,
 };
 
+static u64 bpf_probe_write_user(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
+{
+        void *unsafe_ptr = (void *) (long) r1;
+        void *src = (void *) (long) r2;
+        int size = (int) r3;
+
+        /*
+         * Ensure we're in user context which is safe for the helper to
+         * run. This helper has no business in a kthread.
+         *
+         * access_ok() should prevent writing to non-user memory, but in
+         * some situations (nommu, temporary switch, etc) access_ok() does
+         * not provide enough validation, hence the check on KERNEL_DS.
+         */
+
+        if (unlikely(in_interrupt() ||
+                     current->flags & (PF_KTHREAD | PF_EXITING)))
+                return -EPERM;
+        if (unlikely(segment_eq(get_fs(), KERNEL_DS)))
+                return -EPERM;
+        if (!access_ok(VERIFY_WRITE, unsafe_ptr, size))
+                return -EPERM;
+
+        return probe_kernel_write(unsafe_ptr, src, size);
+}
+
+static const struct bpf_func_proto bpf_probe_write_user_proto = {
+        .func           = bpf_probe_write_user,
+        .gpl_only       = true,
+        .ret_type       = RET_INTEGER,
+        .arg1_type      = ARG_ANYTHING,
+        .arg2_type      = ARG_PTR_TO_STACK,
+        .arg3_type      = ARG_CONST_STACK_SIZE,
+};
+
+static const struct bpf_func_proto *bpf_get_probe_write_proto(void)
+{
+        pr_warn_ratelimited("%s[%d] is installing a program with bpf_probe_write_user helper that may corrupt user memory!",
+                            current->comm, task_pid_nr(current));
+
+        return &bpf_probe_write_user_proto;
+}
+
 /*
  * limited trace_printk()
  * only %d %u %x %ld %lu %lx %lld %llu %llx %p %s conversion specifiers allowed
@@ -362,6 +405,8 @@ static const struct bpf_func_proto *tracing_func_proto(enum bpf_func_id func_id)
                 return &bpf_get_smp_processor_id_proto;
         case BPF_FUNC_perf_event_read:
                 return &bpf_perf_event_read_proto;
+        case BPF_FUNC_probe_write_user:
+                return bpf_get_probe_write_proto();
         default:
                 return NULL;
         }
diff --git a/samples/bpf/bpf_helpers.h b/samples/bpf/bpf_helpers.h
index 84e3fd919a06..217c8d507f2e 100644
--- a/samples/bpf/bpf_helpers.h
+++ b/samples/bpf/bpf_helpers.h
@@ -41,6 +41,8 @@ static int (*bpf_perf_event_output)(void *ctx, void *map, int index, void *data,
         (void *) BPF_FUNC_perf_event_output;
 static int (*bpf_get_stackid)(void *ctx, void *map, int flags) =
         (void *) BPF_FUNC_get_stackid;
+static int (*bpf_probe_write_user)(void *dst, void *src, int size) =
+        (void *) BPF_FUNC_probe_write_user;
 
 /* llvm builtin functions that eBPF C program may use to
  * emit BPF_LD_ABS and BPF_LD_IND instructions