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authorSargun Dhillon <sargun@sargun.me>2016-07-25 05:54:46 -0700
committerDavid S. Miller <davem@davemloft.net>2016-07-25 18:07:48 -0700
commit96ae52279594470622ff0585621a13e96b700600 (patch)
tree72b6be55be49c626dfd6d1b1ac2673b4a0cd649b
parent9b022a6e0f26af108b9105b16b310393c898d9bd (diff)
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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>
-rw-r--r--include/uapi/linux/bpf.h10
-rw-r--r--kernel/trace/bpf_trace.c45
-rw-r--r--samples/bpf/bpf_helpers.h2
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