Merge branch 'Fix bpf_probe_read_user_str() overcopying'

Daniel Xu says:

====================

6ae08ae ("bpf: Add probe_read_{user, kernel} and probe_read_{user,
kernel}_str helpers") introduced a subtle bug where
bpf_probe_read_user_str() would potentially copy a few extra bytes after
the NUL terminator.

This issue is particularly nefarious when strings are used as map keys,
as seemingly identical strings can occupy multiple entries in a map.

This patchset fixes the issue and introduces a selftest to prevent
future regressions.

v6 -> v7:
* Add comments

v5 -> v6:
* zero-pad up to sizeof(unsigned long) after NUL

v4 -> v5:
* don't read potentially uninitialized memory

v3 -> v4:
* directly pass userspace pointer to prog
* test more strings of different length

v2 -> v3:
* set pid filter before attaching prog in selftest
* use long instead of int as bpf_probe_read_user_str() retval
* style changes

v1 -> v2:
* add Fixes: tag
* add selftest
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Alexei Starovoitov

kernel/trace/bpf_trace.c

@@ -181,6 +181,16 @@ bpf_probe_read_user_str_common(void *dst, u32 size,
 {
 	int ret;
 
+	/*
+	 * NB: We rely on strncpy_from_user() not copying junk past the NUL
+	 * terminator into `dst`.
+	 *
+	 * strncpy_from_user() does long-sized strides in the fast path. If the
+	 * strncpy does not mask out the bytes after the NUL in `unsafe_ptr`,
+	 * then there could be junk after the NUL in `dst`. If user takes `dst`
+	 * and keys a hash map with it, then semantically identical strings can
+	 * occupy multiple entries in the map.
+	 */
 	ret = strncpy_from_user_nofault(dst, unsafe_ptr, size);
 	if (unlikely(ret < 0))
 		memset(dst, 0, size);

lib/strncpy_from_user.c

@@ -35,17 +35,32 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src,
 		goto byte_at_a_time;
 
 	while (max >= sizeof(unsigned long)) {
-		unsigned long c, data;
+		unsigned long c, data, mask;
 
 		/* Fall back to byte-at-a-time if we get a page fault */
 		unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
 
-		*(unsigned long *)(dst+res) = c;
+		/*
+		 * Note that we mask out the bytes following the NUL. This is
+		 * important to do because string oblivious code may read past
+		 * the NUL. For those routines, we don't want to give them
+		 * potentially random bytes after the NUL in `src`.
+		 *
+		 * One example of such code is BPF map keys. BPF treats map keys
+		 * as an opaque set of bytes. Without the post-NUL mask, any BPF
+		 * maps keyed by strings returned from strncpy_from_user() may
+		 * have multiple entries for semantically identical strings.
+		 */
 		if (has_zero(c, &data, &constants)) {
 			data = prep_zero_mask(c, data, &constants);
 			data = create_zero_mask(data);
+			mask = zero_bytemask(data);
+			*(unsigned long *)(dst+res) = c & mask;
 			return res + find_zero(data);
 		}
+
+		*(unsigned long *)(dst+res) = c;
+
 		res += sizeof(unsigned long);
 		max -= sizeof(unsigned long);
 	}

tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c

@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include "test_probe_read_user_str.skel.h"
+
+static const char str1[] = "mestring";
+static const char str2[] = "mestringalittlebigger";
+static const char str3[] = "mestringblubblubblubblubblub";
+
+static int test_one_str(struct test_probe_read_user_str *skel, const char *str,
+			size_t len)
+{
+	int err, duration = 0;
+	char buf[256];
+
+	/* Ensure bytes after string are ones */
+	memset(buf, 1, sizeof(buf));
+	memcpy(buf, str, len);
+
+	/* Give prog our userspace pointer */
+	skel->bss->user_ptr = buf;
+
+	/* Trigger tracepoint */
+	usleep(1);
+
+	/* Did helper fail? */
+	if (CHECK(skel->bss->ret < 0, "prog_ret", "prog returned: %ld\n",
+		  skel->bss->ret))
+		return 1;
+
+	/* Check that string was copied correctly */
+	err = memcmp(skel->bss->buf, str, len);
+	if (CHECK(err, "memcmp", "prog copied wrong string"))
+		return 1;
+
+	/* Now check that no extra trailing bytes were copied */
+	memset(buf, 0, sizeof(buf));
+	err = memcmp(skel->bss->buf + len, buf, sizeof(buf) - len);
+	if (CHECK(err, "memcmp", "trailing bytes were not stripped"))
+		return 1;
+
+	return 0;
+}
+
+void test_probe_read_user_str(void)
+{
+	struct test_probe_read_user_str *skel;
+	int err, duration = 0;
+
+	skel = test_probe_read_user_str__open_and_load();
+	if (CHECK(!skel, "test_probe_read_user_str__open_and_load",
+		  "skeleton open and load failed\n"))
+		return;
+
+	/* Give pid to bpf prog so it doesn't read from anyone else */
+	skel->bss->pid = getpid();
+
+	err = test_probe_read_user_str__attach(skel);
+	if (CHECK(err, "test_probe_read_user_str__attach",
+		  "skeleton attach failed: %d\n", err))
+		goto out;
+
+	if (test_one_str(skel, str1, sizeof(str1)))
+		goto out;
+	if (test_one_str(skel, str2, sizeof(str2)))
+		goto out;
+	if (test_one_str(skel, str3, sizeof(str3)))
+		goto out;
+
+out:
+	test_probe_read_user_str__destroy(skel);
+}

tools/testing/selftests/bpf/progs/test_probe_read_user_str.c

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+#include <sys/types.h>
+
+pid_t pid = 0;
+long ret = 0;
+void *user_ptr = 0;
+char buf[256] = {};
+
+SEC("tracepoint/syscalls/sys_enter_nanosleep")
+int on_write(void *ctx)
+{
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	ret = bpf_probe_read_user_str(buf, sizeof(buf), user_ptr);
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";