[PATCH bpf 3/3] samples/bpf: syscall_tp_user: Fix array out-of-bound access

Jinghao Jia
2023-08-18T16:47:07Z

Commit 06744f24696e ("samples/bpf: Add openat2() enter/exit tracepoint
to syscall_tp sample") added two more eBPF programs to support the
openat2() syscall. However, it did not increase the size of the array
that holds the corresponding bpf_links. This leads to an out-of-bound
access on that array in the bpf_object__for_each_program loop and could
corrupt other variables on the stack. On our testing QEMU, it corrupts
the map1_fds array and causes the sample to fail:

# ./syscall_tp
prog #0: map ids 4 5
verify map:4 val: 5
map_lookup failed: Bad file descriptor

Dynamically allocate the array based on the number of programs reported
by libbpf to prevent similar inconsistencies in the future

Fixes: 06744f24696e ("samples/bpf: Add openat2() enter/exit tracepoint to syscall_tp sample")
Signed-off-by: Jinghao Jia <jinghao@linux.ibm.com>
---
samples/bpf/syscall_tp_user.c | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/samples/bpf/syscall_tp_user.c b/samples/bpf/syscall_tp_user.c
index 18c94c7e8a40..8855d2c1290d 100644
--- a/samples/bpf/syscall_tp_user.c
+++ b/samples/bpf/syscall_tp_user.c
@@ -48,7 +48,7 @@ static void verify_map(int map_id)
static int test(char *filename, int nr_tests)
{
int map0_fds[nr_tests], map1_fds[nr_tests], fd, i, j = 0;
- struct bpf_link *links[nr_tests * 4];
+ struct bpf_link **links = NULL;
struct bpf_object *objs[nr_tests];
struct bpf_program *prog;

@@ -60,6 +60,17 @@ static int test(char *filename, int nr_tests)
goto cleanup;
}

+ /* One-time initialization */
+ if (!links) {
+ int nr_progs = 0;
+
+ bpf_object__for_each_program(prog, objs[i])
+ nr_progs += 1;
+
+ links = calloc(nr_progs * nr_tests,
+ sizeof(struct bpf_link *));
+ }
+
/* load BPF program */
if (bpf_object__load(objs[i])) {
fprintf(stderr, "loading BPF object file failed\n");
@@ -107,8 +118,13 @@ static int test(char *filename, int nr_tests)
}

cleanup:
- for (j--; j >= 0; j--)
- bpf_link__destroy(links[j]);
+ if (links) {
+ for (j--; j >= 0; j--)
+ bpf_link__destroy(links[j]);
+
+ free(links);
+ links = NULL;
+ }

for (i--; i >= 0; i--)
bpf_object__close(objs[i]);
--
2.41.0