[PATCH v5 2/6] selftests/x86: fix fork bug

[Bill Roberts <bill.roberts@arm.com>]

The shadow stack ptrace test does not check whether fork() fails
before proceeding with the parent-side ptrace operations.

If fork() fails, it returns -1, which is subsequently passed to
kill() on the test's error path. Calling kill(-1, SIGKILL) sends
SIGKILL to every process the caller has permission to signal,
potentially terminating unrelated processes.

Check for fork() failure immediately and return an error before
entering the ptrace test's parent-side logic.

Shashiko pointed out this fork bug, and the relevant snippets from the
report are below:

[ ... ]
> +int test_ptrace(void)
> +{
> +     unsigned long saved_ssp, ssp = 0;
> +     struct sigaction sa = {};
> +     struct iovec iov;
> +     int status;
> +     int pid;
> +
> +     iov.iov_base = &ssp;
> +     iov.iov_len = sizeof(ssp);
> +
> +     pid = fork();
> +     if (!pid) {

[Severity: High]
This is a pre-existing issue, but could this code broadcast SIGKILL to
all user processes if fork() fails?

If fork() returns -1 due to resource limits, execution will proceed with
pid = -1. When ptrace() subsequently fails, the error path is triggered:

out_kill:
        kill(pid, SIGKILL);
        return 1;

Since pid is -1, POSIX specifies that this will kill all processes the
current user has permission to terminate, potentially crashing the host
environment.

Signed-off-by: Bill Roberts <bill.roberts@arm.com>
---
 tools/testing/selftests/x86/test_shadow_stack.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/testing/selftests/x86/test_shadow_stack.c b/tools/testing/selftests/x86/test_shadow_stack.c
index 3d6ca33edba4..b52c5420c137 100644
--- a/tools/testing/selftests/x86/test_shadow_stack.c
+++ b/tools/testing/selftests/x86/test_shadow_stack.c
@@ -971,6 +971,11 @@ int test_ptrace(void)
 	iov.iov_len = sizeof(ssp);
 
 	pid = fork();
+	if (pid < 0) {
+		printf("[FAIL]\tFork failed for %s\n", __func__);
+		return 1;
+	}
+
 	if (!pid) {
 		ssp = get_ssp();
 
-- 
2.55.0