Re: [PATCH] perf record: Fix segmentation fault in record__read_lost_samples()

[Namhyung Kim]

Hi Leo,

On Sat, Sep 24, 2022 at 4:34 AM Leo Yan <leo.yan@xxxxxxxxxx> wrote:
>
> Commit a49aa8a54e86 ("perf record: Read and inject LOST_SAMPLES events")
> causes segmentation fault when run the "perf mem record" command in
> unprivileged mode, the output log is:
>
>   $ ./perf mem record --all-user -o perf_test.data -- ./test_program
>   Error:
>   Access to performance monitoring and observability operations is limited.
>   Consider adjusting /proc/sys/kernel/perf_event_paranoid setting to open
>   access to performance monitoring and observability operations for processes
>   without CAP_PERFMON, CAP_SYS_PTRACE or CAP_SYS_ADMIN Linux capability.
>   More information can be found at 'Perf events and tool security' document:
>   https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html
>   perf_event_paranoid setting is 4:
>     -1: Allow use of (almost) all events by all users
>         Ignore mlock limit after perf_event_mlock_kb without CAP_IPC_LOCK
>   >= 0: Disallow raw and ftrace function tracepoint access
>   >= 1: Disallow CPU event access
>   >= 2: Disallow kernel profiling
>   To make the adjusted perf_event_paranoid setting permanent preserve it
>   in /etc/sysctl.conf (e.g. kernel.perf_event_paranoid = <setting>)
>   perf: Segmentation fault
>   Obtained 16 stack frames.
>   ./perf(dump_stack+0x31) [0x55b7aa1e8070]
>   ./perf(sighandler_dump_stack+0x36) [0x55b7aa1e815e]
>   ./perf(+0xc9120) [0x55b7aa0a9120]
>   /lib/x86_64-linux-gnu/libc.so.6(+0x4251f) [0x7fd03ef8151f]
>   ./perf(+0xccaca) [0x55b7aa0acaca]
>   ./perf(+0xcf4ab) [0x55b7aa0af4ab]
>   ./perf(cmd_record+0xd50) [0x55b7aa0b28df]
>   ./perf(+0x112f77) [0x55b7aa0f2f77]
>   ./perf(cmd_mem+0x53b) [0x55b7aa0f406c]
>   ./perf(+0x19979c) [0x55b7aa17979c]
>   ./perf(+0x199a37) [0x55b7aa179a37]
>   ./perf(+0x199b95) [0x55b7aa179b95]
>   ./perf(main+0x2c7) [0x55b7aa179fbd]
>   /lib/x86_64-linux-gnu/libc.so.6(+0x29d8f) [0x7fd03ef68d8f]
>   /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x7f) [0x7fd03ef68e3f]
>   ./perf(_start+0x24) [0x55b7aa089974]
>   Segmentation fault (core dumped)
>
> In the unprivileged mode perf fails to open PMU event, the function
> record__open() returns error and "session->evlist" is NULL; this leads
> to segmentation fault when iterates "session->evlist" in the function
> record__read_lost_samples().
>
> This patch checks "session->evlist" in record__read_lost_samples(), if
> "session->evlist" is NULL then the function directly bails out to avoid
> segmentation fault.
>
> Fixes: a49aa8a54e86 ("perf record: Read and inject LOST_SAMPLES events")
> Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>

Thanks for the fix and sorry for the inconvenience.
Actually I sent the same fix a few weeks ago.

https://lore.kernel.org/r/20220909235024.278281-1-namhyung@xxxxxxxxxx

Thanks,
Namhyung


> ---
>  tools/perf/builtin-record.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 02e38f50a138..012b46dd4999 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -1888,6 +1888,10 @@ static void record__read_lost_samples(struct record *rec)
>         struct perf_record_lost_samples *lost;
>         struct evsel *evsel;
>
> +       /* No any event is opened, directly bail out */
> +       if (!session->evlist)
> +               return;
> +
>         lost = zalloc(PERF_SAMPLE_MAX_SIZE);
>         if (lost == NULL) {
>                 pr_debug("Memory allocation failed\n");
> --
> 2.34.1
>