Re: [PATCH v3 3/4] perf: Use CAP_SYSLOG with kptr_restrict checks

[Mathieu Poirier]

On Wed, 7 Aug 2019 at 08:44, Igor Lubashev <ilubashe@xxxxxxxxxx> wrote:
>
> Kernel is using CAP_SYSLOG capability instead of uid==0 and euid==0 when
> checking kptr_restrict. Make perf do the same.
>
> Also, the kernel is a more restrictive than "no restrictions" in case of
> kptr_restrict==0, so add the same logic to perf.
>
> Signed-off-by: Igor Lubashev <ilubashe@xxxxxxxxxx>
> ---
>  tools/perf/util/symbol.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> index 173f3378aaa0..046271103499 100644
> --- a/tools/perf/util/symbol.c
> +++ b/tools/perf/util/symbol.c
> @@ -4,6 +4,7 @@
>  #include <stdlib.h>
>  #include <stdio.h>
>  #include <string.h>
> +#include <linux/capability.h>
>  #include <linux/kernel.h>
>  #include <linux/mman.h>
>  #include <linux/time64.h>
> @@ -15,8 +16,10 @@
>  #include <inttypes.h>
>  #include "annotate.h"
>  #include "build-id.h"
> +#include "cap.h"
>  #include "util.h"
>  #include "debug.h"
> +#include "event.h"
>  #include "machine.h"
>  #include "map.h"
>  #include "symbol.h"
> @@ -890,7 +893,11 @@ bool symbol__restricted_filename(const char *filename,
>  {
>         bool restricted = false;
>
> -       if (symbol_conf.kptr_restrict) {
> +       /* Per kernel/kallsyms.c:
> +        * we also restrict when perf_event_paranoid > 1 w/o CAP_SYSLOG
> +        */
> +       if (symbol_conf.kptr_restrict ||
> +           (perf_event_paranoid() > 1 && !perf_cap__capable(CAP_SYSLOG))) {
>                 char *r = realpath(filename, NULL);
>

# echo 0 > /proc/sys/kernel/kptr_restrict
# ./tools/perf/perf record -e instructions:k uname
perf: Segmentation fault
Obtained 10 stack frames.
./tools/perf/perf(sighandler_dump_stack+0x44) [0x55af9e5da5d4]
/lib/x86_64-linux-gnu/libc.so.6(+0x3ef20) [0x7fd31efb6f20]
./tools/perf/perf(perf_event__synthesize_kernel_mmap+0xa7) [0x55af9e590337]
./tools/perf/perf(+0x1cf5be) [0x55af9e50c5be]
./tools/perf/perf(cmd_record+0x1022) [0x55af9e50dff2]
./tools/perf/perf(+0x23f98d) [0x55af9e57c98d]
./tools/perf/perf(+0x23fc9e) [0x55af9e57cc9e]
./tools/perf/perf(main+0x369) [0x55af9e4f6bc9]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7) [0x7fd31ef99b97]
./tools/perf/perf(_start+0x2a) [0x55af9e4f704a]
Segmentation fault

I can reproduce this on both x86 and ARM64.

>                 if (r != NULL) {
> @@ -2190,9 +2197,9 @@ static bool symbol__read_kptr_restrict(void)
>                 char line[8];
>
>                 if (fgets(line, sizeof(line), fp) != NULL)
> -                       value = ((geteuid() != 0) || (getuid() != 0)) ?
> -                                       (atoi(line) != 0) :
> -                                       (atoi(line) == 2);
> +                       value = perf_cap__capable(CAP_SYSLOG) ?
> +                                       (atoi(line) >= 2) :
> +                                       (atoi(line) != 0);
>
>                 fclose(fp);
>         }
> --
> 2.7.4
>