Re: [Qemu-trivial] [Qemu-devel] [PATCH v2] linux-user: fix is_proc_myself to check the paths via realpath

[Laurent Vivier]

Le 25/10/2017 à 05:34, Zach Riggle a écrit :
> Previously, it was possible to get a handle to the "real" /proc/self/mem
> by creating a symlink to it and opening the symlink, or opening e.g.
> "./mem" after chdir'ing to "/proc/self".
> 
>     $ ln -s /proc/self self
>     $ cat self/maps
>     60000000-602bc000 r-xp 00000000 fc:01 270375                             
> /usr/bin/qemu-arm-static
>     604bc000-6050f000 rw-p 002bc000 fc:01 270375                             
> /usr/bin/qemu-arm-static
>     ...
> 
> Signed-off-by: Zach Riggle <address@hidden>
> ---
>  linux-user/syscall.c | 47 ++++++++++++++++++++++++++++-------------------
>  1 file changed, 28 insertions(+), 19 deletions(-)
> 
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 9bf901fa11..6c1f28a1f7 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -7496,26 +7496,35 @@ static int open_self_auxv(void *cpu_env, int fd)
> 
>  static int is_proc_myself(const char *filename, const char *entry)
>  {
> -    if (!strncmp(filename, "/proc/", strlen("/proc/"))) {
> -        filename += strlen("/proc/");
> -        if (!strncmp(filename, "self/", strlen("self/"))) {
> -            filename += strlen("self/");
> -        } else if (*filename >= '1' && *filename <= '9') {
> -            char myself[80];
> -            snprintf(myself, sizeof(myself), "%d/", getpid());
> -            if (!strncmp(filename, myself, strlen(myself))) {
> -                filename += strlen(myself);
> -            } else {
> -                return 0;
> -            }
> -        } else {
> -            return 0;
> -        }
> -        if (!strcmp(filename, entry)) {
> -            return 1;
> -        }
> +    char proc_self_entry[PATH_MAX + 1];
> +    char proc_self_entry_realpath[PATH_MAX + 1];
> +    char filename_realpath[PATH_MAX + 1];
> +
> +    if (PATH_MAX < snprintf(proc_self_entry,
> +                            sizeof(proc_self_entry),
> +                            "/proc/self/%s",
> +                            entry)) {
> +        /* Full path to "entry" is too long to fit in the buffer */
> +        return 0;
>      }
> -    return 0;
> +
> +    if (!realpath(filename, filename_realpath)) {
> +        /* File does not exist, or can't be canonicalized */
> +        return 0;
> +    }
> +
> +    if (!realpath(proc_self_entry, proc_self_entry_realpath)) {
> +        /* Procfs entry does not exist */
> +        return 0;
> +    }
> +
> +    if (strcmp(filename_realpath, proc_self_entry_realpath) != 0) {
> +        /* Paths are different */

I think it doesn't work with /proc/map/exe (or other soft link in
/proc/self) as realpath will give the path of the executable and not the
path inside /proc so it will be true for any process with the same
executable (which in our case is qemu for all).

Thanks,
Laurent