Re: [PATCH v3 2/2] linux-user: Fix openat() emulation to not modify atime

[Shu-Chun Weng]

Yup, reproduced on a system with glibc built with fortified source. Sending out version 4.

On Thu, Dec 28, 2023 at 7:49 AM Helge Deller <deller@gmx.de> wrote:

On 12/8/23 23:42, Shu-Chun Weng wrote:
> Commit b8002058 strengthened openat()'s /proc detection by calling
> realpath(3) on the given path, which allows various paths and symlinks
> that points to the /proc file system to be intercepted correctly.
>
> Using realpath(3), though, has a side effect that it reads the symlinks
> along the way, and thus changes their atime. The results in the
> following code snippet already get ~now instead of the real atime:
>
>    int fd = open("/path/to/a/symlink", O_PATH | O_NOFOLLOW);
>    struct stat st;
>    fstat(fd, st);
>    return st.st_atime;
>
> This change opens a path that doesn't appear to be part of /proc
> directly and checks the destination of /proc/self/fd/n to determine if
> it actually refers to a file in /proc.
>
> Neither this nor the existing code works with symlinks or indirect paths
> (e.g.  /tmp/../proc/self/exe) that points to /proc/self/exe because it
> is itself a symlink, and both realpath(3) and /proc/self/fd/n will
> resolve into the location of QEMU.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2004
> Signed-off-by: Shu-Chun Weng <scw@google.com>
> ---
>   linux-user/syscall.c | 47 +++++++++++++++++++++++++++++++++++---------
>   1 file changed, 38 insertions(+), 9 deletions(-)
>
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index e384e14248..7c3772301f 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -8308,8 +8308,7 @@ static int open_net_route(CPUArchState *cpu_env, int fd)
>   int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname,
>                       int flags, mode_t mode, bool safe)
>   {
> -    g_autofree char *proc_name = NULL;
> -    const char *pathname;
> +    g_autofree char *pathname = NULL;
>       struct fake_open {
>           const char *filename;
>           int (*fill)(CPUArchState *cpu_env, int fd);
> @@ -8334,12 +8333,42 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname,
>           { NULL, NULL, NULL }
>       };
>
> -    /* if this is a file from /proc/ filesystem, expand full name */
> -    proc_name = realpath(fname, NULL);
> -    if (proc_name && strncmp(proc_name, "/proc/", 6) == 0) {
> -        pathname = proc_name;
> +    if (strncmp(fname, "/proc/", 6) == 0) {
> +        pathname = g_strdup(fname);
>       } else {
> -        pathname = fname;
> +        g_autofree char *proc_name = NULL;
> +        struct stat proc_stat;
> +        int fd;
> +
> +        if (safe) {
> +            fd = safe_openat(dirfd, path(fname), flags, mode);
> +        } else {
> +            fd = openat(dirfd, path(fname), flags, mode);
> +        }
> +        if (fd < 0) {
> +            return fd;
> +        }
> +
> +        /*
> +         * Try to get the real path of the file we just opened. We avoid calling
> +         * `realpath(3)` because it calls `readlink(2)` on symlinks which
> +         * changes their atime. Note that since `/proc/self/exe` is a symlink,
> +         * `pathname` will never resolve to it (neither will `realpath(3)`).
> +         * That's why we check `fname` against the "/proc/" prefix first.
> +         */
> +        proc_name = g_strdup_printf("/proc/self/fd/%d", fd);
> +        if (lstat(proc_name, &proc_stat) < 0 || !S_ISLNK(proc_stat.st_mode)) {
> +            /* No procfs or something weird. Not going to dig further. */
> +            return fd;
> +        }
> +        pathname = g_new(char, proc_stat.st_size + 1);
> +        readlink(proc_name, pathname, proc_stat.st_size + 1);

this gives a build error for me:
./qemu/linux-user/syscall.c:8365:9: error: ignoring return value of ‘readlink’ declared with attribute ‘warn_unused_result’ [-Werror=unused-result]

Other than that, this patch indeed fixes the issue #2004

Reviewed-by: Helge Deller <deller@gmx.de>


Helge

> +
> +        /* if this is not a file from /proc/ filesystem, the fd is good as-is */
> +        if (strncmp(pathname, "/proc/", 6) != 0) {
> +            return fd;
> +        }
> +        close(fd);
>       }
>
>       if (is_proc_myself(pathname, "exe")) {
> @@ -8390,9 +8419,9 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname,
>       }
>
>       if (safe) {
> -        return safe_openat(dirfd, path(pathname), flags, mode);
> +        return safe_openat(dirfd, pathname, flags, mode);
>       } else {
> -        return openat(dirfd, path(pathname), flags, mode);
> +        return openat(dirfd, pathname, flags, mode);
>       }
>   }
>
>

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