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 <del...@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 <s...@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 <del...@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);
> > }
> > }
> >
> >
>
>
smime.p7s
Description: S/MIME Cryptographic Signature
