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

[Daniel P . Berrangé]

On Thu, Nov 30, 2023 at 07:21:40PM -0800, 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.

I wonder if we can detect that by opening with O_NOFOLLOW, then
calling fstatfs() on the FD, and checking f_type == PROCFS_SUPER_MAGIC


> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index e384e14248..25e2cda10a 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -8308,8 +8308,6 @@ 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;
>      struct fake_open {
>          const char *filename;
>          int (*fill)(CPUArchState *cpu_env, int fd);
> @@ -8333,13 +8331,39 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, 
> const char *fname,
>  #endif
>          { NULL, NULL, NULL }
>      };
> +    char pathname[PATH_MAX];
>  
> -    /* 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) {
> +        pstrcpy(pathname, sizeof(pathname), fname);
>      } else {
> -        pathname = fname;
> +        char procpath[PATH_MAX];
> +        int fd, n;
> +
> +        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 resolves to it (neither will `realpath(3)`).
> +         * That's why we check `fname` against the "/proc/" prefix first.
> +         */
> +        snprintf(procpath, sizeof(procpath), "/proc/self/fd/%d", fd);

g_strdup_printf() + g_autofree to avoid this PATH_MAX buffer

> +        n = readlink(procpath, pathname, sizeof(pathname));
> +        pathname[n < sizeof(pathname) ? n : sizeof(pathname)] = '\0';

If you call lstat() then sb_size will tell you how big the buffer
needs to be for a subsequent readlink(), whcih can be allocated
on the heap and released with g_autofree, avoiding the othuer PATH_MAX
buffer

> +
> +        /* 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 +8414,9 @@ 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);
>      }
>  }
>  
> 

With regards,
Daniel
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