On 03/06/16 06:28, Tobias Stoeckmann wrote:
> It is possible (but unlikely) to encounter very long symbolic
> links, which would take a few mega bytes in size. In general,
> such long symbolic links are impossible to create because the
> kernel will only accept up to PATH_MAX in length. BUT lstat()
> on a maliciously modified symbolic link can return a much
> larger size than that.
>
> In function check_file_link (src/pacman/check.c), the length
> of a symbolic link is used in a variable length array:
>
> char link[length]; // length being st->st_size + 1
>
> This doesn't just allow a very theoretical overflow on 32 bit
> systems with 64 bit off_t (which is st_size's type), it can also
> overflow available stack space, leading to a segmentation fault.
>
> Symbolic links pointing to something that is longer than PATH_MAX
> can be considered illegal, so just get a fixed size array and
> reject symbolic links which are longer than that.
Interesting possibility!
I get this warning when building:
check.c: In function ‘check_file_link’:
check.c:133:16: error: unused parameter ‘st’ [-Werror=unused-parameter]
struct stat *st, struct archive_entry *entry)
^~
cc1: all warnings being treated as errors
Should we still be checking "len != st->st_size"?
> ---
> src/pacman/check.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/src/pacman/check.c b/src/pacman/check.c
> index d282cc2..6e8cda7 100644
> --- a/src/pacman/check.c
> +++ b/src/pacman/check.c
> @@ -132,15 +132,16 @@ static int check_file_time(const char *pkgname, const
> char *filepath,
> static int check_file_link(const char *pkgname, const char *filepath,
> struct stat *st, struct archive_entry *entry)
> {
> - size_t length = st->st_size + 1;
> - char link[length];
> + ssize_t len;
> + char link[PATH_MAX];
>
> - if(readlink(filepath, link, length) != st->st_size) {
> + len = readlink(filepath, link, sizeof(link));
> + if (len == -1 || len >= PATH_MAX) {
> /* this should not happen */
> pm_printf(ALPM_LOG_ERROR, _("unable to read symlink contents:
> %s\n"), filepath);
> return 1;
> }
> - link[length - 1] = '\0';
> + link[len] = '\0';
>
> if(strcmp(link, archive_entry_symlink(entry)) != 0) {
> if(!config->quiet) {
>
