Jan Kara <jack@xxxxxxx> writes: > On Thu 19-11-15 16:10:43, Kees Cook wrote: >> Normally, when a user can modify a file that has setuid or setgid bits, >> those bits are cleared when they are not the file owner or a member of the >> group. This is enforced when using write() directly but not when writing >> to a shared mmap on the file. This could allow the file writer to gain >> privileges by changing the binary without losing the setuid/setgid bits. >> >> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> >> Cc: stable@xxxxxxxxxxxxxxx > > So I had another look at this and now I understand why we didn't do it from > the start: > > To call file_remove_privs() safely, we need to hold inode->i_mutex since > that operations is going to modify file mode / extended attributes and > i_mutex protects those. However we cannot get i_mutex in the page fault > path as that ranks above mmap_sem which we hold during the whole page > fault. > > So calling file_remove_privs() when opening the file is probably as good as > it can get. It doesn't catch the case when suid bits / IMA attrs are set > while the file is already open but I don't see easy way around this. Could we perhaps do this on mmap MAP_WRITE instead of open, and simply deny adding these attributes if a file is mapped for write? That would seem to be a little more compatible with what we already do, and guards against the races you mention as well. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
