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. BTW: This is another example where page fault locking is constraining us and life would be simpler for filesystems we they get called without mmap_sem held... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>