Messages in this thread | | | Date | Mon, 23 Nov 2015 13:26:24 +0100 | From | Jan Kara <> | Subject | Re: [PATCH] fs: clear file set[ug]id when writing via mmap |
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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@chromium.org> > Cc: stable@vger.kernel.org
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@suse.com> SUSE Labs, CR
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