Re: [PATCH] fs: clear file set[ug]id when writing via mmap

From: Jan Kara
Date: Mon Nov 23 2015 - 07:26:33 EST

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

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...

Jan Kara <jack@xxxxxxxx>
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