Re: [PATCH v2] mm: Split critical region in remap_file_pages() and invoke LSMs in between

[Paul Moore <paul@paul-moore.com>]

On Fri, Oct 18, 2024 at 12:15 PM Roberto Sassu
<roberto.sassu@huaweicloud.com> wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>
> Commit ea7e2d5e49c0 ("mm: call the security_mmap_file() LSM hook in
> remap_file_pages()") fixed a security issue, it added an LSM check when
> trying to remap file pages, so that LSMs have the opportunity to evaluate
> such action like for other memory operations such as mmap() and mprotect().
>
> However, that commit called security_mmap_file() inside the mmap_lock lock,
> while the other calls do it before taking the lock, after commit
> 8b3ec6814c83 ("take security_mmap_file() outside of ->mmap_sem").
>
> This caused lock inversion issue with IMA which was taking the mmap_lock
> and i_mutex lock in the opposite way when the remap_file_pages() system
> call was called.
>
> Solve the issue by splitting the critical region in remap_file_pages() in
> two regions: the first takes a read lock of mmap_lock, retrieves the VMA
> and the file descriptor associated, and calculates the 'prot' and 'flags'
> variables; the second takes a write lock on mmap_lock, checks that the VMA
> flags and the VMA file descriptor are the same as the ones obtained in the
> first critical region (otherwise the system call fails), and calls
> do_mmap().
>
> In between, after releasing the read lock and before taking the write lock,
> call security_mmap_file(), and solve the lock inversion issue.
>
> Cc: stable@vger.kernel.org # v6.12-rcx
> Fixes: ea7e2d5e49c0 ("mm: call the security_mmap_file() LSM hook in remap_file_pages()")
> Reported-by: syzbot+1cd571a672400ef3a930@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/linux-security-module/66f7b10e.050a0220.46d20.0036.GAE@google.com/
> Reviewed-by: Roberto Sassu <roberto.sassu@huawei.com>
> Reviewed-by: Jann Horn <jannh@google.com>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Tested-by: Roberto Sassu <roberto.sassu@huawei.com>
> Tested-by: syzbot+1cd571a672400ef3a930@syzkaller.appspotmail.com
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  mm/mmap.c | 69 +++++++++++++++++++++++++++++++++++++++++--------------
>  1 file changed, 52 insertions(+), 17 deletions(-)

Thanks for working on this Roberto, Kirill, and everyone else who had
a hand in reviewing and testing.

Reviewed-by: Paul Moore <paul@paul-moore.com>

Andrew, I see you're pulling this into the MM/hotfixes-unstable
branch, do you also plan to send this up to Linus soon/next-week?  If
so, great, if not let me know and I can send it up via the LSM tree.

We need to get clarity around Roberto's sign-off, but I think that is
more of an administrative mistake rather than an intentional omission
:)

-- 
paul-moore.com