[RFC][PATCH] mm: Split locks in remap_file_pages()

[Jann Horn]

On Fri, Oct 18, 2024 at 4:48 PM Roberto Sassu
<roberto.sassu at huaweicloud.com> wrote:
> 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 and retrieves the VMA
> and the file associated, and calculate the 'prot' and 'flags' variable; 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 taking the write lock, call
> security_mmap_file(), and solve the lock inversion issue.
>
> Cc: stable at vger.kernel.org
> Fixes: ea7e2d5e49c0 ("mm: call the security_mmap_file() LSM hook in remap_file_pages()")
> Reported-by: syzbot+91ae49e1c1a2634d20c0 at syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/linux-security-module/66f7b10e.050a0220.46d20.0036.GAE@google.com/
> Reviewed-by: Roberto Sassu <roberto.sassu at huawei.com> (Calculate prot and flags earlier)
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>

Reviewed-by: Jann Horn <jannh at google.com>