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

On Fri, Oct 18, 2024 at 04:47:10PM +0200, Roberto Sassu 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 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.

Great description!

Cc: stable@vger.kernel.org Fixes: ea7e2d5e49c0 ("mm: call the security_mmap_file() LSM hook in remap_file_pages()") Reported-by: syzbot+91ae49e1c1a2634d20c0@syzkaller.appspotmail.com Closes: https://lore.kernel.org/linux-security-module/66f7b10e.050a0220.46d20.0036.G... Reviewed-by: Roberto Sassu roberto.sassu@huawei.com (Calculate prot and flags earlier) Signed-off-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com

Other than some nits below:

Reviewed-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com

I think you're definitely good to un-RFC here.

---

mm/mmap.c | 62 ++++++++++++++++++++++++++++++++++++++++--------------- 1 file changed, 45 insertions(+), 17 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c index 9c0fb43064b5..762944427e03 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -1640,6 +1640,7 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size, unsigned long populate = 0; unsigned long ret = -EINVAL; struct file *file;

• vm_flags_t vm_flags;

  pr_warn_once("%s (%d) uses deprecated remap_file_pages() syscall. See Documentation/mm/remap_file_pages.rst.\n", current->comm, current->pid);

@@ -1656,12 +1657,53 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size, if (pgoff + (size >> PAGE_SHIFT) < pgoff) return ret;

• if (mmap_write_lock_killable(mm))

• if (mmap_read_lock_killable(mm))

• return -EINTR;
  

I'm kinda verbose generally, but I'd love a comment like:

/* * Look up VMA under read lock first so we can perform the security * without holding locks (which can be problematic). We reacquire a * write lock later and check nothing changed underneath us. */

• vma = vma_lookup(mm, start);
• if (!vma || !(vma->vm_flags & VM_SHARED)) {

• mmap_read_unlock(mm);
  

• return -EINVAL;
  

• }
• prot |= vma->vm_flags & VM_READ ? PROT_READ : 0;
• prot |= vma->vm_flags & VM_WRITE ? PROT_WRITE : 0;
• prot |= vma->vm_flags & VM_EXEC ? PROT_EXEC : 0;
• flags &= MAP_NONBLOCK;
• flags |= MAP_SHARED | MAP_FIXED | MAP_POPULATE;
• if (vma->vm_flags & VM_LOCKED)

• flags |= MAP_LOCKED;
  

• /* Save vm_flags used to calculate prot and flags, and recheck later. */
• vm_flags = vma->vm_flags;
• file = get_file(vma->vm_file);
• mmap_read_unlock(mm);

Maybe worth adding a comment to explain why you're doing this without the lock so somebody looking at this later can understand the dance?

• ret = security_mmap_file(file, prot, flags);
• if (ret) {

• fput(file);
  

• return ret;
  

• }
• ret = -EINVAL;

Again, being verbose, I'd put something here like:

/* OK security check passed, take write lock + let it rip */

• if (mmap_write_lock_killable(mm)) {

• fput(file);
  

  return -EINTR;

• }

  vma = vma_lookup(mm, start);

• if (!vma || !(vma->vm_flags & VM_SHARED))

• if (!vma)

• goto out;
  

I'd also add something like:

/* Make sure things didn't change under us. */

• if (vma->vm_flags != vm_flags)

• goto out;
  

And drop this newline to group them together (super nitty I know, sorry!)

• if (vma->vm_file != file) goto out;

  if (start + size > vma->vm_end) {

@@ -1689,25 +1731,11 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size, goto out; }

• prot |= vma->vm_flags & VM_READ ? PROT_READ : 0;
• prot |= vma->vm_flags & VM_WRITE ? PROT_WRITE : 0;
• prot |= vma->vm_flags & VM_EXEC ? PROT_EXEC : 0;
• flags &= MAP_NONBLOCK;
• flags |= MAP_SHARED | MAP_FIXED | MAP_POPULATE;
• if (vma->vm_flags & VM_LOCKED)

• flags |= MAP_LOCKED;
  

• file = get_file(vma->vm_file);
• ret = security_mmap_file(vma->vm_file, prot, flags);
• if (ret)

• goto out_fput;
  

  ret = do_mmap(vma->vm_file, start, size, prot, flags, 0, pgoff, &populate, NULL);

-out_fput:

• fput(file);

out: mmap_write_unlock(mm);

• fput(file); if (populate) mm_populate(ret, populate); if (!IS_ERR_VALUE(ret))

-- 2.34.1

These are just nits, this looks good to me!