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

[Roberto Sassu]

From: "Kirill A. Shutemov" <kirill.shutemov at 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 at vger.kernel.org # v6.12-rcx
Fixes: ea7e2d5e49c0 ("mm: call the security_mmap_file() LSM hook in remap_file_pages()")
Reported-by: syzbot+1cd571a672400ef3a930 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>
Reviewed-by: Jann Horn <jannh at google.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes at oracle.com>
Tested-by: Roberto Sassu <roberto.sassu at huawei.com>
Tested-by: syzbot+1cd571a672400ef3a930 at syzkaller.appspotmail.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
---
 mm/mmap.c | 69 +++++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 52 insertions(+), 17 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 9c0fb43064b5..f731dd69e162 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 +1657,60 @@ 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;
 
+	/*
+	 * 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))
+	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);
+
+	/* Call outside mmap_lock to be consistent with other callers. */
+	ret = security_mmap_file(file, prot, flags);
+	if (ret) {
+		fput(file);
+		return ret;
+	}
+
+	ret = -EINVAL;
+
+	/* 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)
+		goto out;
+
+	/* Make sure things didn't change under us. */
+	if (vma->vm_flags != vm_flags)
+		goto out;
+	if (vma->vm_file != file)
 		goto out;
 
 	if (start + size > vma->vm_end) {
@@ -1689,25 +1738,11 @@ 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