[RFC,v2] mm: move security_mmap_file() back into do_mmap()

Commit Message

Shu Han Sept. 25, 2024, 11:57 a.m. UTC

This patch moves the security_mmap_file() back into do_mmap(), which
revert(It is conceptually a revert of the commit(revert the feature
change in the commit), but not in the git sense(revert lines))
the commit 8b3ec6814c83d76b85bd13badc48552836c24839
("take security_mmap_file() outside of ->mmap_sem"). Below is the reason.

Some logic may call do_mmap() without calling security_mmap_file(),
without being aware of the harm this poses to LSM.
For example, CVE-2016-10044[1] was reported many years ago, but the
remap_file_pages() can still bypass the W^X policy enforced by SELinux[2]
for a long time.

Adding checks is easy(and has been done in all required call sites for
the current version), but there may have more calls to do_mmap() in the
future. Moving security_mmap_file() back into do_mmap() can avoid
forgetting, and avoid repeated logic for whether READ_IMPLIES_EXEC should
add PROT_EXEC for the mapping or not(In current, the !MMU case won't
imply exec if the file's mmap_capabilities is not exist, but the
security check logic is different).

It is noteworthy that moving the security check in do_mmap() will let it
in the mmap_write_lock, which slows down the performance and even have
deadlocks if someone depends on it(Since security_file_mprotect() is
already in the lock, this possibility is tiny), which requires LSM modules
to check.

Link: https://project-zero.issues.chromium.org/issues/42452389 [1]
Link: https://lore.kernel.org/all/[e-mail]/ [2]
Signed-off-by: Shu Han <[e-mail]>
---
V2: Add RFC tag as [e-mail] suggested, and refine the
comment in patch.
V1: https://lore.kernel.org/all/[e-mail]/
Alternatives:
1. mm: move the check of READ_IMPLIES_EXEC out of do_mmap()
2. Add sufficient comments for do_mmap()
---
 include/linux/security.h |  8 ++++----
 ipc/shm.c                |  4 ----
 mm/mmap.c                |  9 +++++----
 mm/nommu.c               |  5 ++++-
 mm/util.c                | 19 ++++++++-----------
 security/security.c      | 41 ++++------------------------------------
 6 files changed, 25 insertions(+), 61 deletions(-)


base-commit: f89722faa31466ff41aed21bdeb9cf34c2312858

Patch

diff --git a/include/linux/security.h b/include/linux/security.h
index c37c32ebbdcd..e061bc9a0331 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -423,8 +423,8 @@  void security_file_free(struct file *file);
 int security_file_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
 int security_file_ioctl_compat(struct file *file, unsigned int cmd,
 			       unsigned long arg);
-int security_mmap_file(struct file *file, unsigned long prot,
-			unsigned long flags);
+int security_mmap_file(struct file *file, unsigned long reqprot,
+		       unsigned long prot, unsigned long flags);
 int security_mmap_addr(unsigned long addr);
 int security_file_mprotect(struct vm_area_struct *vma, unsigned long reqprot,
 			   unsigned long prot);
@@ -1077,8 +1077,8 @@  static inline int security_file_ioctl_compat(struct file *file,
 	return 0;
 }
 
-static inline int security_mmap_file(struct file *file, unsigned long prot,
-				     unsigned long flags)
+static inline int security_mmap_file(struct file *file, unsigned long reqprot,
+				     unsigned long prot, unsigned long flags)
 {
 	return 0;
 }
diff --git a/ipc/shm.c b/ipc/shm.c
index 3e3071252dac..ce02560b856f 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1636,10 +1636,6 @@  long do_shmat(int shmid, char __user *shmaddr, int shmflg,
 	sfd->vm_ops = NULL;
 	file->private_data = sfd;
 
-	err = security_mmap_file(file, prot, flags);
-	if (err)
-		goto out_fput;
-
 	if (mmap_write_lock_killable(current->mm)) {
 		err = -EINTR;
 		goto out_fput;
diff --git a/mm/mmap.c b/mm/mmap.c
index 18fddcce03b8..56f9520f85ab 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1260,6 +1260,7 @@  unsigned long do_mmap(struct file *file, unsigned long addr,
 {
 	struct mm_struct *mm = current->mm;
 	int pkey = 0;
+	unsigned long reqprot = prot, err;
 
 	*populate = 0;
 
@@ -1276,6 +1277,10 @@  unsigned long do_mmap(struct file *file, unsigned long addr,
 		if (!(file && path_noexec(&file->f_path)))
 			prot |= PROT_EXEC;
 
+	err = security_mmap_file(file, reqprot, prot, flags);
+	if (err)
+		return err;
+
 	/* force arch specific MAP_FIXED handling in get_unmapped_area */
 	if (flags & MAP_FIXED_NOREPLACE)
 		flags |= MAP_FIXED;
@@ -3198,12 +3203,8 @@  SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
 		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);
diff --git a/mm/nommu.c b/mm/nommu.c
index 7296e775e04e..e632f3105a5a 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -681,7 +681,7 @@  static int validate_mmap_request(struct file *file,
 				 unsigned long pgoff,
 				 unsigned long *_capabilities)
 {
-	unsigned long capabilities, rlen;
+	unsigned long capabilities, rlen, reqprot = prot;
 	int ret;
 
 	/* do the simple checks first */
@@ -818,6 +818,9 @@  static int validate_mmap_request(struct file *file,
 	}
 
 	/* allow the security API to have its say */
+	ret = security_mmap_file(file, reqprot, prot, flags);
+	if (ret < 0)
+		return ret;
 	ret = security_mmap_addr(addr);
 	if (ret < 0)
 		return ret;
diff --git a/mm/util.c b/mm/util.c
index bd283e2132e0..47345e927a8f 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -581,17 +581,14 @@  unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,
 	unsigned long populate;
 	LIST_HEAD(uf);
 
-	ret = security_mmap_file(file, prot, flag);
-	if (!ret) {
-		if (mmap_write_lock_killable(mm))
-			return -EINTR;
-		ret = do_mmap(file, addr, len, prot, flag, 0, pgoff, &populate,
-			      &uf);
-		mmap_write_unlock(mm);
-		userfaultfd_unmap_complete(mm, &uf);
-		if (populate)
-			mm_populate(ret, populate);
-	}
+	if (mmap_write_lock_killable(mm))
+		return -EINTR;
+	ret = do_mmap(file, addr, len, prot, flag, 0, pgoff, &populate,
+		      &uf);
+	mmap_write_unlock(mm);
+	userfaultfd_unmap_complete(mm, &uf);
+	if (populate)
+		mm_populate(ret, populate);
 	return ret;
 }
 
diff --git a/security/security.c b/security/security.c
index 4564a0a1e4ef..25556629f588 100644
--- a/security/security.c
+++ b/security/security.c
@@ -2927,42 +2927,10 @@  int security_file_ioctl_compat(struct file *file, unsigned int cmd,
 }
 EXPORT_SYMBOL_GPL(security_file_ioctl_compat);
 
-static inline unsigned long mmap_prot(struct file *file, unsigned long prot)
-{
-	/*
-	 * Does we have PROT_READ and does the application expect
-	 * it to imply PROT_EXEC?  If not, nothing to talk about...
-	 */
-	if ((prot & (PROT_READ | PROT_EXEC)) != PROT_READ)
-		return prot;
-	if (!(current->personality & READ_IMPLIES_EXEC))
-		return prot;
-	/*
-	 * if that's an anonymous mapping, let it.
-	 */
-	if (!file)
-		return prot | PROT_EXEC;
-	/*
-	 * ditto if it's not on noexec mount, except that on !MMU we need
-	 * NOMMU_MAP_EXEC (== VM_MAYEXEC) in this case
-	 */
-	if (!path_noexec(&file->f_path)) {
-#ifndef CONFIG_MMU
-		if (file->f_op->mmap_capabilities) {
-			unsigned caps = file->f_op->mmap_capabilities(file);
-			if (!(caps & NOMMU_MAP_EXEC))
-				return prot;
-		}
-#endif
-		return prot | PROT_EXEC;
-	}
-	/* anything on noexec mount won't get PROT_EXEC */
-	return prot;
-}
-
 /**
  * security_mmap_file() - Check if mmap'ing a file is allowed
  * @file: file
+ * @reqprot: protection requested by user
  * @prot: protection applied by the kernel
  * @flags: flags
  *
@@ -2971,11 +2939,10 @@  static inline unsigned long mmap_prot(struct file *file, unsigned long prot)
  *
  * Return: Returns 0 if permission is granted.
  */
-int security_mmap_file(struct file *file, unsigned long prot,
-		       unsigned long flags)
+int security_mmap_file(struct file *file, unsigned long reqprot,
+		       unsigned long prot, unsigned long flags)
 {
-	return call_int_hook(mmap_file, file, prot, mmap_prot(file, prot),
-			     flags);
+	return call_int_hook(mmap_file, file, reqprot, prot, flags);
 }
 
 /**