linux-kernel - [PATCH v4 2/3] mm: update memfd seal write check to include F_SEAL

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Message-ID: <913628168ce6cce77df7d13a63970bae06a526e0.1697116581.git.lstoakes@gmail.com>
Date:   Thu, 12 Oct 2023 18:04:29 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Andy Lutomirski <luto@...nel.org>, Jan Kara <jack@...e.cz>,
        linux-fsdevel@...r.kernel.org, bpf@...r.kernel.org,
        Lorenzo Stoakes <lstoakes@...il.com>
Subject: [PATCH v4 2/3] mm: update memfd seal write check to include F_SEAL_WRITE

The seal_check_future_write() function is called by shmem_mmap() or
hugetlbfs_file_mmap() to disallow any future writable mappings of an memfd
sealed this way.

The F_SEAL_WRITE flag is not checked here, as that is handled via the
mapping->i_mmap_writable mechanism and so any attempt at a mapping would
fail before this could be run.

However we intend to change this, meaning this check can be performed for
F_SEAL_WRITE mappings also.

The logic here is equally applicable to both flags, so update this function
to accommodate both and rename it accordingly.

Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
---
 fs/hugetlbfs/inode.c |  2 +-
 include/linux/mm.h   | 15 ++++++++-------
 mm/shmem.c           |  2 +-
 3 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 06693bb1153d..5c333373dcc9 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -112,7 +112,7 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
 	vm_flags_set(vma, VM_HUGETLB | VM_DONTEXPAND);
 	vma->vm_ops = &hugetlb_vm_ops;
 
-	ret = seal_check_future_write(info->seals, vma);
+	ret = seal_check_write(info->seals, vma);
 	if (ret)
 		return ret;
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index bae234d18d81..26d7dc3b342b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4078,25 +4078,26 @@ static inline void mem_dump_obj(void *object) {}
 #endif
 
 /**
- * seal_check_future_write - Check for F_SEAL_FUTURE_WRITE flag and handle it
+ * seal_check_write - Check for F_SEAL_WRITE or F_SEAL_FUTURE_WRITE flags and
+ *                    handle them.
  * @seals: the seals to check
  * @vma: the vma to operate on
  *
- * Check whether F_SEAL_FUTURE_WRITE is set; if so, do proper check/handling on
- * the vma flags.  Return 0 if check pass, or <0 for errors.
+ * Check whether F_SEAL_WRITE or F_SEAL_FUTURE_WRITE are set; if so, do proper
+ * check/handling on the vma flags.  Return 0 if check pass, or <0 for errors.
  */
-static inline int seal_check_future_write(int seals, struct vm_area_struct *vma)
+static inline int seal_check_write(int seals, struct vm_area_struct *vma)
 {
-	if (seals & F_SEAL_FUTURE_WRITE) {
+	if (seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
 		/*
 		 * New PROT_WRITE and MAP_SHARED mmaps are not allowed when
-		 * "future write" seal active.
+		 * write seals are active.
 		 */
 		if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE))
 			return -EPERM;
 
 		/*
-		 * Since an F_SEAL_FUTURE_WRITE sealed memfd can be mapped as
+		 * Since an F_SEAL_[FUTURE_]WRITE sealed memfd can be mapped as
 		 * MAP_SHARED and read-only, take care to not allow mprotect to
 		 * revert protections on such mappings. Do this only for shared
 		 * mappings. For private mappings, don't need to mask
diff --git a/mm/shmem.c b/mm/shmem.c
index 6503910b0f54..cab053831fea 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2405,7 +2405,7 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	int ret;
 
-	ret = seal_check_future_write(info->seals, vma);
+	ret = seal_check_write(info->seals, vma);
 	if (ret)
 		return ret;
 
-- 
2.42.0