[PATCH v2 02/16] fsnotify: introduce pre-content permission event

Josef Bacik
2024-08-08T19:28:21Z

From: Amir Goldstein <amir73il@gmail.com>

The new FS_PRE_ACCESS permission event is similar to FS_ACCESS_PERM,
but it meant for a different use case of filling file content before
access to a file range, so it has slightly different semantics.

Generate FS_PRE_ACCESS/FS_ACCESS_PERM as two seperate events, same as
we did for FS_OPEN_PERM/FS_OPEN_EXEC_PERM.

FS_PRE_MODIFY is a new permission event, with similar semantics as
FS_PRE_ACCESS, which is called before a file is modified.

FS_ACCESS_PERM is reported also on blockdev and pipes, but the new
pre-content events are only reported for regular files and dirs.

The pre-content events are meant to be used by hierarchical storage
managers that want to fill the content of files on first access.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
fs/notify/fsnotify.c | 2 +-
include/linux/fsnotify.h | 27 ++++++++++++++++++++++++---
include/linux/fsnotify_backend.h | 13 +++++++++++--
security/selinux/hooks.c | 3 ++-
4 files changed, 38 insertions(+), 7 deletions(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 272c8a1dab3c..1ca4a8da7f29 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -621,7 +621,7 @@ static __init int fsnotify_init(void)
{
int ret;

- BUILD_BUG_ON(HWEIGHT32(ALL_FSNOTIFY_BITS) != 23);
+ BUILD_BUG_ON(HWEIGHT32(ALL_FSNOTIFY_BITS) != 25);

ret = init_srcu_struct(&fsnotify_mark_srcu);
if (ret)
diff --git a/include/linux/fsnotify.h b/include/linux/fsnotify.h
index 278620e063ab..028ce807805a 100644
--- a/include/linux/fsnotify.h
+++ b/include/linux/fsnotify.h
@@ -133,12 +133,13 @@ static inline int fsnotify_file(struct file *file, __u32 mask)

#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
/*
- * fsnotify_file_area_perm - permission hook before access to file range
+ * fsnotify_file_area_perm - permission hook before access/modify of file range
*/
static inline int fsnotify_file_area_perm(struct file *file, int perm_mask,
const loff_t *ppos, size_t count)
{
- __u32 fsnotify_mask = FS_ACCESS_PERM;
+ struct inode *inode = file_inode(file);
+ __u32 fsnotify_mask;

/*
* filesystem may be modified in the context of permission events
@@ -147,7 +148,27 @@ static inline int fsnotify_file_area_perm(struct file *file, int perm_mask,
*/
lockdep_assert_once(file_write_not_started(file));

- if (!(perm_mask & MAY_READ))
+ /*
+ * Generate FS_PRE_ACCESS/FS_ACCESS_PERM as two seperate events.
+ */
+ if (perm_mask & MAY_READ) {
+ int ret = fsnotify_file(file, FS_ACCESS_PERM);
+
+ if (ret)
+ return ret;
+ }
+
+ /*
+ * Pre-content events are only reported for regular files and dirs.
+ */
+ if (!S_ISDIR(inode->i_mode) && !S_ISREG(inode->i_mode))
+ return 0;
+
+ if (perm_mask & MAY_WRITE)
+ fsnotify_mask = FS_PRE_MODIFY;
+ else if (perm_mask & MAY_READ)
+ fsnotify_mask = FS_PRE_ACCESS;
+ else
return 0;

return fsnotify_file(file, fsnotify_mask);
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 8be029bc50b1..200a5e3b1cd4 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -56,6 +56,9 @@
#define FS_ACCESS_PERM 0x00020000 /* access event in a permissions hook */
#define FS_OPEN_EXEC_PERM 0x00040000 /* open/exec event in a permission hook */

+#define FS_PRE_ACCESS 0x00080000 /* Pre-content access hook */
+#define FS_PRE_MODIFY 0x00100000 /* Pre-content modify hook */
+
/*
* Set on inode mark that cares about things that happen to its children.
* Always set for dnotify and inotify.
@@ -77,8 +80,14 @@
*/
#define ALL_FSNOTIFY_DIRENT_EVENTS (FS_CREATE | FS_DELETE | FS_MOVE | FS_RENAME)

-#define ALL_FSNOTIFY_PERM_EVENTS (FS_OPEN_PERM | FS_ACCESS_PERM | \
- FS_OPEN_EXEC_PERM)
+/* Content events can be used to inspect file content */
+#define FSNOTIFY_CONTENT_PERM_EVENTS (FS_OPEN_PERM | FS_OPEN_EXEC_PERM | \
+ FS_ACCESS_PERM)
+/* Pre-content events can be used to fill file content */
+#define FSNOTIFY_PRE_CONTENT_EVENTS (FS_PRE_ACCESS | FS_PRE_MODIFY)
+
+#define ALL_FSNOTIFY_PERM_EVENTS (FSNOTIFY_CONTENT_PERM_EVENTS | \
+ FSNOTIFY_PRE_CONTENT_EVENTS)

/*
* This is a list of all events that may get sent to a parent that is watching
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 55c78c318ccd..2997edf3e7cd 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -3406,7 +3406,8 @@ static int selinux_path_notify(const struct path *path, u64 mask,
perm |= FILE__WATCH_WITH_PERM;

/* watches on read-like events need the file:watch_reads permission */
- if (mask & (FS_ACCESS | FS_ACCESS_PERM | FS_CLOSE_NOWRITE))
+ if (mask & (FS_ACCESS | FS_ACCESS_PERM | FS_PRE_ACCESS |
+ FS_CLOSE_NOWRITE))
perm |= FILE__WATCH_READS;

return path_has_perm(current_cred(), path, perm);
--
2.43.0