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fanotify: allow reporting errors on failure to open fd

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When working in "fd mode", fanotify_read() needs to open an fd
from a dentry to report event->fd to userspace.

Opening an fd from dentry can fail for several reasons.
For example, when tasks are gone and we try to open their
/proc files or we try to open a WRONLY file like in sysfs
or when trying to open a file that was deleted on the
remote network server.

Add a new flag FAN_REPORT_FD_ERROR for fanotify_init().
For a group with FAN_REPORT_FD_ERROR, we will send the
event with the error instead of the open fd, otherwise
userspace may not get the error at all.

For an overflow event, we report -EBADF to avoid confusing FAN_NOFD
with -EPERM.  Similarly for pidfd open errors we report either -ESRCH
or the open error instead of FAN_NOPIDFD and FAN_EPIDFD.

In any case, userspace will not know which file failed to
open, so add a debug print for further investigation.

Reported-by: Krishna Vivek Vitta <kvitta@microsoft.com>
Link: https://lore.kernel.org/linux-fsdevel/SI2P153MB07182F3424619EDDD1F393EED46D2@SI2P153MB0718.APCP153.PROD.OUTLOOK.COM/
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20241003142922.111539-1-amir73il@gmail.com
This commit is contained in:
Amir Goldstein 2024-10-03 16:29:22 +02:00 committed by Jan Kara
parent 1cda52f1b4
commit 522249f05c
3 changed files with 50 additions and 37 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

85
fs/notify/fanotify/fanotify_user.c
View File

@ -266,13 +266,6 @@ static int create_fd(struct fsnotify_group *group, const struct path *path,
group->fanotify_data.f_flags | __FMODE_NONOTIFY,
current_cred());
if (IS_ERR(new_file)) {
/*
* we still send an event even if we can't open the file. this
* can happen when say tasks are gone and we try to open their
* /proc files or we try to open a WRONLY file like in sysfs
* we just send the errno to userspace since there isn't much
* else we can do.
*/
put_unused_fd(client_fd);
client_fd = PTR_ERR(new_file);
} else {
@ -663,7 +656,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
unsigned int info_mode = FAN_GROUP_FLAG(group, FANOTIFY_INFO_MODES);
unsigned int pidfd_mode = info_mode & FAN_REPORT_PIDFD;
struct file *f = NULL, *pidfd_file = NULL;
int ret, pidfd = FAN_NOPIDFD, fd = FAN_NOFD;
int ret, pidfd = -ESRCH, fd = -EBADF;
pr_debug("%s: group=%p event=%p\n", __func__, group, event);
@ -691,10 +684,39 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
if (!FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV) &&
path && path->mnt && path->dentry) {
fd = create_fd(group, path, &f);
if (fd < 0)
return fd;
/*
* Opening an fd from dentry can fail for several reasons.
* For example, when tasks are gone and we try to open their
* /proc files or we try to open a WRONLY file like in sysfs
* or when trying to open a file that was deleted on the
* remote network server.
*
* For a group with FAN_REPORT_FD_ERROR, we will send the
* event with the error instead of the open fd, otherwise
* Userspace may not get the error at all.
* In any case, userspace will not know which file failed to
* open, so add a debug print for further investigation.
*/
if (fd < 0) {
pr_debug("fanotify: create_fd(%pd2) failed err=%d\n",
path->dentry, fd);
if (!FAN_GROUP_FLAG(group, FAN_REPORT_FD_ERROR)) {
/*
* Historically, we've handled EOPENSTALE in a
* special way and silently dropped such
* events. Now we have to keep it to maintain
* backward compatibility...
*/
if (fd == -EOPENSTALE)
fd = 0;
return fd;
}
}
}
metadata.fd = fd;
if (FAN_GROUP_FLAG(group, FAN_REPORT_FD_ERROR))
metadata.fd = fd;
else
metadata.fd = fd >= 0 ? fd : FAN_NOFD;
if (pidfd_mode) {
/*
@ -709,18 +731,16 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
* The PIDTYPE_TGID check for an event->pid is performed
* preemptively in an attempt to catch out cases where the event
* listener reads events after the event generating process has
* already terminated. Report FAN_NOPIDFD to the event listener
* in those cases, with all other pidfd creation errors being
* reported as FAN_EPIDFD.
* already terminated. Depending on flag FAN_REPORT_FD_ERROR,
* report either -ESRCH or FAN_NOPIDFD to the event listener in
* those cases with all other pidfd creation errors reported as
* the error code itself or as FAN_EPIDFD.
*/
if (metadata.pid == 0 ||
!pid_has_task(event->pid, PIDTYPE_TGID)) {
pidfd = FAN_NOPIDFD;
} else {
if (metadata.pid && pid_has_task(event->pid, PIDTYPE_TGID))
pidfd = pidfd_prepare(event->pid, 0, &pidfd_file);
if (pidfd < 0)
pidfd = FAN_EPIDFD;
}
if (!FAN_GROUP_FLAG(group, FAN_REPORT_FD_ERROR) && pidfd < 0)
pidfd = pidfd == -ESRCH ? FAN_NOPIDFD : FAN_EPIDFD;
}
ret = -EFAULT;
@ -737,9 +757,6 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
buf += FAN_EVENT_METADATA_LEN;
count -= FAN_EVENT_METADATA_LEN;
if (fanotify_is_perm_event(event->mask))
FANOTIFY_PERM(event)->fd = fd;
if (info_mode) {
ret = copy_info_records_to_user(event, info, info_mode, pidfd,
buf, count);
@ -753,15 +770,18 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
if (pidfd_file)
fd_install(pidfd, pidfd_file);
if (fanotify_is_perm_event(event->mask))
FANOTIFY_PERM(event)->fd = fd;
return metadata.event_len;
out_close_fd:
if (fd != FAN_NOFD) {
if (f) {
put_unused_fd(fd);
fput(f);
}
if (pidfd >= 0) {
if (pidfd_file) {
put_unused_fd(pidfd);
fput(pidfd_file);
}
@ -828,15 +848,6 @@ static ssize_t fanotify_read(struct file *file, char __user *buf,
}
ret = copy_event_to_user(group, event, buf, count);
if (unlikely(ret == -EOPENSTALE)) {
/*
* We cannot report events with stale fd so drop it.
* Setting ret to 0 will continue the event loop and
* do the right thing if there are no more events to
* read (i.e. return bytes read, -EAGAIN or wait).
*/
ret = 0;
}
/*
* Permission events get queued to wait for response. Other
@ -845,7 +856,7 @@ static ssize_t fanotify_read(struct file *file, char __user *buf,
if (!fanotify_is_perm_event(event->mask)) {
fsnotify_destroy_event(group, &event->fse);
} else {
if (ret <= 0) {
if (ret <= 0 || FANOTIFY_PERM(event)->fd < 0) {
spin_lock(&group->notification_lock);
finish_permission_event(group,
FANOTIFY_PERM(event), FAN_DENY, NULL);
@ -1954,7 +1965,7 @@ static int __init fanotify_user_setup(void)
FANOTIFY_DEFAULT_MAX_USER_MARKS);
BUILD_BUG_ON(FANOTIFY_INIT_FLAGS & FANOTIFY_INTERNAL_GROUP_FLAGS);
BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 12);
BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 13);
BUILD_BUG_ON(HWEIGHT32(FANOTIFY_MARK_FLAGS) != 11);
fanotify_mark_cache = KMEM_CACHE(fanotify_mark,

1
include/linux/fanotify.h
View File

@ -36,6 +36,7 @@
#define FANOTIFY_ADMIN_INIT_FLAGS (FANOTIFY_PERM_CLASSES | \
FAN_REPORT_TID | \
FAN_REPORT_PIDFD | \
FAN_REPORT_FD_ERROR | \
FAN_UNLIMITED_QUEUE | \
FAN_UNLIMITED_MARKS)

1
include/uapi/linux/fanotify.h
View File

@ -60,6 +60,7 @@
#define FAN_REPORT_DIR_FID 0x00000400 /* Report unique directory id */
#define FAN_REPORT_NAME 0x00000800 /* Report events with name */
#define FAN_REPORT_TARGET_FID 0x00001000 /* Report dirent target id */
#define FAN_REPORT_FD_ERROR 0x00002000 /* event->fd can report error */
/* Convenience macro - FAN_REPORT_NAME requires FAN_REPORT_DIR_FID */
#define FAN_REPORT_DFID_NAME (FAN_REPORT_DIR_FID | FAN_REPORT_NAME)