Callers holding the files->file_lock lock can call iterate_fd_locked instead of
iterate_fd
Signed-off-by: aloktiagi <[email protected]>
Reviewed-by: Tycho Andersen <[email protected]>
---
fs/file.c | 21 +++++++++++++++------
include/linux/fdtable.h | 3 +++
2 files changed, 18 insertions(+), 6 deletions(-)
diff --git a/fs/file.c b/fs/file.c
index c942c89ca4cd..4b2346b8a5ee 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1295,15 +1295,12 @@ int f_dupfd(unsigned int from, struct file *file, unsigned flags)
return err;
}
-int iterate_fd(struct files_struct *files, unsigned n,
- int (*f)(const void *, struct file *, unsigned),
- const void *p)
+int iterate_fd_locked(struct files_struct *files, unsigned n,
+ int (*f)(const void *, struct file *, unsigned),
+ const void *p)
{
struct fdtable *fdt;
int res = 0;
- if (!files)
- return 0;
- spin_lock(&files->file_lock);
for (fdt = files_fdtable(files); n < fdt->max_fds; n++) {
struct file *file;
file = rcu_dereference_check_fdtable(files, fdt->fd[n]);
@@ -1313,6 +1310,18 @@ int iterate_fd(struct files_struct *files, unsigned n,
if (res)
break;
}
+ return res;
+}
+
+int iterate_fd(struct files_struct *files, unsigned n,
+ int (*f)(const void *, struct file *, unsigned),
+ const void *p)
+{
+ int res = 0;
+ if (!files)
+ return 0;
+ spin_lock(&files->file_lock);
+ res = iterate_fd_locked(files, n, f, p);
spin_unlock(&files->file_lock);
return res;
}
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index e066816f3519..14882520d1fe 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -122,6 +122,9 @@ void do_close_on_exec(struct files_struct *);
int iterate_fd(struct files_struct *, unsigned,
int (*)(const void *, struct file *, unsigned),
const void *);
+int iterate_fd_locked(struct files_struct *, unsigned,
+ int (*)(const void *, struct file *, unsigned),
+ const void *);
extern int close_fd(unsigned int fd);
extern int __close_range(unsigned int fd, unsigned int max_fd, unsigned int flags);
--
2.34.1
Allow callers of do_dup2 to get a reference to the file pointer being dup'ed
over. The callers can then replace the file with the new file in the eventpoll
interface or the file table before freeing it.
Signed-off-by: aloktiagi <[email protected]>
---
Changes in v2:
- Make the commit message more clearer.
- Address review comment to make the interface cleaner so that the caller cannot
forget to initialize the fdfile.
---
fs/file.c | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)
diff --git a/fs/file.c b/fs/file.c
index 4b2346b8a5ee..1716f07103d8 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1086,7 +1086,7 @@ bool get_close_on_exec(unsigned int fd)
}
static int do_dup2(struct files_struct *files,
- struct file *file, unsigned fd, unsigned flags)
+ struct file *file, unsigned fd, struct file **fdfile, unsigned flags)
__releases(&files->file_lock)
{
struct file *tofree;
@@ -1119,8 +1119,12 @@ __releases(&files->file_lock)
__clear_close_on_exec(fd, fdt);
spin_unlock(&files->file_lock);
- if (tofree)
- filp_close(tofree, files);
+ if (fdfile) {
+ *fdfile = tofree;
+ } else {
+ if (tofree)
+ filp_close(tofree, files);
+ }
return fd;
@@ -1132,6 +1136,7 @@ __releases(&files->file_lock)
int replace_fd(unsigned fd, struct file *file, unsigned flags)
{
int err;
+ struct file *fdfile = NULL;
struct files_struct *files = current->files;
if (!file)
@@ -1144,7 +1149,10 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
err = expand_files(files, fd);
if (unlikely(err < 0))
goto out_unlock;
- return do_dup2(files, file, fd, flags);
+ err = do_dup2(files, file, fd, &fdfile, flags);
+ if (fdfile)
+ filp_close(fdfile, files);
+ return err;
out_unlock:
spin_unlock(&files->file_lock);
@@ -1237,7 +1245,7 @@ static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
goto Ebadf;
goto out_unlock;
}
- return do_dup2(files, file, newfd, flags);
+ return do_dup2(files, file, newfd, NULL, flags);
Ebadf:
err = -EBADF;
--
2.34.1
Introduce a mechanism to replace a file linked in the epoll interface or a file
that has been dup'ed by a file received via the replace_fd() interface.
eventpoll_replace() is called from do_replace() and finds all instances of the
file to be replaced and replaces them with the new file.
do_replace() also replaces the file in the file descriptor table for all fd
numbers referencing it with the new file.
We have a use case where multiple IPv6 only network namespaces can use a single
IPv4 network namespace for IPv4 only egress connectivity by switching their
sockets from IPv6 to IPv4 network namespace. This allows for migration of
systems to IPv6 only while keeping their connectivity to IPv4 only destinations
intact.
Today, we achieve this by setting up seccomp filter to intercept network system
calls like connect() from a container in a container manager which runs in an
IPv4 only network namespace. The container manager creates a new IPv4 connection
and injects the new file descriptor through SECCOMP_NOTIFY_IOCTL_ADDFD replacing
the original file descriptor from the connect() call. This does not work for
cases where the original file descriptor is handed off to a system like epoll
before the connect() call. After a new file descriptor is injected the original
file descriptor being referenced by the epoll fd is not longer valid leading to
failures. As a workaround the container manager when intercepting connect()
loops through all open socket file descriptors to check if they are referencing
the socket attempting the connect() and replace the reference with the to be
injected file descriptor. This workaround is cumbersome and makes the solution
prone to similar yet to be discovered issues.
The above change will enable us remove the workaround in the container manager
and let the kernel handle the replacement correctly.
Signed-off-by: aloktiagi <[email protected]>
---
fs/eventpoll.c | 38 ++++++++
fs/file.c | 54 +++++++++++
include/linux/eventpoll.h | 18 ++++
include/linux/file.h | 1 +
tools/testing/selftests/seccomp/seccomp_bpf.c | 97 +++++++++++++++++++
5 files changed, 208 insertions(+)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 64659b110973..958ad995fd45 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -935,6 +935,44 @@ void eventpoll_release_file(struct file *file)
mutex_unlock(&epmutex);
}
+static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
+ struct file *tfile, int fd, int full_check);
+
+/*
+ * This is called from eventpoll_replace() to replace a linked file in the epoll
+ * interface with a new file received from another process. This is useful in
+ * cases where a process is trying to install a new file for an existing one
+ * that is linked in the epoll interface
+ */
+void eventpoll_replace_file(struct file *toreplace, struct file *file)
+{
+ int fd;
+ struct eventpoll *ep;
+ struct epitem *epi;
+ struct hlist_node *next;
+ struct epoll_event event;
+
+ if (!file_can_poll(file))
+ return;
+
+ mutex_lock(&epmutex);
+ if (unlikely(!toreplace->f_ep)) {
+ mutex_unlock(&epmutex);
+ return;
+ }
+
+ hlist_for_each_entry_safe(epi, next, toreplace->f_ep, fllink) {
+ ep = epi->ep;
+ mutex_lock(&ep->mtx);
+ fd = epi->ffd.fd;
+ event = epi->event;
+ ep_remove(ep, epi);
+ ep_insert(ep, &event, file, fd, 1);
+ mutex_unlock(&ep->mtx);
+ }
+ mutex_unlock(&epmutex);
+}
+
static int ep_alloc(struct eventpoll **pep)
{
int error;
diff --git a/fs/file.c b/fs/file.c
index 1716f07103d8..ce691dae1f0e 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -20,10 +20,18 @@
#include <linux/spinlock.h>
#include <linux/rcupdate.h>
#include <linux/close_range.h>
+#include <linux/eventpoll.h>
#include <net/sock.h>
#include "internal.h"
+struct replace_filefd {
+ struct files_struct *files;
+ unsigned fd;
+ struct file *fdfile;
+ struct file *file;
+};
+
unsigned int sysctl_nr_open __read_mostly = 1024*1024;
unsigned int sysctl_nr_open_min = BITS_PER_LONG;
/* our min() is unusable in constant expressions ;-/ */
@@ -1133,6 +1141,50 @@ __releases(&files->file_lock)
return -EBUSY;
}
+static int do_replace_fd_array(const void *v, struct file *tofree, unsigned int n)
+{
+ struct replace_filefd *ffd = (void *)v;
+ struct fdtable *fdt;
+
+ fdt = files_fdtable(ffd->files);
+
+ if ((n != ffd->fd) && (tofree == ffd->fdfile)) {
+ get_file(ffd->file);
+ rcu_assign_pointer(fdt->fd[n], ffd->file);
+ tofree = pick_file(ffd->files, n);
+ filp_close(tofree, ffd->files);
+ }
+ return 0;
+}
+
+static void do_replace(struct files_struct *files,
+ struct file *file, unsigned fd, struct file *fdfile)
+{
+ unsigned n = 0;
+ struct replace_filefd ffd = {
+ .files = files,
+ .fd = fd,
+ .fdfile = fdfile,
+ .file = file
+ };
+
+ /*
+ * Check if the file referenced by the fd number is linked to the epoll
+ * interface. If yes, replace the reference with the received file in
+ * the epoll interface.
+ */
+ if (fdfile && fdfile->f_ep) {
+ eventpoll_replace(fdfile, file);
+ }
+ /*
+ * Install the received file in the file descriptor table for all fd
+ * numbers referencing the same file as the one we are trying to
+ * replace. Do not install it for the fd number received since that is
+ * handled in do_dup2()
+ */
+ iterate_fd_locked(files, n, do_replace_fd_array, &ffd);
+}
+
int replace_fd(unsigned fd, struct file *file, unsigned flags)
{
int err;
@@ -1150,8 +1202,10 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
if (unlikely(err < 0))
goto out_unlock;
err = do_dup2(files, file, fd, &fdfile, flags);
+ do_replace(files, file, fd, fdfile);
if (fdfile)
filp_close(fdfile, files);
+
return err;
out_unlock:
diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h
index 3337745d81bd..38904fce3840 100644
--- a/include/linux/eventpoll.h
+++ b/include/linux/eventpoll.h
@@ -25,6 +25,8 @@ struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd, unsigned long t
/* Used to release the epoll bits inside the "struct file" */
void eventpoll_release_file(struct file *file);
+void eventpoll_replace_file(struct file *toreplace, struct file *file);
+
/*
* This is called from inside fs/file_table.c:__fput() to unlink files
* from the eventpoll interface. We need to have this facility to cleanup
@@ -53,6 +55,22 @@ static inline void eventpoll_release(struct file *file)
eventpoll_release_file(file);
}
+
+/*
+ * This is called from fs/file.c:do_replace() to replace a linked file in the
+ * epoll interface with a new file received from another process. This is useful
+ * in cases where a process is trying to install a new file for an existing one
+ * that is linked in the epoll interface
+ */
+static inline void eventpoll_replace(struct file *toreplace, struct file *file)
+{
+ /*
+ * toreplace is the file being replaced. Install the new file for the
+ * existing one that is linked in the epoll interface
+ */
+ eventpoll_replace_file(toreplace, file);
+}
+
int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
bool nonblock);
diff --git a/include/linux/file.h b/include/linux/file.h
index 39704eae83e2..80e56b2b44fb 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -36,6 +36,7 @@ struct fd {
struct file *file;
unsigned int flags;
};
+
#define FDPUT_FPUT 1
#define FDPUT_POS_UNLOCK 2
diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index 61386e499b77..caf68682519c 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -47,6 +47,7 @@
#include <linux/kcmp.h>
#include <sys/resource.h>
#include <sys/capability.h>
+#include <sys/epoll.h>
#include <unistd.h>
#include <sys/syscall.h>
@@ -4179,6 +4180,102 @@ TEST(user_notification_addfd)
close(memfd);
}
+TEST(user_notification_addfd_with_epoll_replace)
+{
+ char c;
+ pid_t pid;
+ long ret;
+ int status, listener, fd;
+ int efd, sfd[4];
+ struct epoll_event e;
+ struct seccomp_notif_addfd addfd = {};
+ struct seccomp_notif req = {};
+ struct seccomp_notif_resp resp = {};
+
+ ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+ ASSERT_EQ(0, ret) {
+ TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
+ }
+
+ listener = user_notif_syscall(__NR_getppid,
+ SECCOMP_FILTER_FLAG_NEW_LISTENER);
+
+ /* Create two socket pairs sfd[0] <-> sfd[1] and sfd[2] <-> sfd[3] */
+ ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, &sfd[0]), 0);
+ ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, &sfd[2]), 0);
+
+ pid = fork();
+ ASSERT_GE(pid, 0);
+
+ if (pid == 0) {
+ efd = epoll_create(1);
+ if (efd == -1)
+ exit(1);
+
+ e.events = EPOLLIN;
+ if (epoll_ctl(efd, EPOLL_CTL_ADD, sfd[0], &e) != 0)
+ exit(1);
+
+ /*
+ * fd will be added here to replace an existing one linked
+ * in the epoll interface.
+ */
+ if (syscall(__NR_getppid) != USER_NOTIF_MAGIC)
+ exit(1);
+
+ /*
+ * Write data to the sfd[3] connected to sfd[2], but due to
+ * the swap, we should see data on sfd[0]
+ */
+ if (write(sfd[3], "w", 1) != 1)
+ exit(1);
+
+ if (epoll_wait(efd, &e, 1, 0) != 1)
+ exit(1);
+
+ if (read(sfd[0], &c, 1) != 1)
+ exit(1);
+
+ if ('w' != c)
+ exit(1);
+
+ if (epoll_ctl(efd, EPOLL_CTL_DEL, sfd[0], &e) != 0)
+ exit(1);
+
+ close(efd);
+ close(sfd[0]);
+ close(sfd[1]);
+ close(sfd[2]);
+ close(sfd[3]);
+ exit(0);
+ }
+
+ ASSERT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+
+ addfd.srcfd = sfd[2];
+ addfd.newfd = sfd[0];
+ addfd.id = req.id;
+ addfd.flags = SECCOMP_ADDFD_FLAG_SETFD;
+ addfd.newfd_flags = O_CLOEXEC;
+
+ /*
+ * Verfiy we can install and replace a file that is linked in the
+ * epoll interface. Replace the socket sfd[0] with sfd[2]
+ */
+ fd = ioctl(listener, SECCOMP_IOCTL_NOTIF_ADDFD, &addfd);
+ EXPECT_EQ(fd, sfd[0]);
+
+ resp.id = req.id;
+ resp.error = 0;
+ resp.val = USER_NOTIF_MAGIC;
+ EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
+
+ /* Wait for child to finish. */
+ EXPECT_EQ(waitpid(pid, &status, 0), pid);
+ EXPECT_EQ(true, WIFEXITED(status));
+ EXPECT_EQ(0, WEXITSTATUS(status));
+}
+
TEST(user_notification_addfd_rlimit)
{
pid_t pid;
--
2.34.1
On Sat, Mar 18, 2023 at 06:02:48AM +0000, aloktiagi wrote:
> Introduce a mechanism to replace a file linked in the epoll interface or a file
> that has been dup'ed by a file received via the replace_fd() interface.
>
> eventpoll_replace() is called from do_replace() and finds all instances of the
> file to be replaced and replaces them with the new file.
>
> do_replace() also replaces the file in the file descriptor table for all fd
> numbers referencing it with the new file.
>
> We have a use case where multiple IPv6 only network namespaces can use a single
> IPv4 network namespace for IPv4 only egress connectivity by switching their
> sockets from IPv6 to IPv4 network namespace. This allows for migration of
> systems to IPv6 only while keeping their connectivity to IPv4 only destinations
> intact.
>
> Today, we achieve this by setting up seccomp filter to intercept network system
> calls like connect() from a container in a container manager which runs in an
> IPv4 only network namespace. The container manager creates a new IPv4 connection
> and injects the new file descriptor through SECCOMP_NOTIFY_IOCTL_ADDFD replacing
> the original file descriptor from the connect() call. This does not work for
> cases where the original file descriptor is handed off to a system like epoll
> before the connect() call. After a new file descriptor is injected the original
> file descriptor being referenced by the epoll fd is not longer valid leading to
> failures. As a workaround the container manager when intercepting connect()
> loops through all open socket file descriptors to check if they are referencing
> the socket attempting the connect() and replace the reference with the to be
> injected file descriptor. This workaround is cumbersome and makes the solution
> prone to similar yet to be discovered issues.
>
> The above change will enable us remove the workaround in the container manager
> and let the kernel handle the replacement correctly.
>
> Signed-off-by: aloktiagi <[email protected]>
> ---
> fs/eventpoll.c | 38 ++++++++
> fs/file.c | 54 +++++++++++
> include/linux/eventpoll.h | 18 ++++
> include/linux/file.h | 1 +
> tools/testing/selftests/seccomp/seccomp_bpf.c | 97 +++++++++++++++++++
> 5 files changed, 208 insertions(+)
>
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index 64659b110973..958ad995fd45 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -935,6 +935,44 @@ void eventpoll_release_file(struct file *file)
> mutex_unlock(&epmutex);
> }
>
> +static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
> + struct file *tfile, int fd, int full_check);
> +
> +/*
> + * This is called from eventpoll_replace() to replace a linked file in the epoll
> + * interface with a new file received from another process. This is useful in
> + * cases where a process is trying to install a new file for an existing one
> + * that is linked in the epoll interface
> + */
> +void eventpoll_replace_file(struct file *toreplace, struct file *file)
> +{
> + int fd;
> + struct eventpoll *ep;
> + struct epitem *epi;
> + struct hlist_node *next;
> + struct epoll_event event;
> +
> + if (!file_can_poll(file))
> + return;
> +
> + mutex_lock(&epmutex);
> + if (unlikely(!toreplace->f_ep)) {
> + mutex_unlock(&epmutex);
> + return;
> + }
> +
> + hlist_for_each_entry_safe(epi, next, toreplace->f_ep, fllink) {
> + ep = epi->ep;
> + mutex_lock(&ep->mtx);
> + fd = epi->ffd.fd;
> + event = epi->event;
> + ep_remove(ep, epi);
> + ep_insert(ep, &event, file, fd, 1);
So, ep_remove() can't fail but ep_insert() can. Maybe that doesn't
matter...
> + mutex_unlock(&ep->mtx);
> + }
> + mutex_unlock(&epmutex);
> +}
Andrew carries a patchset that may impact the locking here:
https://lore.kernel.org/linux-fsdevel/323de732635cc3513c1837c6cbb98f012174f994.1678312201.git.pabeni@redhat.com
I have to say that this whole thing has a very unpleasant taste to it.
Replacing a single fd from seccomp is fine, wading through the fdtable
to replace all fds referencing the same file is pretty nasty. Especially
with the global epoll mutex involved in all of this.
And what limits this to epoll. I'm seriously asking aren't there
potentially issues for fds somehow referenced in io_uring instances as
well?
I'm not convinced this belongs here yet...
> +
> static int ep_alloc(struct eventpoll **pep)
> {
> int error;
> diff --git a/fs/file.c b/fs/file.c
> index 1716f07103d8..ce691dae1f0e 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -20,10 +20,18 @@
> #include <linux/spinlock.h>
> #include <linux/rcupdate.h>
> #include <linux/close_range.h>
> +#include <linux/eventpoll.h>
> #include <net/sock.h>
>
> #include "internal.h"
>
> +struct replace_filefd {
> + struct files_struct *files;
> + unsigned fd;
> + struct file *fdfile;
> + struct file *file;
> +};
> +
> unsigned int sysctl_nr_open __read_mostly = 1024*1024;
> unsigned int sysctl_nr_open_min = BITS_PER_LONG;
> /* our min() is unusable in constant expressions ;-/ */
> @@ -1133,6 +1141,50 @@ __releases(&files->file_lock)
> return -EBUSY;
> }
>
> +static int do_replace_fd_array(const void *v, struct file *tofree, unsigned int n)
> +{
> + struct replace_filefd *ffd = (void *)v;
> + struct fdtable *fdt;
> +
> + fdt = files_fdtable(ffd->files);
> +
> + if ((n != ffd->fd) && (tofree == ffd->fdfile)) {
> + get_file(ffd->file);
> + rcu_assign_pointer(fdt->fd[n], ffd->file);
> + tofree = pick_file(ffd->files, n);
> + filp_close(tofree, ffd->files);
> + }
> + return 0;
> +}
> +
> +static void do_replace(struct files_struct *files,
> + struct file *file, unsigned fd, struct file *fdfile)
> +{
> + unsigned n = 0;
> + struct replace_filefd ffd = {
> + .files = files,
> + .fd = fd,
> + .fdfile = fdfile,
> + .file = file
> + };
> +
> + /*
> + * Check if the file referenced by the fd number is linked to the epoll
> + * interface. If yes, replace the reference with the received file in
> + * the epoll interface.
> + */
> + if (fdfile && fdfile->f_ep) {
> + eventpoll_replace(fdfile, file);
> + }
> + /*
> + * Install the received file in the file descriptor table for all fd
> + * numbers referencing the same file as the one we are trying to
> + * replace. Do not install it for the fd number received since that is
> + * handled in do_dup2()
> + */
> + iterate_fd_locked(files, n, do_replace_fd_array, &ffd);
> +}
> +
> int replace_fd(unsigned fd, struct file *file, unsigned flags)
> {
> int err;
> @@ -1150,8 +1202,10 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
> if (unlikely(err < 0))
> goto out_unlock;
> err = do_dup2(files, file, fd, &fdfile, flags);
> + do_replace(files, file, fd, fdfile);
I might be missing context here but from what I can see:
Above you call iterate_fd_locked() in do_replace() implying that the
caller is holding spin_lock(&files->file_lock) from my reading.
But you call do_replace() after do_dup2() which drops
spin_lock(&files->file_lock).
So spin_lock(&files->file_lock) isn't held anymore and you call
iterate_fd_locked() non-locked. So it seems misnamed?
But then, I documented that pick_file() requires
spin_lock(&files->file_lock) to be held but then you don't seem to be
holding it. And I wonder why that's safe.
Then you call
get_file(ffd->file)
...
filp_close(tofree)
in do_replace_fd_array()
So if you call filp_close you might be hitting a filesystems' ->flush()
method (nfs, cifs, etc...) and cifs for example calls
filemap_write_and_wait(). And iirc that can take a while. So even if you
know that there are still references to the file and so you won't be
doing the final fput() you might still end up waiting which would get
you into issues if you have to be holding spin_lock(&files->file_lock).
> if (fdfile)
> filp_close(fdfile, files);
> +
> return err;
>
> out_unlock:
> diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h
> index 3337745d81bd..38904fce3840 100644
> --- a/include/linux/eventpoll.h
> +++ b/include/linux/eventpoll.h
> @@ -25,6 +25,8 @@ struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd, unsigned long t
> /* Used to release the epoll bits inside the "struct file" */
> void eventpoll_release_file(struct file *file);
>
> +void eventpoll_replace_file(struct file *toreplace, struct file *file);
> +
> /*
> * This is called from inside fs/file_table.c:__fput() to unlink files
> * from the eventpoll interface. We need to have this facility to cleanup
> @@ -53,6 +55,22 @@ static inline void eventpoll_release(struct file *file)
> eventpoll_release_file(file);
> }
>
> +
> +/*
> + * This is called from fs/file.c:do_replace() to replace a linked file in the
> + * epoll interface with a new file received from another process. This is useful
> + * in cases where a process is trying to install a new file for an existing one
> + * that is linked in the epoll interface
> + */
> +static inline void eventpoll_replace(struct file *toreplace, struct file *file)
> +{
> + /*
> + * toreplace is the file being replaced. Install the new file for the
> + * existing one that is linked in the epoll interface
> + */
> + eventpoll_replace_file(toreplace, file);
> +}
> +
> int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
> bool nonblock);
>
> diff --git a/include/linux/file.h b/include/linux/file.h
> index 39704eae83e2..80e56b2b44fb 100644
> --- a/include/linux/file.h
> +++ b/include/linux/file.h
> @@ -36,6 +36,7 @@ struct fd {
> struct file *file;
> unsigned int flags;
> };
> +
> #define FDPUT_FPUT 1
> #define FDPUT_POS_UNLOCK 2
>
> diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> index 61386e499b77..caf68682519c 100644
> --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> @@ -47,6 +47,7 @@
> #include <linux/kcmp.h>
> #include <sys/resource.h>
> #include <sys/capability.h>
> +#include <sys/epoll.h>
>
> #include <unistd.h>
> #include <sys/syscall.h>
> @@ -4179,6 +4180,102 @@ TEST(user_notification_addfd)
> close(memfd);
> }
>
> +TEST(user_notification_addfd_with_epoll_replace)
> +{
> + char c;
> + pid_t pid;
> + long ret;
> + int status, listener, fd;
> + int efd, sfd[4];
> + struct epoll_event e;
> + struct seccomp_notif_addfd addfd = {};
> + struct seccomp_notif req = {};
> + struct seccomp_notif_resp resp = {};
> +
> + ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
> + ASSERT_EQ(0, ret) {
> + TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
> + }
> +
> + listener = user_notif_syscall(__NR_getppid,
> + SECCOMP_FILTER_FLAG_NEW_LISTENER);
> +
> + /* Create two socket pairs sfd[0] <-> sfd[1] and sfd[2] <-> sfd[3] */
> + ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, &sfd[0]), 0);
> + ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, &sfd[2]), 0);
> +
> + pid = fork();
> + ASSERT_GE(pid, 0);
> +
> + if (pid == 0) {
> + efd = epoll_create(1);
> + if (efd == -1)
> + exit(1);
> +
> + e.events = EPOLLIN;
> + if (epoll_ctl(efd, EPOLL_CTL_ADD, sfd[0], &e) != 0)
> + exit(1);
> +
> + /*
> + * fd will be added here to replace an existing one linked
> + * in the epoll interface.
> + */
> + if (syscall(__NR_getppid) != USER_NOTIF_MAGIC)
> + exit(1);
> +
> + /*
> + * Write data to the sfd[3] connected to sfd[2], but due to
> + * the swap, we should see data on sfd[0]
> + */
> + if (write(sfd[3], "w", 1) != 1)
> + exit(1);
> +
> + if (epoll_wait(efd, &e, 1, 0) != 1)
> + exit(1);
> +
> + if (read(sfd[0], &c, 1) != 1)
> + exit(1);
> +
> + if ('w' != c)
> + exit(1);
> +
> + if (epoll_ctl(efd, EPOLL_CTL_DEL, sfd[0], &e) != 0)
> + exit(1);
> +
> + close(efd);
> + close(sfd[0]);
> + close(sfd[1]);
> + close(sfd[2]);
> + close(sfd[3]);
> + exit(0);
> + }
> +
> + ASSERT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
> +
> + addfd.srcfd = sfd[2];
> + addfd.newfd = sfd[0];
> + addfd.id = req.id;
> + addfd.flags = SECCOMP_ADDFD_FLAG_SETFD;
> + addfd.newfd_flags = O_CLOEXEC;
> +
> + /*
> + * Verfiy we can install and replace a file that is linked in the
> + * epoll interface. Replace the socket sfd[0] with sfd[2]
> + */
> + fd = ioctl(listener, SECCOMP_IOCTL_NOTIF_ADDFD, &addfd);
> + EXPECT_EQ(fd, sfd[0]);
> +
> + resp.id = req.id;
> + resp.error = 0;
> + resp.val = USER_NOTIF_MAGIC;
> + EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
> +
> + /* Wait for child to finish. */
> + EXPECT_EQ(waitpid(pid, &status, 0), pid);
> + EXPECT_EQ(true, WIFEXITED(status));
> + EXPECT_EQ(0, WEXITSTATUS(status));
> +}
> +
> TEST(user_notification_addfd_rlimit)
> {
> pid_t pid;
> --
> 2.34.1
>
On Mon, Mar 20, 2023 at 03:51:03PM +0100, Christian Brauner wrote:
> On Sat, Mar 18, 2023 at 06:02:48AM +0000, aloktiagi wrote:
> > Introduce a mechanism to replace a file linked in the epoll interface or a file
> > that has been dup'ed by a file received via the replace_fd() interface.
> >
> > eventpoll_replace() is called from do_replace() and finds all instances of the
> > file to be replaced and replaces them with the new file.
> >
> > do_replace() also replaces the file in the file descriptor table for all fd
> > numbers referencing it with the new file.
> >
> > We have a use case where multiple IPv6 only network namespaces can use a single
> > IPv4 network namespace for IPv4 only egress connectivity by switching their
> > sockets from IPv6 to IPv4 network namespace. This allows for migration of
> > systems to IPv6 only while keeping their connectivity to IPv4 only destinations
> > intact.
> >
> > Today, we achieve this by setting up seccomp filter to intercept network system
> > calls like connect() from a container in a container manager which runs in an
> > IPv4 only network namespace. The container manager creates a new IPv4 connection
> > and injects the new file descriptor through SECCOMP_NOTIFY_IOCTL_ADDFD replacing
> > the original file descriptor from the connect() call. This does not work for
> > cases where the original file descriptor is handed off to a system like epoll
> > before the connect() call. After a new file descriptor is injected the original
> > file descriptor being referenced by the epoll fd is not longer valid leading to
> > failures. As a workaround the container manager when intercepting connect()
> > loops through all open socket file descriptors to check if they are referencing
> > the socket attempting the connect() and replace the reference with the to be
> > injected file descriptor. This workaround is cumbersome and makes the solution
> > prone to similar yet to be discovered issues.
> >
> > The above change will enable us remove the workaround in the container manager
> > and let the kernel handle the replacement correctly.
> >
> > Signed-off-by: aloktiagi <[email protected]>
> > ---
> > fs/eventpoll.c | 38 ++++++++
> > fs/file.c | 54 +++++++++++
> > include/linux/eventpoll.h | 18 ++++
> > include/linux/file.h | 1 +
> > tools/testing/selftests/seccomp/seccomp_bpf.c | 97 +++++++++++++++++++
> > 5 files changed, 208 insertions(+)
> >
> > diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> > index 64659b110973..958ad995fd45 100644
> > --- a/fs/eventpoll.c
> > +++ b/fs/eventpoll.c
> > @@ -935,6 +935,44 @@ void eventpoll_release_file(struct file *file)
> > mutex_unlock(&epmutex);
> > }
> >
> > +static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
> > + struct file *tfile, int fd, int full_check);
> > +
> > +/*
> > + * This is called from eventpoll_replace() to replace a linked file in the epoll
> > + * interface with a new file received from another process. This is useful in
> > + * cases where a process is trying to install a new file for an existing one
> > + * that is linked in the epoll interface
> > + */
> > +void eventpoll_replace_file(struct file *toreplace, struct file *file)
> > +{
> > + int fd;
> > + struct eventpoll *ep;
> > + struct epitem *epi;
> > + struct hlist_node *next;
> > + struct epoll_event event;
> > +
> > + if (!file_can_poll(file))
> > + return;
> > +
> > + mutex_lock(&epmutex);
> > + if (unlikely(!toreplace->f_ep)) {
> > + mutex_unlock(&epmutex);
> > + return;
> > + }
> > +
> > + hlist_for_each_entry_safe(epi, next, toreplace->f_ep, fllink) {
> > + ep = epi->ep;
> > + mutex_lock(&ep->mtx);
> > + fd = epi->ffd.fd;
> > + event = epi->event;
> > + ep_remove(ep, epi);
> > + ep_insert(ep, &event, file, fd, 1);
>
> So, ep_remove() can't fail but ep_insert() can. Maybe that doesn't
> matter...
>
> > + mutex_unlock(&ep->mtx);
> > + }
> > + mutex_unlock(&epmutex);
> > +}
>
> Andrew carries a patchset that may impact the locking here:
>
> https://lore.kernel.org/linux-fsdevel/323de732635cc3513c1837c6cbb98f012174f994.1678312201.git.pabeni@redhat.com
>
> I have to say that this whole thing has a very unpleasant taste to it.
> Replacing a single fd from seccomp is fine, wading through the fdtable
> to replace all fds referencing the same file is pretty nasty. Especially
> with the global epoll mutex involved in all of this.
>
> And what limits this to epoll. I'm seriously asking aren't there
> potentially issues for fds somehow referenced in io_uring instances as
> well?
>
> I'm not convinced this belongs here yet...
Thank you for reviewing and proving a link to Andrew's patch.
I think just replacing a single fd from seccomp leaves this feature in an
incomplete state. As a user of this feature, it means I need to figure out what
all file descriptors are referencing this file eg. epoll, dup'ed fds etc. This
patch is an attempt to complete this seccomp feature and also move the logic of
figuring out the references to the kernel.
The epmutex is taken when the file is replaced in the epoll interface. This is
similar to what would happen when eventpoll_release would be called for this
same file when it is ultimately released from __fput().
This is indeed not limited to epoll and the file descriptor table, but this
current patch addresses is limited to these interfaces. We can create a separate
one for io_uring.
>
> > +
> > static int ep_alloc(struct eventpoll **pep)
> > {
> > int error;
> > diff --git a/fs/file.c b/fs/file.c
> > index 1716f07103d8..ce691dae1f0e 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -20,10 +20,18 @@
> > #include <linux/spinlock.h>
> > #include <linux/rcupdate.h>
> > #include <linux/close_range.h>
> > +#include <linux/eventpoll.h>
> > #include <net/sock.h>
> >
> > #include "internal.h"
> >
> > +struct replace_filefd {
> > + struct files_struct *files;
> > + unsigned fd;
> > + struct file *fdfile;
> > + struct file *file;
> > +};
> > +
> > unsigned int sysctl_nr_open __read_mostly = 1024*1024;
> > unsigned int sysctl_nr_open_min = BITS_PER_LONG;
> > /* our min() is unusable in constant expressions ;-/ */
> > @@ -1133,6 +1141,50 @@ __releases(&files->file_lock)
> > return -EBUSY;
> > }
> >
> > +static int do_replace_fd_array(const void *v, struct file *tofree, unsigned int n)
> > +{
> > + struct replace_filefd *ffd = (void *)v;
> > + struct fdtable *fdt;
> > +
> > + fdt = files_fdtable(ffd->files);
> > +
> > + if ((n != ffd->fd) && (tofree == ffd->fdfile)) {
> > + get_file(ffd->file);
> > + rcu_assign_pointer(fdt->fd[n], ffd->file);
> > + tofree = pick_file(ffd->files, n);
> > + filp_close(tofree, ffd->files);
> > + }
> > + return 0;
> > +}
> > +
> > +static void do_replace(struct files_struct *files,
> > + struct file *file, unsigned fd, struct file *fdfile)
> > +{
> > + unsigned n = 0;
> > + struct replace_filefd ffd = {
> > + .files = files,
> > + .fd = fd,
> > + .fdfile = fdfile,
> > + .file = file
> > + };
> > +
> > + /*
> > + * Check if the file referenced by the fd number is linked to the epoll
> > + * interface. If yes, replace the reference with the received file in
> > + * the epoll interface.
> > + */
> > + if (fdfile && fdfile->f_ep) {
> > + eventpoll_replace(fdfile, file);
> > + }
> > + /*
> > + * Install the received file in the file descriptor table for all fd
> > + * numbers referencing the same file as the one we are trying to
> > + * replace. Do not install it for the fd number received since that is
> > + * handled in do_dup2()
> > + */
> > + iterate_fd_locked(files, n, do_replace_fd_array, &ffd);
> > +}
> > +
> > int replace_fd(unsigned fd, struct file *file, unsigned flags)
> > {
> > int err;
> > @@ -1150,8 +1202,10 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
> > if (unlikely(err < 0))
> > goto out_unlock;
> > err = do_dup2(files, file, fd, &fdfile, flags);
> > + do_replace(files, file, fd, fdfile);
>
> I might be missing context here but from what I can see:
>
> Above you call iterate_fd_locked() in do_replace() implying that the
> caller is holding spin_lock(&files->file_lock) from my reading.
> But you call do_replace() after do_dup2() which drops
> spin_lock(&files->file_lock).
> So spin_lock(&files->file_lock) isn't held anymore and you call
> iterate_fd_locked() non-locked. So it seems misnamed?
>
> But then, I documented that pick_file() requires
> spin_lock(&files->file_lock) to be held but then you don't seem to be
> holding it. And I wonder why that's safe.
>
> Then you call
> get_file(ffd->file)
> ...
> filp_close(tofree)
>
> in do_replace_fd_array()
>
> So if you call filp_close you might be hitting a filesystems' ->flush()
> method (nfs, cifs, etc...) and cifs for example calls
> filemap_write_and_wait(). And iirc that can take a while. So even if you
> know that there are still references to the file and so you won't be
> doing the final fput() you might still end up waiting which would get
> you into issues if you have to be holding spin_lock(&files->file_lock).
You are correct. I had re-ordered the code after moving the free out of dup2 and
this was not how I intended it to be. I'll fix this in v3 and filp_close() will
have to be done outside the spinlock. Also I do not think I need pick_file()
anymore. Thank you for catching this.
>
> > if (fdfile)
> > filp_close(fdfile, files);
> > +
> > return err;
> >
> > out_unlock:
> > diff --git a/include/linux/eventpoll.h b/include/linux/eventpoll.h
> > index 3337745d81bd..38904fce3840 100644
> > --- a/include/linux/eventpoll.h
> > +++ b/include/linux/eventpoll.h
> > @@ -25,6 +25,8 @@ struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd, unsigned long t
> > /* Used to release the epoll bits inside the "struct file" */
> > void eventpoll_release_file(struct file *file);
> >
> > +void eventpoll_replace_file(struct file *toreplace, struct file *file);
> > +
> > /*
> > * This is called from inside fs/file_table.c:__fput() to unlink files
> > * from the eventpoll interface. We need to have this facility to cleanup
> > @@ -53,6 +55,22 @@ static inline void eventpoll_release(struct file *file)
> > eventpoll_release_file(file);
> > }
> >
> > +
> > +/*
> > + * This is called from fs/file.c:do_replace() to replace a linked file in the
> > + * epoll interface with a new file received from another process. This is useful
> > + * in cases where a process is trying to install a new file for an existing one
> > + * that is linked in the epoll interface
> > + */
> > +static inline void eventpoll_replace(struct file *toreplace, struct file *file)
> > +{
> > + /*
> > + * toreplace is the file being replaced. Install the new file for the
> > + * existing one that is linked in the epoll interface
> > + */
> > + eventpoll_replace_file(toreplace, file);
> > +}
> > +
> > int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
> > bool nonblock);
> >
> > diff --git a/include/linux/file.h b/include/linux/file.h
> > index 39704eae83e2..80e56b2b44fb 100644
> > --- a/include/linux/file.h
> > +++ b/include/linux/file.h
> > @@ -36,6 +36,7 @@ struct fd {
> > struct file *file;
> > unsigned int flags;
> > };
> > +
> > #define FDPUT_FPUT 1
> > #define FDPUT_POS_UNLOCK 2
> >
> > diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > index 61386e499b77..caf68682519c 100644
> > --- a/tools/testing/selftests/seccomp/seccomp_bpf.c
> > +++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
> > @@ -47,6 +47,7 @@
> > #include <linux/kcmp.h>
> > #include <sys/resource.h>
> > #include <sys/capability.h>
> > +#include <sys/epoll.h>
> >
> > #include <unistd.h>
> > #include <sys/syscall.h>
> > @@ -4179,6 +4180,102 @@ TEST(user_notification_addfd)
> > close(memfd);
> > }
> >
> > +TEST(user_notification_addfd_with_epoll_replace)
> > +{
> > + char c;
> > + pid_t pid;
> > + long ret;
> > + int status, listener, fd;
> > + int efd, sfd[4];
> > + struct epoll_event e;
> > + struct seccomp_notif_addfd addfd = {};
> > + struct seccomp_notif req = {};
> > + struct seccomp_notif_resp resp = {};
> > +
> > + ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
> > + ASSERT_EQ(0, ret) {
> > + TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
> > + }
> > +
> > + listener = user_notif_syscall(__NR_getppid,
> > + SECCOMP_FILTER_FLAG_NEW_LISTENER);
> > +
> > + /* Create two socket pairs sfd[0] <-> sfd[1] and sfd[2] <-> sfd[3] */
> > + ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, &sfd[0]), 0);
> > + ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, &sfd[2]), 0);
> > +
> > + pid = fork();
> > + ASSERT_GE(pid, 0);
> > +
> > + if (pid == 0) {
> > + efd = epoll_create(1);
> > + if (efd == -1)
> > + exit(1);
> > +
> > + e.events = EPOLLIN;
> > + if (epoll_ctl(efd, EPOLL_CTL_ADD, sfd[0], &e) != 0)
> > + exit(1);
> > +
> > + /*
> > + * fd will be added here to replace an existing one linked
> > + * in the epoll interface.
> > + */
> > + if (syscall(__NR_getppid) != USER_NOTIF_MAGIC)
> > + exit(1);
> > +
> > + /*
> > + * Write data to the sfd[3] connected to sfd[2], but due to
> > + * the swap, we should see data on sfd[0]
> > + */
> > + if (write(sfd[3], "w", 1) != 1)
> > + exit(1);
> > +
> > + if (epoll_wait(efd, &e, 1, 0) != 1)
> > + exit(1);
> > +
> > + if (read(sfd[0], &c, 1) != 1)
> > + exit(1);
> > +
> > + if ('w' != c)
> > + exit(1);
> > +
> > + if (epoll_ctl(efd, EPOLL_CTL_DEL, sfd[0], &e) != 0)
> > + exit(1);
> > +
> > + close(efd);
> > + close(sfd[0]);
> > + close(sfd[1]);
> > + close(sfd[2]);
> > + close(sfd[3]);
> > + exit(0);
> > + }
> > +
> > + ASSERT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
> > +
> > + addfd.srcfd = sfd[2];
> > + addfd.newfd = sfd[0];
> > + addfd.id = req.id;
> > + addfd.flags = SECCOMP_ADDFD_FLAG_SETFD;
> > + addfd.newfd_flags = O_CLOEXEC;
> > +
> > + /*
> > + * Verfiy we can install and replace a file that is linked in the
> > + * epoll interface. Replace the socket sfd[0] with sfd[2]
> > + */
> > + fd = ioctl(listener, SECCOMP_IOCTL_NOTIF_ADDFD, &addfd);
> > + EXPECT_EQ(fd, sfd[0]);
> > +
> > + resp.id = req.id;
> > + resp.error = 0;
> > + resp.val = USER_NOTIF_MAGIC;
> > + EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0);
> > +
> > + /* Wait for child to finish. */
> > + EXPECT_EQ(waitpid(pid, &status, 0), pid);
> > + EXPECT_EQ(true, WIFEXITED(status));
> > + EXPECT_EQ(0, WEXITSTATUS(status));
> > +}
> > +
> > TEST(user_notification_addfd_rlimit)
> > {
> > pid_t pid;
> > --
> > 2.34.1
> >