[PATCH 5.14 156/432] userfaultfd: prevent concurrent API initialization

Greg Kroah-Hartman
2021-09-16T17:26:41Z

From: Nadav Amit <namit@vmware.com>

[ Upstream commit 22e5fe2a2a279d9a6fcbdfb4dffe73821bef1c90 ]

userfaultfd assumes that the enabled features are set once and never
changed after UFFDIO_API ioctl succeeded.

However, currently, UFFDIO_API can be called concurrently from two
different threads, succeed on both threads and leave userfaultfd's
features in non-deterministic state. Theoretically, other uffd operations
(ioctl's and page-faults) can be dispatched while adversely affected by
such changes of features.

Moreover, the writes to ctx->state and ctx->features are not ordered,
which can - theoretically, again - let userfaultfd_ioctl() think that
userfaultfd API completed, while the features are still not initialized.

To avoid races, it is arguably best to get rid of ctx->state. Since there
are only 2 states, record the API initialization in ctx->features as the
uppermost bit and remove ctx->state.

Link: https://lkml.kernel.org/r/20210808020724.1022515-3-namit@vmware.com
Fixes: 9cd75c3cd4c3d ("userfaultfd: non-cooperative: add ability to report non-PF events from uffd descriptor")
Signed-off-by: Nadav Amit <namit@vmware.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/userfaultfd.c | 91 +++++++++++++++++++++++-------------------------
1 file changed, 44 insertions(+), 47 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 5c2d806e6ae5..c830cc4ea60f 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -33,11 +33,6 @@ int sysctl_unprivileged_userfaultfd __read_mostly;

static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;

-enum userfaultfd_state {
- UFFD_STATE_WAIT_API,
- UFFD_STATE_RUNNING,
-};
-
/*
* Start with fault_pending_wqh and fault_wqh so they're more likely
* to be in the same cacheline.
@@ -69,8 +64,6 @@ struct userfaultfd_ctx {
unsigned int flags;
/* features requested from the userspace */
unsigned int features;
- /* state machine */
- enum userfaultfd_state state;
/* released */
bool released;
/* memory mappings are changing because of non-cooperative event */
@@ -104,6 +97,14 @@ struct userfaultfd_wake_range {
unsigned long len;
};

+/* internal indication that UFFD_API ioctl was successfully executed */
+#define UFFD_FEATURE_INITIALIZED (1u << 31)
+
+static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx)
+{
+ return ctx->features & UFFD_FEATURE_INITIALIZED;
+}
+
static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
int wake_flags, void *key)
{
@@ -666,7 +667,6 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)

refcount_set(&ctx->refcount, 1);
ctx->flags = octx->flags;
- ctx->state = UFFD_STATE_RUNNING;
ctx->features = octx->features;
ctx->released = false;
ctx->mmap_changing = false;
@@ -943,38 +943,33 @@ static __poll_t userfaultfd_poll(struct file *file, poll_table *wait)

poll_wait(file, &ctx->fd_wqh, wait);

- switch (ctx->state) {
- case UFFD_STATE_WAIT_API:
+ if (!userfaultfd_is_initialized(ctx))
return EPOLLERR;
- case UFFD_STATE_RUNNING:
- /*
- * poll() never guarantees that read won't block.
- * userfaults can be waken before they're read().
- */
- if (unlikely(!(file->f_flags & O_NONBLOCK)))
- return EPOLLERR;
- /*
- * lockless access to see if there are pending faults
- * __pollwait last action is the add_wait_queue but
- * the spin_unlock would allow the waitqueue_active to
- * pass above the actual list_add inside
- * add_wait_queue critical section. So use a full
- * memory barrier to serialize the list_add write of
- * add_wait_queue() with the waitqueue_active read
- * below.
- */
- ret = 0;
- smp_mb();
- if (waitqueue_active(&ctx->fault_pending_wqh))
- ret = EPOLLIN;
- else if (waitqueue_active(&ctx->event_wqh))
- ret = EPOLLIN;

- return ret;
- default:
- WARN_ON_ONCE(1);
+ /*
+ * poll() never guarantees that read won't block.
+ * userfaults can be waken before they're read().
+ */
+ if (unlikely(!(file->f_flags & O_NONBLOCK)))
return EPOLLERR;
- }
+ /*
+ * lockless access to see if there are pending faults
+ * __pollwait last action is the add_wait_queue but
+ * the spin_unlock would allow the waitqueue_active to
+ * pass above the actual list_add inside
+ * add_wait_queue critical section. So use a full
+ * memory barrier to serialize the list_add write of
+ * add_wait_queue() with the waitqueue_active read
+ * below.
+ */
+ ret = 0;
+ smp_mb();
+ if (waitqueue_active(&ctx->fault_pending_wqh))
+ ret = EPOLLIN;
+ else if (waitqueue_active(&ctx->event_wqh))
+ ret = EPOLLIN;
+
+ return ret;
}

static const struct file_operations userfaultfd_fops;
@@ -1169,7 +1164,7 @@ static ssize_t userfaultfd_read(struct file *file, char __user *buf,
int no_wait = file->f_flags & O_NONBLOCK;
struct inode *inode = file_inode(file);

- if (ctx->state == UFFD_STATE_WAIT_API)
+ if (!userfaultfd_is_initialized(ctx))
return -EINVAL;

for (;;) {
@@ -1908,9 +1903,10 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
static inline unsigned int uffd_ctx_features(__u64 user_features)
{
/*
- * For the current set of features the bits just coincide
+ * For the current set of features the bits just coincide. Set
+ * UFFD_FEATURE_INITIALIZED to mark the features as enabled.
*/
- return (unsigned int)user_features;
+ return (unsigned int)user_features | UFFD_FEATURE_INITIALIZED;
}

/*
@@ -1923,12 +1919,10 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
{
struct uffdio_api uffdio_api;
void __user *buf = (void __user *)arg;
+ unsigned int ctx_features;
int ret;
__u64 features;

- ret = -EINVAL;
- if (ctx->state != UFFD_STATE_WAIT_API)
- goto out;
ret = -EFAULT;
if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
goto out;
@@ -1952,9 +1946,13 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
ret = -EFAULT;
if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
goto out;
- ctx->state = UFFD_STATE_RUNNING;
+
/* only enable the requested features for this uffd context */
- ctx->features = uffd_ctx_features(features);
+ ctx_features = uffd_ctx_features(features);
+ ret = -EINVAL;
+ if (cmpxchg(&ctx->features, 0, ctx_features) != 0)
+ goto err_out;
+
ret = 0;
out:
return ret;
@@ -1971,7 +1969,7 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
int ret = -EINVAL;
struct userfaultfd_ctx *ctx = file->private_data;

- if (cmd != UFFDIO_API && ctx->state == UFFD_STATE_WAIT_API)
+ if (cmd != UFFDIO_API && !userfaultfd_is_initialized(ctx))
return -EINVAL;

switch(cmd) {
@@ -2085,7 +2083,6 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
refcount_set(&ctx->refcount, 1);
ctx->flags = flags;
ctx->features = 0;
- ctx->state = UFFD_STATE_WAIT_API;
ctx->released = false;
ctx->mmap_changing = false;
ctx->mm = current->mm;
--
2.30.2