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userfaultfd: prevent concurrent API initialization
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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: 9cd75c3 ("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>
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Nadav Amit authored and Linus Torvalds committed Sep 3, 2021
1 parent a759a90 commit 22e5fe2
Showing 1 changed file with 44 additions and 47 deletions.
91 changes: 44 additions & 47 deletions fs/userfaultfd.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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.
Expand Down Expand Up @@ -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 */
Expand Down Expand Up @@ -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)
{
Expand Down Expand Up @@ -667,7 +668,6 @@ 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;
atomic_set(&ctx->mmap_changing, 0);
Expand Down Expand Up @@ -944,38 +944,33 @@ 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;
Expand Down Expand Up @@ -1170,7 +1165,7 @@ 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 (;;) {
Expand Down Expand Up @@ -1909,9 +1904,10 @@ 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;
}

/*
Expand All @@ -1924,12 +1920,10 @@ 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;
Expand All @@ -1953,9 +1947,13 @@ 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;
Expand All @@ -1972,7 +1970,7 @@ 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) {
Expand Down Expand Up @@ -2086,7 +2084,6 @@ 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;
atomic_set(&ctx->mmap_changing, 0);
ctx->mm = current->mm;
Expand Down

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