If we look up the kbuf, ensure that it doesn't get unregistered until
after we're done with it. Since we're inside mmap, we cannot safely use
the io_uring lock. Rely on the fact that we can lookup the buffer list
under RCU now and grab a reference to it, preventing it from being
unregistered until we're done with it. The lookup returns the
io_buffer_list directly with it referenced.
Cc: stable@xxxxxxxxxxxxxxx # v6.4+
Fixes: 5cf4f52e6d8a ("io_uring: free io_buffer_list entries via RCU")
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
---
io_uring/io_uring.c | 11 ++++++-----
io_uring/kbuf.c | 35 +++++++++++++++++++++++++++--------
io_uring/kbuf.h | 4 +++-
3 files changed, 36 insertions(+), 14 deletions(-)
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index bc730f59265f..4521c2b66b98 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -3447,14 +3447,15 @@ static void *io_uring_validate_mmap_request(struct file *file,
ptr = ctx->sq_sqes;
break;
case IORING_OFF_PBUF_RING: {
+ struct io_buffer_list *bl;
unsigned int bgid;
bgid = (offset & ~IORING_OFF_MMAP_MASK) >> IORING_OFF_PBUF_SHIFT;
- rcu_read_lock();
- ptr = io_pbuf_get_address(ctx, bgid);
- rcu_read_unlock();
- if (!ptr)
- return ERR_PTR(-EINVAL);
+ bl = io_pbuf_get_bl(ctx, bgid);
+ if (IS_ERR(bl))
+ return bl;
+ ptr = bl->buf_ring;
+ io_put_bl(ctx, bl);
break;
}
default:
diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
index 2edc6854f6f3..3aa16e27f509 100644
--- a/io_uring/kbuf.c
+++ b/io_uring/kbuf.c
@@ -266,7 +266,7 @@ static int __io_remove_buffers(struct io_ring_ctx *ctx,
return i;
}
-static void io_put_bl(struct io_ring_ctx *ctx, struct io_buffer_list *bl)
+void io_put_bl(struct io_ring_ctx *ctx, struct io_buffer_list *bl)
{
if (atomic_dec_and_test(&bl->refs)) {
__io_remove_buffers(ctx, bl, -1U);
@@ -719,16 +719,35 @@ int io_register_pbuf_status(struct io_ring_ctx *ctx, void __user *arg)
return 0;
}
-void *io_pbuf_get_address(struct io_ring_ctx *ctx, unsigned long bgid)
+struct io_buffer_list *io_pbuf_get_bl(struct io_ring_ctx *ctx,
+ unsigned long bgid)
{
struct io_buffer_list *bl;
+ bool ret;
- bl = __io_buffer_get_list(ctx, bgid);
-
- if (!bl || !bl->is_mmap)
- return NULL;
-
- return bl->buf_ring;
+ /*
+ * We have to be a bit careful here - we're inside mmap and cannot grab
+ * the uring_lock. This means the buffer_list could be simultaneously
+ * going away, if someone is trying to be sneaky. Look it up under rcu
+ * so we know it's not going away, and attempt to grab a reference to
+ * it. If the ref is already zero, then fail the mapping. If successful,
+ * the caller will call io_put_bl() to drop the the reference at at the
+ * end. This may then safely free the buffer_list (and drop the pages)
+ * at that point, vm_insert_pages() would've already grabbed the
+ * necessary vma references.
+ */
+ rcu_read_lock();
+ bl = xa_load(&ctx->io_bl_xa, bgid);
+ /* must be a mmap'able buffer ring and have pages */
+ ret = false;
+ if (bl && bl->is_mmap)
+ ret = atomic_inc_not_zero(&bl->refs);
+ rcu_read_unlock();
+
+ if (ret)
+ return bl;
+
+ return ERR_PTR(-EINVAL);
}
/*
diff --git a/io_uring/kbuf.h b/io_uring/kbuf.h
index 8b868a1744e2..df365b8860cf 100644
--- a/io_uring/kbuf.h
+++ b/io_uring/kbuf.h
@@ -61,7 +61,9 @@ void __io_put_kbuf(struct io_kiocb *req, unsigned issue_flags);
bool io_kbuf_recycle_legacy(struct io_kiocb *req, unsigned issue_flags);
-void *io_pbuf_get_address(struct io_ring_ctx *ctx, unsigned long bgid);
+void io_put_bl(struct io_ring_ctx *ctx, struct io_buffer_list *bl);
+struct io_buffer_list *io_pbuf_get_bl(struct io_ring_ctx *ctx,
+ unsigned long bgid);
static inline bool io_kbuf_recycle_ring(struct io_kiocb *req)
{
--
2.43.0
