[PATCH v3] misc: fastrpc: Fix double free and races in fastrpc_req_mmap() error path

[Yao Yiqi]

fastrpc_req_mmap() links the newly allocated buffer into fl->mmaps before
copying the response back to userspace. If copy_to_user() fails, the
err_assign path calls fastrpc_req_munmap_impl() to unmap the buffer from
the DSP and, on success, to free it.

Since commit 6102ceb4eab8 ("misc: fastrpc: Remove buffer from list prior
to unmap operation"), fastrpc_req_munmap_impl() no longer removes the
buffer from fl->mmaps, as that is now the responsibility of the caller.
fastrpc_req_mmap() was not updated accordingly, so the buffer is freed
while still linked in the list. The fl->mmaps cleanup in
fastrpc_user_free() then calls list_del() and fastrpc_buf_free() on the
already freed buffer, resulting in a use-after-free and a double free.

Publishing the buffer before copy_to_user() returns is also racy:
another thread of the same process can read the DSP address from the
partially written user buffer (or via userfaultfd) and issue
FASTRPC_IOCTL_MUNMAP, which unlinks and frees the buffer while
fastrpc_req_mmap() is still using it.

Fix this by adding the buffer to fl->mmaps only after copy_to_user() has
succeeded, so the buffer is not visible to a concurrent unmap while the
response is still being copied. If unmapping the buffer fails on the
error path, fastrpc_buf_free() is not called, so re-add the buffer to
fl->mmaps to keep it tracked and let fastrpc_user_free() reclaim it on
release instead of leaking the DMA allocation.

Fixes: 6102ceb4eab8 ("misc: fastrpc: Remove buffer from list prior to unmap operation")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Yao Yiqi <yaoyiqi3@xxxxxxxxxx>
---
Changes in v3:
 - Re-add the buffer to fl->mmaps if unmapping it on the error path
   fails, since fastrpc_buf_free() is not called in that case and the
   DMA allocation would otherwise be leaked.

Changes in v2:
 - Link the buffer into fl->mmaps only after copy_to_user() succeeds.
   Besides fixing the double free, this also closes the concurrent
   FASTRPC_IOCTL_MUNMAP race, where a partially written user buffer let
   another thread unmap and free the buffer while fastrpc_req_mmap() was
   still using it.

 drivers/misc/fastrpc.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index d4fac2caca86..917a5dfe3774 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -2153,22 +2153,26 @@ static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
 		}
 	}
 
-	spin_lock(&fl->lock);
-	list_add_tail(&buf->node, &fl->mmaps);
-	spin_unlock(&fl->lock);
-
 	if (copy_to_user((void __user *)argp, &req, sizeof(req))) {
 		err = -EFAULT;
 		goto err_assign;
 	}
 
+	spin_lock(&fl->lock);
+	list_add_tail(&buf->node, &fl->mmaps);
+	spin_unlock(&fl->lock);
+
 	dev_dbg(dev, "mmap\t\tpt 0x%09lx OK [len 0x%08llx]\n",
 		buf->raddr, buf->size);
 
 	return 0;
 
 err_assign:
-	fastrpc_req_munmap_impl(fl, buf);
+	if (fastrpc_req_munmap_impl(fl, buf)) {
+		spin_lock(&fl->lock);
+		list_add_tail(&buf->node, &fl->mmaps);
+		spin_unlock(&fl->lock);
+	}
 
 	return err;
 }
-- 
2.34.1