[PATCH] Userfaultfd: Avoid double free of userfault_ctx and remove O_CLOEXEC

[Lokesh Gidra]

when get_unused_fd_flags returns error, ctx will be freed by
userfaultfd's release function, which is indirectly called by fput().
Also, if anon_inode_getfile_secure() returns an error, then
userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx.

Also, the O_CLOEXEC was inadvertently added to the call to
get_unused_fd_flags() [1].

Adding Al Viro's suggested-by, based on [2].

[1] https://lore.kernel.org/lkml/1f69c0ab-5791-974f-8bc0-3997ab1d61ea@xxxxxxxxxx/
[2] https://lore.kernel.org/lkml/20200719165746.GJ2786714@xxxxxxxxxxxxxxxxxx/

Fixes: d08ac70b1e0d (Wire UFFD up to SELinux)
Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Reported-by: syzbot+75867c44841cb6373570@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
---
 fs/userfaultfd.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index ae859161908f..e15eb8fdc083 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -2042,24 +2042,18 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
 		O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS),
 		NULL);
 	if (IS_ERR(file)) {
-		fd = PTR_ERR(file);
-		goto out;
+		userfaultfd_ctx_put(ctx);
+		return PTR_ERR(file);
 	}
 
-	fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC);
+	fd = get_unused_fd_flags(O_RDONLY);
 	if (fd < 0) {
 		fput(file);
-		goto out;
+		return fd;
 	}
 
 	ctx->owner = file_inode(file);
 	fd_install(fd, file);
-
-out:
-	if (fd < 0) {
-		mmdrop(ctx->mm);
-		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
-	}
 	return fd;
 }
 
-- 
2.28.0.163.g6104cc2f0b6-goog