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

Lokesh Gidra
2020-08-04T20:32:10Z

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@dancol.org/
[2] https://lore.kernel.org/lkml/20200719165746.GJ2786714@ZenIV.linux.org.uk/

Fixes: d08ac70b1e0d (Wire UFFD up to SELinux)
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: syzbot+75867c44841cb6373570@syzkaller.appspotmail.com
Signed-off-by: Lokesh Gidra <lokeshgidra@google.com>
---
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