Messages in this thread | | | Date | Fri, 17 Jul 2020 07:45:32 -0400 | From | Peilin Ye <> | Subject | Reporting a use-after-free read bug in userfaultfd_release() |
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Hi all,
Syzbot reported the following use-after-free bug in userfaultfd_release():
https://syzkaller.appspot.com/bug?id=4b9e5aea757b678d9939c364e50212354a3480a6
It seems to be caused by this patch. I took a look at the stack trace. In the patch:
fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC); if (fd < 0) { fput(file); goto out; }
If get_unused_fd_flags() fails, `ctx` is freed. Later however, before returning back to userland, userfaultfd_release() is called and tries to use `ctx` again, causing a use-after-free bug.
The syzbot reproducer does a setrlimit() then a userfaultfd(). The former sets a hard limit on number of open files to zero, which causes get_unused_fd_flags() to fail.
Thank you,
Peilin Ye
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