Re: [PATCH] bpf: fix double-fdput in replace_map_fd_with_map_ptr()

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Apr 26, 2016 at 10:26:26PM +0200, Jann Horn wrote:
> When bpf(BPF_PROG_LOAD, ...) was invoked with a BPF program whose bytecode
> references a non-map file descriptor as a map file descriptor, the error
> handling code called fdput() twice instead of once (in __bpf_map_get() and
> in replace_map_fd_with_map_ptr()). If the file descriptor table of the
> current task is shared, this causes f_count to be decremented too much,
> allowing the struct file to be freed while it is still in use
> (use-after-free). This can be exploited to gain root privileges by an
> unprivileged user.
> 
> This bug was introduced in
> commit 0246e64d9a5f ("bpf: handle pseudo BPF_LD_IMM64 insn"), but is only
> exploitable since
> commit 1be7f75d1668 ("bpf: enable non-root eBPF programs") because
> previously, CAP_SYS_ADMIN was required to reach the vulnerable code.
> 
> (posted publicly according to request by maintainer)
> 
> Signed-off-by: Jann Horn <jannh@google.com>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Acked-by: Alexei Starovoitov <ast@kernel.org>