netdev - Re: [PATCH] bpf: fix double-fdput in replace_map_fd_with_map

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Message-ID: <20160426204444.GA45613@ast-mbp.thefacebook.com>
Date:	Tue, 26 Apr 2016 13:44:46 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Jann Horn <jannh@...gle.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] bpf: fix double-fdput in replace_map_fd_with_map_ptr()

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@...gle.com>
> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>

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