linux-kernel - [PATCH 4.5 020/101] bpf: fix double-fdput in replace_map_fd_with_map

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Message-Id: <20160517011507.109626208@linuxfoundation.org>
Date:	Mon, 16 May 2016 18:20:25 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Jann Horn <jannh@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	"David S. Miller" <davem@...emloft.net>
Subject: [PATCH 4.5 020/101] bpf: fix double-fdput in replace_map_fd_with_map_ptr()

4.5-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@...gle.com>

[ Upstream commit 8358b02bf67d3a5d8a825070e1aa73f25fb2e4c7 ]

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>
Acked-by: Daniel Borkmann <daniel@...earbox.net>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/bpf/verifier.c |    1 -
 1 file changed, 1 deletion(-)

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2004,7 +2004,6 @@ static int replace_map_fd_with_map_ptr(s
 			if (IS_ERR(map)) {
 				verbose("fd %d is not pointing to valid bpf_map\n",
 					insn->imm);
-				fdput(f);
 				return PTR_ERR(map);
 			}