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

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

[Jann Horn]

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>
---
 kernel/bpf/verifier.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2e08f8e..8291251 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2029,7 +2029,6 @@ static int replace_map_fd_with_map_ptr(struct verifier_env *env)
 			if (IS_ERR(map)) {
 				verbose("fd %d is not pointing to valid bpf_map\n",
 					insn->imm);
-				fdput(f);
 				return PTR_ERR(map);
 			}
 
-- 
2.8.0.rc3.226.g39d4020

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

[Alexei Starovoitov]

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>

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

[Daniel Borkmann]

On 04/26/2016 10:26 PM, 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: Daniel Borkmann <daniel@iogearbox.net>

Thanks!

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

[David Miller]

From: Jann Horn <jannh@google.com>
Date: Tue, 26 Apr 2016 22:26:26 +0200

> 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>

Applied and queued up for -stable, thanks Jann.