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


From: Jann Horn <ja...@google.com>

[ Upstream commit 6ab405114b0b229151ef06f4e31c7834dd09d0c0 ]

Check whether inputs from userspace are too long (explicit length field too
big or string not null-terminated) to avoid out-of-bounds reads.

As far as I can tell, this can at worst lead to very limited kernel heap
memory disclosure or oopses.

This bug can be triggered by an unprivileged user even if the xt_bpf module
is not loaded: iptables is available in network namespaces, and the xt_bpf
module can be autoloaded.

Triggering the bug with a classic BPF filter with fake length 0x1000 causes
the following KASAN report:

BUG: KASAN: slab-out-of-bounds in bpf_prog_create+0x84/0xf0
Read of size 32768 at addr ffff8801eff2c494 by task test/4627

CPU: 0 PID: 4627 Comm: test Not tainted 4.15.0-rc1+ #1
Call Trace:
 ? bpf_prog_create+0x84/0xf0
 bpf_mt_check+0x90/0xd6 [xt_bpf]
Allocated by task 4627:
 xt_alloc_table_info+0x41/0x70 [x_tables]
The buggy address belongs to the object at ffff8801eff2c3c0
                which belongs to the cache kmalloc-2048 of size 2048
The buggy address is located 212 bytes inside of
                2048-byte region [ffff8801eff2c3c0, ffff8801eff2cbc0)

Fixes: e6f30c731718 ("netfilter: x_tables: add xt_bpf match")
Signed-off-by: Jann Horn <ja...@google.com>
Signed-off-by: Pablo Neira Ayuso <pa...@netfilter.org>
Signed-off-by: Sasha Levin <alexander.le...@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
 net/netfilter/xt_bpf.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/net/netfilter/xt_bpf.c
+++ b/net/netfilter/xt_bpf.c
@@ -27,6 +27,9 @@ static int __bpf_mt_check_bytecode(struc
        struct sock_fprog_kern program;
+       if (len > XT_BPF_MAX_NUM_INSTR)
+               return -EINVAL;
        program.len = len;
        program.filter = insns;
@@ -55,6 +58,9 @@ static int __bpf_mt_check_path(const cha
        mm_segment_t oldfs = get_fs();
        int retval, fd;
+       if (strnlen(path, XT_BPF_PATH_MAX) == XT_BPF_PATH_MAX)
+               return -EINVAL;
        fd = bpf_obj_get_user(path);

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