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Subject[PATCH AUTOSEL for 4.14 079/100] netfilter: xt_bpf: add overflow checks
Date
From: Jann Horn <jannh@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:
dump_stack+0x5c/0x85
print_address_description+0x6a/0x260
kasan_report+0x254/0x370
? bpf_prog_create+0x84/0xf0
memcpy+0x1f/0x50
bpf_prog_create+0x84/0xf0
bpf_mt_check+0x90/0xd6 [xt_bpf]
[...]
Allocated by task 4627:
kasan_kmalloc+0xa0/0xd0
__kmalloc_node+0x47/0x60
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 <jannh@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
net/netfilter/xt_bpf.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/net/netfilter/xt_bpf.c b/net/netfilter/xt_bpf.c
index 29123934887b..5185ff0f8f58 100644
--- a/net/netfilter/xt_bpf.c
+++ b/net/netfilter/xt_bpf.c
@@ -27,6 +27,9 @@ static int __bpf_mt_check_bytecode(struct sock_filter *insns, __u16 len,
{
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 char *path, struct bpf_prog **ret)
mm_segment_t oldfs = get_fs();
int retval, fd;

+ if (strnlen(path, XT_BPF_PATH_MAX) == XT_BPF_PATH_MAX)
+ return -EINVAL;
+
set_fs(KERNEL_DS);
fd = bpf_obj_get_user(path);
set_fs(oldfs);
--
2.11.0
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