On Thu, Nov 30, 2017 at 7:46 PM, Jann Horn <ja...@google.com> wrote:
> 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 <ja...@google.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 041da0d9c06f..1f7fbd3c7e5a 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;
Next, this calls bpf_prog_create, which calls bpf_check_basics_ok to verify len.
> @@ -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;
> +
Good catch. It looks like this code needs a more thorough revision.
https://lkml.kernel.org/r/<20171201034859.gn21...@zeniv.linux.org.uk>
