[PATCH 4.14 08/45] bpf: fix panic due to oob in bpf_prog_test_run_skb

Greg Kroah-Hartman
2021-12-20T14:41:23Z

From: Daniel Borkmann <daniel@iogearbox.net>

commit 6e6fddc78323533be570873abb728b7e0ba7e024 upstream.

sykzaller triggered several panics similar to the below:

[...]
[ 248.851531] BUG: KASAN: use-after-free in _copy_to_user+0x5c/0x90
[ 248.857656] Read of size 985 at addr ffff8808017ffff2 by task a.out/1425
[...]
[ 248.865902] CPU: 1 PID: 1425 Comm: a.out Not tainted 4.18.0-rc4+ #13
[ 248.865903] Hardware name: Supermicro SYS-5039MS-H12TRF/X11SSE-F, BIOS 2.1a 03/08/2018
[ 248.865905] Call Trace:
[ 248.865910] dump_stack+0xd6/0x185
[ 248.865911] ? show_regs_print_info+0xb/0xb
[ 248.865913] ? printk+0x9c/0xc3
[ 248.865915] ? kmsg_dump_rewind_nolock+0xe4/0xe4
[ 248.865919] print_address_description+0x6f/0x270
[ 248.865920] kasan_report+0x25b/0x380
[ 248.865922] ? _copy_to_user+0x5c/0x90
[ 248.865924] check_memory_region+0x137/0x190
[ 248.865925] kasan_check_read+0x11/0x20
[ 248.865927] _copy_to_user+0x5c/0x90
[ 248.865930] bpf_test_finish.isra.8+0x4f/0xc0
[ 248.865932] bpf_prog_test_run_skb+0x6a0/0xba0
[...]

After scrubbing the BPF prog a bit from the noise, turns out it called
bpf_skb_change_head() for the lwt_xmit prog with headroom of 2. Nothing
wrong in that, however, this was run with repeat >> 0 in bpf_prog_test_run_skb()
and the same skb thus keeps changing until the pskb_expand_head() called
from skb_cow() keeps bailing out in atomic alloc context with -ENOMEM.
So upon return we'll basically have 0 headroom left yet blindly do the
__skb_push() of 14 bytes and keep copying data from there in bpf_test_finish()
out of bounds. Fix to check if we have enough headroom and if pskb_expand_head()
fails, bail out with error.

Another bug independent of this fix (but related in triggering above) is
that BPF_PROG_TEST_RUN should be reworked to reset the skb/xdp buffer to
it's original state from input as otherwise repeating the same test in a
loop won't work for benchmarking when underlying input buffer is getting
changed by the prog each time and reused for the next run leading to
unexpected results.

Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
Reported-by: syzbot+709412e651e55ed96498@syzkaller.appspotmail.com
Reported-by: syzbot+54f39d6ab58f39720a55@syzkaller.appspotmail.com
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
[connoro: drop test_verifier.c changes not applicable to 4.14]
Signed-off-by: Connor O'Brien <connoro@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/bpf/test_run.c | 17 ++++++++++++++---
tools/testing/selftests/bpf/test_verifier.c | 18 ++++++++++++++++++
2 files changed, 32 insertions(+), 3 deletions(-)

--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -96,6 +96,7 @@ int bpf_prog_test_run_skb(struct bpf_pro
u32 size = kattr->test.data_size_in;
u32 repeat = kattr->test.repeat;
u32 retval, duration;
+ int hh_len = ETH_HLEN;
struct sk_buff *skb;
void *data;
int ret;
@@ -131,12 +132,22 @@ int bpf_prog_test_run_skb(struct bpf_pro
skb_reset_network_header(skb);

if (is_l2)
- __skb_push(skb, ETH_HLEN);
+ __skb_push(skb, hh_len);
if (is_direct_pkt_access)
bpf_compute_data_end(skb);
retval = bpf_test_run(prog, skb, repeat, &duration);
- if (!is_l2)
- __skb_push(skb, ETH_HLEN);
+ if (!is_l2) {
+ if (skb_headroom(skb) < hh_len) {
+ int nhead = HH_DATA_ALIGN(hh_len - skb_headroom(skb));
+
+ if (pskb_expand_head(skb, nhead, 0, GFP_USER)) {
+ kfree_skb(skb);
+ return -ENOMEM;
+ }
+ }
+ memset(__skb_push(skb, hh_len), 0, hh_len);
+ }
+
size = skb->len;
/* bpf program can never convert linear skb to non-linear */
if (WARN_ON_ONCE(skb_is_nonlinear(skb)))
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/tools/testing/selftests/bpf/test_verifier.c
@@ -4335,6 +4335,24 @@ static struct bpf_test tests[] = {
.prog_type = BPF_PROG_TYPE_LWT_XMIT,
},
{
+ "make headroom for LWT_XMIT",
+ .insns = {
+ BPF_MOV64_REG(BPF_REG_6, BPF_REG_1),
+ BPF_MOV64_IMM(BPF_REG_2, 34),
+ BPF_MOV64_IMM(BPF_REG_3, 0),
+ BPF_EMIT_CALL(BPF_FUNC_skb_change_head),
+ /* split for s390 to succeed */
+ BPF_MOV64_REG(BPF_REG_1, BPF_REG_6),
+ BPF_MOV64_IMM(BPF_REG_2, 42),
+ BPF_MOV64_IMM(BPF_REG_3, 0),
+ BPF_EMIT_CALL(BPF_FUNC_skb_change_head),
+ BPF_MOV64_IMM(BPF_REG_0, 0),
+ BPF_EXIT_INSN(),
+ },
+ .result = ACCEPT,
+ .prog_type = BPF_PROG_TYPE_LWT_XMIT,
+ },
+ {
"invalid access of tc_classid for LWT_IN",
.insns = {
BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,