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bpf: fix panic due to oob in bpf_prog_test_run_skb

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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: 1cf1cae963 ("bpf: introduce BPF_PROG_TEST_RUN command")
Reported-by: [email protected]
Reported-by: [email protected]
Signed-off-by: Daniel Borkmann <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
This commit is contained in:
Daniel Borkmann 2018-07-11 15:30:14 +02:00 • committed by Alexei Starovoitov
commit 6e6fddc783
2 changed files with 36 additions and 4 deletions
Download patch file Download diff file Expand all files Collapse all files

17
net/bpf/test_run.c
View file

@ -96,6 +96,7 @@ int bpf_prog_test_run_skb(struct bpf_prog *prog, const union bpf_attr *kattr,
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_prog *prog, const union bpf_attr *kattr,
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_pointers(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)))