On 9/19/23 5:43 AM, Yafang Shao wrote:
On Mon, Sep 18, 2023 at 7:52 PM Luis Gerhorst <gerhorst@xxxxxxxxx> wrote:
On 15/09/2023 04:26, Yafang Shao wrote:
On Wed, Sep 13, 2023 at 8:30 PM Luis Gerhorst <gerhorst@xxxxxxxxx> wrote:
This reverts commit d75e30dddf73449bc2d10bb8e2f1a2c446bc67a2.
To mitigate Spectre v1, the verifier relies on static analysis to deduct
constant pointer bounds, which can then be enforced by rewriting pointer
arithmetic [1] or index masking [2]. This relies on the fact that every
memory region to be accessed has a static upper bound and every date
below that bound is accessible. The verifier can only rewrite pointer
arithmetic or insert masking instructions to mitigate Spectre v1 if a
static upper bound, below of which every access is valid, can be given.
When allowing packet pointer comparisons, this introduces a way for the
program to effectively construct an accessible pointer for which no
static upper bound is known. Intuitively, this is obvious as a packet
might be of any size and therefore 0 is the only statically known upper
bound below of which every date is always accessible (i.e., none).
To clarify, the problem is not that comparing two pointers can be used
for pointer leaks in the same way in that comparing a pointer to a known
scalar can be used for pointer leaks. That is because the "secret"
components of the addresses cancel each other out if the pointers are
into the same region.
With [3] applied, the following malicious BPF program can be loaded into
the kernel without CAP_PERFMON:
r2 = *(u32 *)(r1 + 76) // data
r3 = *(u32 *)(r1 + 80) // data_end
r4 = r2
r4 += 1
if r4 > r3 goto exit
r5 = *(u8 *)(r2 + 0) // speculatively read secret
r5 &= 1 // choose bit to leak
// ... side channel to leak secret bit
exit:
// ...
This is jited to the following amd64 code which still contains the
gadget:
0: endbr64
4: nopl 0x0(%rax,%rax,1)
9: xchg %ax,%ax
b: push %rbp
c: mov %rsp,%rbp
f: endbr64
13: push %rbx
14: mov 0xc8(%rdi),%rsi // data
1b: mov 0x50(%rdi),%rdx // data_end
1f: mov %rsi,%rcx
22: add $0x1,%rcx
26: cmp %rdx,%rcx
29: ja 0x000000000000003f // branch to mispredict
2b: movzbq 0x0(%rsi),%r8 // speculative load of secret
30: and $0x1,%r8 // choose bit to leak
34: xor %ebx,%ebx
36: cmp %rbx,%r8
39: je 0x000000000000003f // branch based on secret
3b: imul $0x61,%r8,%r8 // leak using port contention side channel
3f: xor %eax,%eax
41: pop %rbx
42: leaveq
43: retq
Here I'm using a port contention side channel because storing the secret
to the stack causes the verifier to insert an lfence for unrelated
reasons (SSB mitigation) which would terminate the speculation.
As Daniel already pointed out to me, data_end is even attacker
controlled as one could send many packets of sufficient length to train
the branch prediction into assuming data_end >= data will never be true.
When the attacker then sends a packet with insufficient data, the
Spectre v1 gadget leaks the chosen bit of some value that lies behind
data_end.
To make it clear that the problem is not the pointer comparison but the
missing masking instruction, it can be useful to transform the code
above into the following equivalent pseudocode:
r2 = data
r3 = data_end
r6 = ... // index to access, constant does not help
r7 = data_end - data // only known at runtime, could be [0,PKT_MAX)
if !(r6 < r7) goto exit
// no masking of index in r6 happens
r2 += r6 // addr. to access
r5 = *(u8 *)(r2 + 0) // speculatively read secret
// ... leak secret as above
One idea to resolve this while still allowing for unprivileged packet
access would be to always allocate a power of 2 for the packet data and
then also pass the respective index mask in the skb structure. The
verifier would then have to check that this index mask is always applied
to the offset before a packet pointer is dereferenced. This patch does
not implement this extension, but only reverts [3].
Hi Luis,
The skb pointer comparison is a reasonable operation in a networking bpf prog.
If we just prohibit a reasonable operation to prevent a possible
spectre v1 attack, it looks a little weird, right ?
Should we figure out a real fix to prevent it ?
I see your point, but this has been the case since Spectre v1 was
mitigated for BPF. I actually did a few statistics on that in [1] and
>50 out of ~350 programs are rejected because of the Spectre v1
mitigations. However, to repeat: The operation is not completely
prohibited, only prohibited without CAP_PERFMON.
Maybe it would be possible to expose the allow_ptr_leaks/bpf_spec_vX
flags in sysfs? It would be helpful for debugging, and you could set it
to 1 permanently for your purposes. However, I'm not sure if the others
would like that...
I really appreciate that idea. I actually shared a similar concept earlier.[1].
Nonetheless, I believe it would be prudent to align with the system
settings regarding CPU security mitigations within the BPF subsystem
as well. In our production environment, we consistently configure it
with "mitigations=off"[2] to enhance performance, essentially
deactivating all optional CPU mitigations. Consequently, if we
implement a system-wide "mitigations=off" setting, it should also
inherently bypass Spectre v1 and Spectre v4 in the BPF subsystem.
Alexei, Daniel, any comments ?
Yes, I think that would be acceptable as a global override. At least I
don't see it would make anything worse if the rest of the system has
mitigations disabled anyway.
[1]. https://lore.kernel.org/bpf/CALOAHbDDT=paFEdTb1Jsqu7eGkFXAh6A+f21VTrMdAeq5F60kg@xxxxxxxxxxxxxx/
[2]. https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html
Another solution I have been working on in [2] is to change the
mitigations to insert an lfence instead of rejecting the program
entirely. That would have bad performance, but would still be better
than completely rejecting the program. However, these patches are far
from going upstream currently.
A detail: The patches in [2] currently do not support the case we are
discussing here, they only insert fences when the speculative paths fail
to verify.
[1]
https://sys.cs.fau.de/extern/person/gerhorst/23-03_fgbs-spring-2023-presentation.pdf
- Slide 9
[2]
https://gitlab.cs.fau.de/un65esoq/linux/-/commits/v6.5-rc6-bpf-spectre-nospec/
--
Luis
--
Regards
Yafang
