On 9/14/23 6:20 PM, Alexei Starovoitov wrote:
On Wed, Sep 13, 2023 at 5:30 AM Luis Gerhorst gerhorst@amazon.de 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: endbr6413: 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.
The above analysis is correct, but unlike traditional spec_v1 the attacker doesn't control data/data_end. The attack can send many large packets to train that data + X < data_end and then send a small packet where CPU will mispredict that branch and data + X will speculatively read past data_end, so the attacker can extract a bit past data_end, but data/data_end themselves cannot be controlled. So whether this bit 0 or 1 has no bearing. The attack cannot be repeated for the same location. The attacker can read one bit 8 times in a row and all of them will be from different locations in the memory. Same as reading 8 random bits from 8 random locations. Hence I don't think this revert is necessary. I don't believe you can craft an actual exploit.
Your patch 3 says: /* Speculative access to be prevented. */
char secret = *((char *) iph);/* Leak the first bit of the secret value that lies behind data_end to a* SMP silbling thread that also executes imul instructions. If the bit* is 1, the silbling will experience a slowdown. */long long x = secret;if (secret & 1) {x *= 97;}the comment is correct, but speculative access alone is not enough to leak data.
What you write makes sense, it will probably be hard to craft an exploit. Where it's a bit more of an unknown to me is whether struct skb_shared_info could have e.g. destructor_arg rather static (at last the upper addr bits) so that you would leak out kernel addresses.