linux/kernel/bpf/verifier.c, branch v4.15

bpf: mark dst unknown on inconsistent {s, u}bounds adjustments

2018-01-18T00:23:17Z

syzkaller generated a BPF proglet and triggered a warning with the following: 0: (b7) r0 = 0 1: (d5) if r0 s<= 0x0 goto pc+0 R0=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0 2: (1f) r0 -= r1 R0=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0 verifier internal error: known but bad sbounds What happens is that in the first insn, r0's min/max value are both 0 due to the immediate assignment, later in the jsle test the bounds are updated for the min value in the false path, meaning, they yield smin_val = 1, smax_val = 0, and when ctx pointer is subtracted from r0, verifier bails out with the internal error and throwing a WARN since smin_val != smax_val for the known constant. For min_val > max_val scenario it means that reg_set_min_max() and reg_set_min_max_inv() (which both refine existing bounds) demonstrated that such branch cannot be taken at runtime. In above scenario for the case where it will be taken, the existing [0, 0] bounds are kept intact. Meaning, the rejection is not due to a verifier internal error, and therefore the WARN() is not necessary either. We could just reject such cases in adjust_{ptr,scalar}_min_max_vals() when either known scalars have smin_val != smax_val or umin_val != umax_val or any scalar reg with bounds smin_val > smax_val or umin_val > umax_val. However, there may be a small risk of breakage of buggy programs, so handle this more gracefully and in adjust_{ptr,scalar}_min_max_vals() just taint the dst reg as unknown scalar when we see ops with such kind of src reg. Reported-by: syzbot+6d362cadd45dc0a12ba4@syzkaller.appspotmail.com Signed-off-by: Daniel Borkmann Signed-off-by: Alexei Starovoitov

bpf: reject stores into ctx via st and xadd

2018-01-16T23:04:58Z

Alexei found that verifier does not reject stores into context via BPF_ST instead of BPF_STX. And while looking at it, we also should not allow XADD variant of BPF_STX. The context rewriter is only assuming either BPF_LDX_MEM- or BPF_STX_MEM-type operations, thus reject anything other than that so that assumptions in the rewriter properly hold. Add test cases as well for BPF selftests. Fixes: d691f9e8d440 ("bpf: allow programs to write to certain skb fields") Reported-by: Alexei Starovoitov Signed-off-by: Daniel Borkmann Signed-off-by: Alexei Starovoitov

bpf: fix 32-bit divide by zero

2018-01-14T22:05:33Z

due to some JITs doing if (src_reg == 0) check in 64-bit mode for div/mod operations mask upper 32-bits of src register before doing the check Fixes: 622582786c9e ("net: filter: x86: internal BPF JIT") Fixes: 7a12b5031c6b ("sparc64: Add eBPF JIT.") Reported-by: syzbot+48340bb518e88849e2e3@syzkaller.appspotmail.com Signed-off-by: Alexei Starovoitov Signed-off-by: Daniel Borkmann

bpf: arsh is not supported in 32 bit alu thus reject it

2018-01-10T22:42:22Z

The following snippet was throwing an 'unknown opcode cc' warning in BPF interpreter: 0: (18) r0 = 0x0 2: (7b) *(u64 *)(r10 -16) = r0 3: (cc) (u32) r0 s>>= (u32) r0 4: (95) exit Although a number of JITs do support BPF_ALU | BPF_ARSH | BPF_{K,X} generation, not all of them do and interpreter does neither. We can leave existing ones and implement it later in bpf-next for the remaining ones, but reject this properly in verifier for the time being. Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)") Reported-by: syzbot+93c4904c5c70348a6890@syzkaller.appspotmail.com Signed-off-by: Daniel Borkmann Signed-off-by: Alexei Starovoitov

bpf: fix spelling mistake: "obusing" -> "abusing"

2018-01-10T22:32:59Z

Trivial fix to spelling mistake in error message text. Signed-off-by: Colin Ian King Signed-off-by: Alexei Starovoitov

bpf: prevent out-of-bounds speculation

2018-01-08T23:53:49Z

Under speculation, CPUs may mis-predict branches in bounds checks. Thus, memory accesses under a bounds check may be speculated even if the bounds check fails, providing a primitive for building a side channel. To avoid leaking kernel data round up array-based maps and mask the index after bounds check, so speculated load with out of bounds index will load either valid value from the array or zero from the padded area. Unconditionally mask index for all array types even when max_entries are not rounded to power of 2 for root user. When map is created by unpriv user generate a sequence of bpf insns that includes AND operation to make sure that JITed code includes the same 'index & index_mask' operation. If prog_array map is created by unpriv user replace bpf_tail_call(ctx, map, index); with if (index >= max_entries) { index &= map->index_mask; bpf_tail_call(ctx, map, index); } (along with roundup to power 2) to prevent out-of-bounds speculation. There is secondary redundant 'if (index >= max_entries)' in the interpreter and in all JITs, but they can be optimized later if necessary. Other array-like maps (cpumap, devmap, sockmap, perf_event_array, cgroup_array) cannot be used by unpriv, so no changes there. That fixes bpf side of "Variant 1: bounds check bypass (CVE-2017-5753)" on all architectures with and without JIT. v2->v3: Daniel noticed that attack potentially can be crafted via syscall commands without loading the program, so add masking to those paths as well. Signed-off-by: Alexei Starovoitov Acked-by: John Fastabend Signed-off-by: Daniel Borkmann

bpf: do not allow root to mangle valid pointers

2017-12-21T01:26:29Z

Do not allow root to convert valid pointers into unknown scalars. In particular disallow: ptr &= reg ptr <<= reg ptr += ptr and explicitly allow: ptr -= ptr since pkt_end - pkt == length 1. This minimizes amount of address leaks root can do. In the future may need to further tighten the leaks with kptr_restrict. 2. If program has such pointer math it's likely a user mistake and when verifier complains about it right away instead of many instructions later on invalid memory access it's easier for users to fix their progs. 3. when register holding a pointer cannot change to scalar it allows JITs to optimize better. Like 32-bit archs could use single register for pointers instead of a pair required to hold 64-bit scalars. 4. reduces architecture dependent behavior. Since code: r1 = r10; r1 &= 0xff; if (r1 ...) will behave differently arm64 vs x64 and offloaded vs native. A significant chunk of ptr mangling was allowed by commit f1174f77b50c ("bpf/verifier: rework value tracking") yet some of it was allowed even earlier. Signed-off-by: Alexei Starovoitov Signed-off-by: Daniel Borkmann

bpf: fix integer overflows

2017-12-21T01:15:41Z

There were various issues related to the limited size of integers used in the verifier: - `off + size` overflow in __check_map_access() - `off + reg->off` overflow in check_mem_access() - `off + reg->var_off.value` overflow or 32-bit truncation of `reg->var_off.value` in check_mem_access() - 32-bit truncation in check_stack_boundary() Make sure that any integer math cannot overflow by not allowing pointer math with large values. Also reduce the scope of "scalar op scalar" tracking. Fixes: f1174f77b50c ("bpf/verifier: rework value tracking") Reported-by: Jann Horn Signed-off-by: Alexei Starovoitov Signed-off-by: Daniel Borkmann

bpf: don't prune branches when a scalar is replaced with a pointer

2017-12-21T01:15:41Z

This could be made safe by passing through a reference to env and checking for env->allow_ptr_leaks, but it would only work one way and is probably not worth the hassle - not doing it will not directly lead to program rejection. Fixes: f1174f77b50c ("bpf/verifier: rework value tracking") Signed-off-by: Jann Horn Signed-off-by: Alexei Starovoitov Signed-off-by: Daniel Borkmann

bpf: force strict alignment checks for stack pointers

2017-12-21T01:15:41Z

Force strict alignment checks for stack pointers because the tracking of stack spills relies on it; unaligned stack accesses can lead to corruption of spilled registers, which is exploitable. Fixes: f1174f77b50c ("bpf/verifier: rework value tracking") Signed-off-by: Jann Horn Signed-off-by: Alexei Starovoitov Signed-off-by: Daniel Borkmann