From 51580e798cb61b0fc63fa3aa6c5c975375aa0550 Mon Sep 17 00:00:00 2001 From: Alexei Starovoitov Date: Fri, 26 Sep 2014 00:17:02 -0700 Subject: bpf: verifier (add docs) this patch adds all of eBPF verfier documentation and empty bpf_check() The end goal for the verifier is to statically check safety of the program. Verifier will catch: - loops - out of range jumps - unreachable instructions - invalid instructions - uninitialized register access - uninitialized stack access - misaligned stack access - out of range stack access - invalid calling convention More details in Documentation/networking/filter.txt Signed-off-by: Alexei Starovoitov Signed-off-by: David S. Miller --- Documentation/networking/filter.txt | 224 ++++++++++++++++++++++++++++++++++++ 1 file changed, 224 insertions(+) (limited to 'Documentation/networking/filter.txt') diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt index 4a01d71785e9..5ce4d07406a5 100644 --- a/Documentation/networking/filter.txt +++ b/Documentation/networking/filter.txt @@ -1001,6 +1001,99 @@ instruction that loads 64-bit immediate value into a dst_reg. Classic BPF has similar instruction: BPF_LD | BPF_W | BPF_IMM which loads 32-bit immediate value into a register. +eBPF verifier +------------- +The safety of the eBPF program is determined in two steps. + +First step does DAG check to disallow loops and other CFG validation. +In particular it will detect programs that have unreachable instructions. +(though classic BPF checker allows them) + +Second step starts from the first insn and descends all possible paths. +It simulates execution of every insn and observes the state change of +registers and stack. + +At the start of the program the register R1 contains a pointer to context +and has type PTR_TO_CTX. +If verifier sees an insn that does R2=R1, then R2 has now type +PTR_TO_CTX as well and can be used on the right hand side of expression. +If R1=PTR_TO_CTX and insn is R2=R1+R1, then R2=UNKNOWN_VALUE, +since addition of two valid pointers makes invalid pointer. +(In 'secure' mode verifier will reject any type of pointer arithmetic to make +sure that kernel addresses don't leak to unprivileged users) + +If register was never written to, it's not readable: + bpf_mov R0 = R2 + bpf_exit +will be rejected, since R2 is unreadable at the start of the program. + +After kernel function call, R1-R5 are reset to unreadable and +R0 has a return type of the function. + +Since R6-R9 are callee saved, their state is preserved across the call. + bpf_mov R6 = 1 + bpf_call foo + bpf_mov R0 = R6 + bpf_exit +is a correct program. If there was R1 instead of R6, it would have +been rejected. + +load/store instructions are allowed only with registers of valid types, which +are PTR_TO_CTX, PTR_TO_MAP, FRAME_PTR. They are bounds and alignment checked. +For example: + bpf_mov R1 = 1 + bpf_mov R2 = 2 + bpf_xadd *(u32 *)(R1 + 3) += R2 + bpf_exit +will be rejected, since R1 doesn't have a valid pointer type at the time of +execution of instruction bpf_xadd. + +At the start R1 type is PTR_TO_CTX (a pointer to generic 'struct bpf_context') +A callback is used to customize verifier to restrict eBPF program access to only +certain fields within ctx structure with specified size and alignment. + +For example, the following insn: + bpf_ld R0 = *(u32 *)(R6 + 8) +intends to load a word from address R6 + 8 and store it into R0 +If R6=PTR_TO_CTX, via is_valid_access() callback the verifier will know +that offset 8 of size 4 bytes can be accessed for reading, otherwise +the verifier will reject the program. +If R6=FRAME_PTR, then access should be aligned and be within +stack bounds, which are [-MAX_BPF_STACK, 0). In this example offset is 8, +so it will fail verification, since it's out of bounds. + +The verifier will allow eBPF program to read data from stack only after +it wrote into it. +Classic BPF verifier does similar check with M[0-15] memory slots. +For example: + bpf_ld R0 = *(u32 *)(R10 - 4) + bpf_exit +is invalid program. +Though R10 is correct read-only register and has type FRAME_PTR +and R10 - 4 is within stack bounds, there were no stores into that location. + +Pointer register spill/fill is tracked as well, since four (R6-R9) +callee saved registers may not be enough for some programs. + +Allowed function calls are customized with bpf_verifier_ops->get_func_proto() +The eBPF verifier will check that registers match argument constraints. +After the call register R0 will be set to return type of the function. + +Function calls is a main mechanism to extend functionality of eBPF programs. +Socket filters may let programs to call one set of functions, whereas tracing +filters may allow completely different set. + +If a function made accessible to eBPF program, it needs to be thought through +from safety point of view. The verifier will guarantee that the function is +called with valid arguments. + +seccomp vs socket filters have different security restrictions for classic BPF. +Seccomp solves this by two stage verifier: classic BPF verifier is followed +by seccomp verifier. In case of eBPF one configurable verifier is shared for +all use cases. + +See details of eBPF verifier in kernel/bpf/verifier.c + eBPF maps --------- 'maps' is a generic storage of different types for sharing data between kernel @@ -1040,6 +1133,137 @@ The map is defined by: . key size in bytes . value size in bytes +Understanding eBPF verifier messages +------------------------------------ + +The following are few examples of invalid eBPF programs and verifier error +messages as seen in the log: + +Program with unreachable instructions: +static struct bpf_insn prog[] = { + BPF_EXIT_INSN(), + BPF_EXIT_INSN(), +}; +Error: + unreachable insn 1 + +Program that reads uninitialized register: + BPF_MOV64_REG(BPF_REG_0, BPF_REG_2), + BPF_EXIT_INSN(), +Error: + 0: (bf) r0 = r2 + R2 !read_ok + +Program that doesn't initialize R0 before exiting: + BPF_MOV64_REG(BPF_REG_2, BPF_REG_1), + BPF_EXIT_INSN(), +Error: + 0: (bf) r2 = r1 + 1: (95) exit + R0 !read_ok + +Program that accesses stack out of bounds: + BPF_ST_MEM(BPF_DW, BPF_REG_10, 8, 0), + BPF_EXIT_INSN(), +Error: + 0: (7a) *(u64 *)(r10 +8) = 0 + invalid stack off=8 size=8 + +Program that doesn't initialize stack before passing its address into function: + BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), + BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), + BPF_LD_MAP_FD(BPF_REG_1, 0), + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), + BPF_EXIT_INSN(), +Error: + 0: (bf) r2 = r10 + 1: (07) r2 += -8 + 2: (b7) r1 = 0x0 + 3: (85) call 1 + invalid indirect read from stack off -8+0 size 8 + +Program that uses invalid map_fd=0 while calling to map_lookup_elem() function: + BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), + BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), + BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), + BPF_LD_MAP_FD(BPF_REG_1, 0), + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), + BPF_EXIT_INSN(), +Error: + 0: (7a) *(u64 *)(r10 -8) = 0 + 1: (bf) r2 = r10 + 2: (07) r2 += -8 + 3: (b7) r1 = 0x0 + 4: (85) call 1 + fd 0 is not pointing to valid bpf_map + +Program that doesn't check return value of map_lookup_elem() before accessing +map element: + BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), + BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), + BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), + BPF_LD_MAP_FD(BPF_REG_1, 0), + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), + BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0), + BPF_EXIT_INSN(), +Error: + 0: (7a) *(u64 *)(r10 -8) = 0 + 1: (bf) r2 = r10 + 2: (07) r2 += -8 + 3: (b7) r1 = 0x0 + 4: (85) call 1 + 5: (7a) *(u64 *)(r0 +0) = 0 + R0 invalid mem access 'map_value_or_null' + +Program that correctly checks map_lookup_elem() returned value for NULL, but +accesses the memory with incorrect alignment: + BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), + BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), + BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), + BPF_LD_MAP_FD(BPF_REG_1, 0), + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), + BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1), + BPF_ST_MEM(BPF_DW, BPF_REG_0, 4, 0), + BPF_EXIT_INSN(), +Error: + 0: (7a) *(u64 *)(r10 -8) = 0 + 1: (bf) r2 = r10 + 2: (07) r2 += -8 + 3: (b7) r1 = 1 + 4: (85) call 1 + 5: (15) if r0 == 0x0 goto pc+1 + R0=map_ptr R10=fp + 6: (7a) *(u64 *)(r0 +4) = 0 + misaligned access off 4 size 8 + +Program that correctly checks map_lookup_elem() returned value for NULL and +accesses memory with correct alignment in one side of 'if' branch, but fails +to do so in the other side of 'if' branch: + BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), + BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), + BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), + BPF_LD_MAP_FD(BPF_REG_1, 0), + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), + BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2), + BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0), + BPF_EXIT_INSN(), + BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 1), + BPF_EXIT_INSN(), +Error: + 0: (7a) *(u64 *)(r10 -8) = 0 + 1: (bf) r2 = r10 + 2: (07) r2 += -8 + 3: (b7) r1 = 1 + 4: (85) call 1 + 5: (15) if r0 == 0x0 goto pc+2 + R0=map_ptr R10=fp + 6: (7a) *(u64 *)(r0 +0) = 0 + 7: (95) exit + + from 5 to 8: R0=imm0 R10=fp + 8: (7a) *(u64 *)(r0 +0) = 1 + R0 invalid mem access 'imm' + Testing ------- -- cgit v1.2.3-59-g8ed1b