Hi, I constructed a BPF prog that can pass the verifier but contains null-ptr-deref. This is achieved by exploiting the following behavior in the verifier: 1. verifier allows direct struct bpf_map access but does not mark the pointer (PTR_TO_BTF_ID) as maybe_null 2. verifier allows pointer CMP in JMP, and marks reg as non-null if one of the pointers is not null in the corresponding path. The following crash can be reproduced on: HEAD commit: 2d14123617f9 Merge branch 'Document some recent core kfunc additions' git tree: bpf-next console log: https://pastebin.com/raw/UkZXqdqs kernel config: https://pastebin.com/raw/BbzHXhkV POC: https://pastebin.com/raw/XFHVa3xV Essentially, the program in POC creates a MAP_HASH, then loads and run the following BPF prog: BPF_LD_MAP_FD(BPF_REG_1, map_fd), // verifier believe R6 is ptr_to_btf_id, after this ldx // but R6 = bpf_map->inner_map_meta, which is NULL, // because we don't have inner map BPF_LDX_MEM(BPF_DW, BPF_REG_6, BPF_REG_1, 8), BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4), BPF_ST_MEM(BPF_W, BPF_REG_2, 0, 0), BPF_CALL_FUNC(BPF_FUNC_map_lookup_elem), // verifier think R0 is map_value_or_null, R6 is ptr_btf_id // at runtime, they all equal to NULL BPF_JMP_REG(BPF_JEQ, BPF_REG_6, BPF_REG_0, 1), BPF_EXIT_INSN(), // here, verifier think R0 this map_value, but it is NULL BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0), BPF_EXIT_INSN(), After loading and running prog, the following is reported: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] CPU: 7 PID: 6781 Comm: a.out Not tainted 6.1.0-rc7-01517-g2d14123617f9 #125 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.1-1-1 04/01/2014 RIP: 0010:___bpf_prog_run+0x66d/0x8fd0 kernel/bpf/core.c:1937 Code: e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 e6 78 00 00 48 b8 00 00 00 00 00 fc ff df 4d 0f bf 66 02 4d 01 ec 4c 89 e2 48 c1 ea 03 <0f> b6 14 02 4c 89 e0 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 a7 RSP: 0018:ffffc900043377e8 EFLAGS: 00010256 RAX: dffffc0000000000 RBX: dffffc0000000000 RCX: 0000000000000000 RDX: 0000000000000000 RSI: ffffffff818efa7c RDI: ffffc90001146092 RBP: ffffc900043378a0 R08: 0000000000000006 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000 R13: 0000000000000000 R14: ffffc90001146090 R15: 0000000000000000 FS: 00007f68c01f6440(0000) GS:ffff88823bd80000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000563424ffa008 CR3: 000000001fd2a000 CR4: 0000000000750ee0 PKRU: 55555554 Call Trace: <TASK> __bpf_prog_run32+0x9d/0xe0 kernel/bpf/core.c:2045 bpf_dispatcher_nop_func include/linux/bpf.h:1082 [inline] __bpf_prog_run include/linux/filter.h:600 [inline] bpf_prog_run include/linux/filter.h:607 [inline] bpf_test_run+0x38e/0x980 net/bpf/test_run.c:402 bpf_prog_test_run_skb+0xb69/0x1dd0 net/bpf/test_run.c:1187 bpf_prog_test_run kernel/bpf/syscall.c:3644 [inline] __sys_bpf+0x1293/0x5840 kernel/bpf/syscall.c:4997 __do_sys_bpf kernel/bpf/syscall.c:5083 [inline] __se_sys_bpf kernel/bpf/syscall.c:5081 [inline] __x64_sys_bpf+0x78/0xc0 kernel/bpf/syscall.c:5081 do_syscall_x64 arch/x86/entry/common.c:50 [inline] do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x63/0xcd RIP: 0033:0x7f68bfae4469 Code: 00 f3 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d ff 49 2b 00 f7 d8 64 89 01 48 RSP: 002b:00007fff992ab838 EFLAGS: 00000246 ORIG_RAX: 0000000000000141 RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f68bfae4469 RDX: 0000000000000090 RSI: 00007fff992ac940 RDI: 000000000000000a RBP: 00007fff992ac9e0 R08: 000000000000000a R09: 0000000300001118 R10: 0000000000000001 R11: 0000000000000246 R12: 00005634238006f0 R13: 00007fff992acac0 R14: 0000000000000000 R15: 0000000000000000 </TASK> Modules linked in: ---[ end trace 0000000000000000 ]--- RIP: 0010:___bpf_prog_run+0x66d/0x8fd0 kernel/bpf/core.c:1937 Code: e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 e6 78 00 00 48 b8 00 00 00 00 00 fc ff df 4d 0f bf 66 02 4d 01 ec 4c 89 e2 48 c1 ea 03 <0f> b6 14 02 4c 89 e0 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 a7 RSP: 0018:ffffc900043377e8 EFLAGS: 00010256 RAX: dffffc0000000000 RBX: dffffc0000000000 RCX: 0000000000000000 RDX: 0000000000000000 RSI: ffffffff818efa7c RDI: ffffc90001146092 RBP: ffffc900043378a0 R08: 0000000000000006 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000 R13: 0000000000000000 R14: ffffc90001146090 R15: 0000000000000000 FS: 00007f68c01f6440(0000) GS:ffff88823bd80000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000563424ffa008 CR3: 000000001fd2a000 CR4: 0000000000750ee0 PKRU: 55555554 ---------------- Code disassembly (best guess): 0: e0 07 loopne 0x9 2: 83 c0 01 add $0x1,%eax 5: 38 d0 cmp %dl,%al 7: 7c 08 jl 0x11 9: 84 d2 test %dl,%dl b: 0f 85 e6 78 00 00 jne 0x78f7 11: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax 18: fc ff df 1b: 4d 0f bf 66 02 movswq 0x2(%r14),%r12 20: 4d 01 ec add %r13,%r12 23: 4c 89 e2 mov %r12,%rdx 26: 48 c1 ea 03 shr $0x3,%rdx * 2a: 0f b6 14 02 movzbl (%rdx,%rax,1),%edx <-- trapping instruction 2e: 4c 89 e0 mov %r12,%rax 31: 83 e0 07 and $0x7,%eax 34: 83 c0 03 add $0x3,%eax 37: 38 d0 cmp %dl,%al 39: 7c 08 jl 0x43 3b: 84 d2 test %dl,%dl 3d: 0f .byte 0xf 3e: 85 .byte 0x85 3f: a7 cmpsl %es:(%rdi),%ds:(%rsi)
