From 59ee4129a279070d8e2f9dc1660330f6593c7808 Mon Sep 17 00:00:00 2001 From: Daniel Borkmann Date: Tue, 10 Jul 2018 00:43:22 +0200 Subject: [PATCH] bpf: fix ldx in ld_abs rewrite for large offsets Mark reported that syzkaller triggered a KASAN detected slab-out-of-bounds bug in ___bpf_prog_run() with a BPF_LD | BPF_ABS word load at offset 0x8001. After further investigation it became clear that the issue was the BPF_LDX_MEM() which takes offset as an argument whereas it cannot encode larger than S16_MAX offsets into it. For this synthetical case we need to move the full address into tmp register instead and do the LDX without immediate value. Fixes: e0cea7ce988c ("bpf: implement ld_abs/ld_ind in native bpf") Reported-by: syzbot Reported-by: Mark Rutland Signed-off-by: Daniel Borkmann Signed-off-by: Alexei Starovoitov --- net/core/filter.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/net/core/filter.c b/net/core/filter.c index 5fa66a3..a13f5b1 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -459,11 +459,21 @@ static bool convert_bpf_ld_abs(struct sock_filter *fp, struct bpf_insn **insnp) (!unaligned_ok && offset >= 0 && offset + ip_align >= 0 && offset + ip_align % size == 0))) { + bool ldx_off_ok = offset <= S16_MAX; + *insn++ = BPF_MOV64_REG(BPF_REG_TMP, BPF_REG_H); *insn++ = BPF_ALU64_IMM(BPF_SUB, BPF_REG_TMP, offset); - *insn++ = BPF_JMP_IMM(BPF_JSLT, BPF_REG_TMP, size, 2 + endian); - *insn++ = BPF_LDX_MEM(BPF_SIZE(fp->code), BPF_REG_A, BPF_REG_D, - offset); + *insn++ = BPF_JMP_IMM(BPF_JSLT, BPF_REG_TMP, + size, 2 + endian + (!ldx_off_ok * 2)); + if (ldx_off_ok) { + *insn++ = BPF_LDX_MEM(BPF_SIZE(fp->code), BPF_REG_A, + BPF_REG_D, offset); + } else { + *insn++ = BPF_MOV64_REG(BPF_REG_TMP, BPF_REG_D); + *insn++ = BPF_ALU64_IMM(BPF_ADD, BPF_REG_TMP, offset); + *insn++ = BPF_LDX_MEM(BPF_SIZE(fp->code), BPF_REG_A, + BPF_REG_TMP, 0); + } if (endian) *insn++ = BPF_ENDIAN(BPF_FROM_BE, BPF_REG_A, size * 8); *insn++ = BPF_JMP_A(8); -- 2.7.4