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 <syzkal...@googlegroups.com>
Reported-by: Mark Rutland <mark.rutl...@arm.com>
Signed-off-by: Daniel Borkmann <dan...@iogearbox.net>
---
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);
--
2.9.5
