From 2339cd6cd0b5401fa3fe886bf1c0cb8822041957 Mon Sep 17 00:00:00 2001 From: Alexei Starovoitov Date: Tue, 3 Sep 2019 15:16:17 -0700 Subject: bpf: fix precision tracking of stack slots The problem can be seen in the following two tests: 0: (bf) r3 = r10 1: (55) if r3 != 0x7b goto pc+0 2: (7a) *(u64 *)(r3 -8) = 0 3: (79) r4 = *(u64 *)(r10 -8) .. 0: (85) call bpf_get_prandom_u32#7 1: (bf) r3 = r10 2: (55) if r3 != 0x7b goto pc+0 3: (7b) *(u64 *)(r3 -8) = r0 4: (79) r4 = *(u64 *)(r10 -8) When backtracking need to mark R4 it will mark slot fp-8. But ST or STX into fp-8 could belong to the same block of instructions. When backtracing is done the parent state may have fp-8 slot as "unallocated stack". Which will cause verifier to warn and incorrectly reject such programs. Writes into stack via non-R10 register are rare. llvm always generates canonical stack spill/fill. For such pathological case fall back to conservative precision tracking instead of rejecting. Reported-by: syzbot+c8d66267fd2b5955287e@syzkaller.appspotmail.com Fixes: b5dc0163d8fd ("bpf: precise scalar_value tracking") Signed-off-by: Alexei Starovoitov Signed-off-by: Daniel Borkmann --- kernel/bpf/verifier.c | 23 ++++++++++++++--------- 1 file changed, 14 insertions(+), 9 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index b5c14c9d7b98..c36a719fee6d 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1772,16 +1772,21 @@ static int __mark_chain_precision(struct bpf_verifier_env *env, int regno, bitmap_from_u64(mask, stack_mask); for_each_set_bit(i, mask, 64) { if (i >= func->allocated_stack / BPF_REG_SIZE) { - /* This can happen if backtracking - * is propagating stack precision where - * caller has larger stack frame - * than callee, but backtrack_insn() should - * have returned -ENOTSUPP. + /* the sequence of instructions: + * 2: (bf) r3 = r10 + * 3: (7b) *(u64 *)(r3 -8) = r0 + * 4: (79) r4 = *(u64 *)(r10 -8) + * doesn't contain jmps. It's backtracked + * as a single block. + * During backtracking insn 3 is not recognized as + * stack access, so at the end of backtracking + * stack slot fp-8 is still marked in stack_mask. + * However the parent state may not have accessed + * fp-8 and it's "unallocated" stack space. + * In such case fallback to conservative. */ - verbose(env, "BUG spi %d stack_size %d\n", - i, func->allocated_stack); - WARN_ONCE(1, "verifier backtracking bug"); - return -EFAULT; + mark_all_scalars_precise(env, st); + return 0; } if (func->stack[i].slot_type[0] != STACK_SPILL) { -- cgit