Commit 2339cd6c authored Sep 03, 2019 by Alexei Starovoitov Committed by Daniel Borkmann Sep 05, 2019

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: b5dc0163 ("bpf: precise scalar_value tracking")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

parent 44580a01

kernel/bpf/verifier.c

+14 −9

Original line number	Diff line number	Diff line
		@@ -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.
		*/
		verbose(env, "BUG spi %d stack_size %d\n",
		i, func->allocated_stack);
		WARN_ONCE(1, "verifier backtracking bug");
		return -EFAULT;
		/* 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.
		*/
		mark_all_scalars_precise(env, st);
		return 0;
		}

		if (func->stack[i].slot_type[0] != STACK_SPILL) {

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