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bpf: prevent r10 register from being marked as precise

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r10 is a special register that is not under BPF program's control and is
always effectively precise. The rest of precision logic assumes that
only r0-r9 SCALAR registers are marked as precise, so prevent r10 from
being marked precise.

This can happen due to signed cast instruction allowing to do something
like `r0 = (s8)r10;`, which later, if r0 needs to be precise, would lead
to an attempt to mark r10 as precise.

Prevent this with an extra check during instruction backtracking.

Fixes: 8100928c88 ("bpf: Support new sign-extension mov insns")
Reported-by: syzbot+148110ee7cf72f39f33e@syzkaller.appspotmail.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240404214536.3551295-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Andrii Nakryiko 2024-04-04 14:45:35 -07:00 committed by Alexei Starovoitov
parent f917170072
commit 1f2a74b41e
1 changed files with 4 additions and 2 deletions
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6
kernel/bpf/verifier.c
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@ -3615,7 +3615,8 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
* sreg needs precision before this insn
*/
bt_clear_reg(bt, dreg);
bt_set_reg(bt, sreg);
if (sreg != BPF_REG_FP)
bt_set_reg(bt, sreg);
} else {
/* dreg = K
* dreg needs precision after this insn.
@ -3631,7 +3632,8 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx, int subseq_idx,
* both dreg and sreg need precision
* before this insn
*/
bt_set_reg(bt, sreg);
if (sreg != BPF_REG_FP)
bt_set_reg(bt, sreg);
} /* else dreg += K
* dreg still needs precision before this insn
*/