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bpf: fix precision bit propagation for BPF_ST instructions 

When backtracking instructions to propagate precision bit for registers
and stack slots, one class of instructions (BPF_ST) weren't handled
causing extra stack slots to be propagated into parent state. Parent
state might not have that much stack allocated, though, which causes
warning on invalid stack slot usage.

This patch adds handling of BPF_ST instructions:

BPF_MEM | <size> | BPF_ST:   *(size *) (dst_reg + off) = imm32

Reported-by: syzbot+4da3ff23081bafe74fc2@syzkaller.appspotmail.com
Fixes: b5dc0163d8 ("bpf: precise scalar_value tracking")
Cc: Alexei Starovoitov <ast@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
This commit is contained in:
Andrii Nakryiko 2019-07-08 20:32:44 -07:00 committed by Daniel Borkmann
parent 327835fb1e
commit b3b50f05dc
1 changed files with 4 additions and 7 deletions
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9
kernel/bpf/verifier.c
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@ -1519,9 +1519,9 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx,
return -EFAULT;
}
*stack_mask |= 1ull << spi;
} else if (class == BPF_STX) {
} else if (class == BPF_STX || class == BPF_ST) {
if (*reg_mask & dreg)
/* stx shouldn't be using _scalar_ dst_reg
/* stx & st shouldn't be using _scalar_ dst_reg
* to access memory. It means backtracking
* encountered a case of pointer subtraction.
*/
@ -1540,6 +1540,7 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx,
if (!(*stack_mask & (1ull << spi)))
return 0;
*stack_mask &= ~(1ull << spi);
if (class == BPF_STX)
*reg_mask |= sreg;
} else if (class == BPF_JMP || class == BPF_JMP32) {
if (opcode == BPF_CALL) {
@ -1569,10 +1570,6 @@ static int backtrack_insn(struct bpf_verifier_env *env, int idx,
if (mode == BPF_IND || mode == BPF_ABS)
/* to be analyzed */
return -ENOTSUPP;
} else if (class == BPF_ST) {
if (*reg_mask & dreg)
/* likely pointer subtraction */
return -ENOTSUPP;
}
return 0;
}