4.20-stable review patch. If anyone has any objections, please let me know.
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[ commit b7137c4eab85c1cf3d46acdde90ce1163b28c873 upstream ]
In check_map_access() we probe actual bounds through __check_map_access() with offset of reg->smin_value + off for lower bound and offset of reg->umax_value + off for the upper bound. However, even though the reg->smin_value could have a negative value, the final result of the sum with off could be positive when pointer arithmetic with known and unknown scalars is combined. In this case we reject the program with an error such as "R<x> min value is negative, either use unsigned index or do a if (index >=0) check." even though the access itself would be fine. Therefore extend the check to probe whether the actual resulting reg->smin_value + off is less than zero.
Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/verifier.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 8e90e795ca68..973ebab5b19d 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1370,13 +1370,17 @@ static int check_map_access(struct bpf_verifier_env *env, u32 regno, */ if (env->log.level) print_verifier_state(env, state); + /* The minimum value is only important with signed * comparisons where we can't assume the floor of a * value is 0. If we are using signed variables for our * index'es we need to make sure that whatever we use * will have a set floor within our range. */ - if (reg->smin_value < 0) { + if (reg->smin_value < 0 && + (reg->smin_value == S64_MIN || + (off + reg->smin_value != (s64)(s32)(off + reg->smin_value)) || + reg->smin_value + off < 0)) { verbose(env, "R%d min value is negative, either use unsigned index or do a if (index >=0) check.\n", regno); return -EACCES;