Thanks for the review!
On Wed, Jun 25, 2025 at 01:19:01PM -0700, Eduard Zingerman wrote:
> On Wed, 2025-06-25 at 20:01 +0200, Paul Chaignon wrote:
> > Commit d6f1c85f2253 ("bpf: Fall back to nospec for Spectre v1") added a
> > WARN_ON_ONCE to check that we're not skipping a nospec due to a jump.
> > It however failed to take into account LDIMM64 instructions as below:
> >
> > 15: (18) r1 = 0x2020200005642020
> > 17: (7b) *(u64 *)(r10 -264) = r1
> >
> > This bytecode snippet generates a warning because the move from the
> > LDIMM64 instruction to the next instruction is seen as a jump. This
> > patch fixes it.
> >
> > Reported-by: syzbot+dc27c5fb8388e38d2d37@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: d6f1c85f2253 ("bpf: Fall back to nospec for Spectre v1")
> > Signed-off-by: Paul Chaignon <paul.chaignon@xxxxxxxxx>
> > ---
> > kernel/bpf/verifier.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 279a64933262..66841ed6dfc0 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -19819,6 +19819,7 @@ static int do_check(struct bpf_verifier_env *env)
> > int insn_cnt = env->prog->len;
> > bool do_print_state = false;
> > int prev_insn_idx = -1;
> > + int insn_sz;
> >
> > for (;;) {
> > struct bpf_insn *insn;
> > @@ -19942,7 +19943,8 @@ static int do_check(struct bpf_verifier_env *env)
> > * to document this in case nospec_result is used
> > * elsewhere in the future.
> > */
> > - WARN_ON_ONCE(env->insn_idx != prev_insn_idx + 1);
> > + insn_sz = bpf_is_ldimm64(insn) ? 2 : 1;
> > + WARN_ON_ONCE(env->insn_idx != prev_insn_idx + insn_sz);
>
> Could you please elaborate a bit?
> The code looks as follows:
>
> prev_insn_idx = env->insn_idx;
> ...
> err = do_check_insn(env, do_print_state: &do_print_state);
> ...
> if (state->speculative && cur_aux(env)->nospec_result) {
> ...
> insn_sz = bpf_is_ldimm64(insn) ? 2 : 1;
> WARN_ON_ONCE(env->insn_idx != prev_insn_idx + insn_sz);
> ...
> }
>
> The `cur_aux(env)->nospec_result` is set to true only for ST/STX
> instructions which are 8-bytes wide. `do_check_insn` moves
> env->isns_idx by 1 for these instructions.
>
> So, suppose there is a program:
>
> 15: (18) r1 = 0x2020200005642020
> 17: (7b) *(u64 *)(r10 -264) = r1
>
> Insn processing sequence would look like (starting from 15):
> - prev_insn_idx <- 15
> - do_check_insn()
> - env->insn_idx <- 17
> - prev_insn_idx <- 17
> - do_check_insn():
> - nospec_result <- true
> - env->insn_idx <- 18
> - state->speculative && cur_aux(env)->nospec_result == true:
> - WARN_ON_ONCE(18 != 17 + 1) // no warning
>
> What do I miss?
In the if condition, "cur_aux(env)" points to the aux data of the next
instruction (#17 here) because we incremented "insn_idx" in
do_check_insn(). In my fix, "insn" points to the previous instruction
because we retrieved it before calling do_check_insn().
Therefore, the processing sequence would look like:
- prev_insn_idx <- 15
- do_check_insn()
- env->insn_idx <- 17
- state->speculative && cur_aux(env)->nospec_result == true:
- WARN_ON_ONCE(17 != 15 + 1) // warning
I added a verbose() and recompiled to confirm those numbers.
If that makes sense, I'll send a v2 with:
- A better description, probably with a walkthrough.
- A test case simplified from the syzkaller repro.
- insn_sz renamed to prev_insn_sz for clarity.
> Could you please add a test case?
>
> > process_bpf_exit:
> > mark_verifier_state_scratched(env);
> > err = update_branch_counts(env, env->cur_state);
