linux.git/kernel/bpf/verifier.c, branch v6.6.16

bpf: keep track of max number of bpf_loop callback iterations

2024-02-01T00:18:59+00:00

commit bb124da69c47dd98d69361ec13244ece50bec63e upstream.

In some cases verifier can't infer convergence of the bpf_loop()
iteration. E.g. for the following program:

    static int cb(__u32 idx, struct num_context* ctx)
    {
        ctx->i++;
        return 0;
    }

    SEC("?raw_tp")
    int prog(void *_)
    {
        struct num_context ctx = { .i = 0 };
        __u8 choice_arr[2] = { 0, 1 };

        bpf_loop(2, cb, &ctx, 0);
        return choice_arr[ctx.i];
    }

Each 'cb' simulation would eventually return to 'prog' and reach
'return choice_arr[ctx.i]' statement. At which point ctx.i would be
marked precise, thus forcing verifier to track multitude of separate
states with {.i=0}, {.i=1}, ... at bpf_loop() callback entry.

This commit allows "brute force" handling for such cases by limiting
number of callback body simulations using 'umax' value of the first
bpf_loop() parameter.

For this, extend bpf_func_state with 'callback_depth' field.
Increment this field when callback visiting state is pushed to states
traversal stack. For frame #N it's 'callback_depth' field counts how
many times callback with frame depth N+1 had been executed.
Use bpf_func_state specifically to allow independent tracking of
callback depths when multiple nested bpf_loop() calls are present.

Signed-off-by: Eduard Zingerman 
Link: https://lore.kernel.org/r/20231121020701.26440-11-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Greg Kroah-Hartman

bpf: widening for callback iterators

2024-02-01T00:18:59+00:00

commit cafe2c21508a38cdb3ed22708842e957b2572c3e upstream.

Callbacks are similar to open coded iterators, so add imprecise
widening logic for callback body processing. This makes callback based
loops behave identically to open coded iterators, e.g. allowing to
verify programs like below:

  struct ctx { u32 i; };
  int cb(u32 idx, struct ctx* ctx)
  {
          ++ctx->i;
          return 0;
  }
  ...
  struct ctx ctx = { .i = 0 };
  bpf_loop(100, cb, &ctx, 0);
  ...

Acked-by: Andrii Nakryiko 
Signed-off-by: Eduard Zingerman 
Link: https://lore.kernel.org/r/20231121020701.26440-9-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Greg Kroah-Hartman

bpf: verify callbacks as if they are called unknown number of times

2024-02-01T00:18:59+00:00

commit ab5cfac139ab8576fb54630d4cca23c3e690ee90 upstream.

Prior to this patch callbacks were handled as regular function calls,
execution of callback body was modeled exactly once.
This patch updates callbacks handling logic as follows:
- introduces a function push_callback_call() that schedules callback
  body verification in env->head stack;
- updates prepare_func_exit() to reschedule callback body verification
  upon BPF_EXIT;
- as calls to bpf_*_iter_next(), calls to callback invoking functions
  are marked as checkpoints;
- is_state_visited() is updated to stop callback based iteration when
  some identical parent state is found.

Paths with callback function invoked zero times are now verified first,
which leads to necessity to modify some selftests:
- the following negative tests required adding release/unlock/drop
  calls to avoid previously masked unrelated error reports:
  - cb_refs.c:underflow_prog
  - exceptions_fail.c:reject_rbtree_add_throw
  - exceptions_fail.c:reject_with_cp_reference
- the following precision tracking selftests needed change in expected
  log trace:
  - verifier_subprog_precision.c:callback_result_precise
    (note: r0 precision is no longer propagated inside callback and
           I think this is a correct behavior)
  - verifier_subprog_precision.c:parent_callee_saved_reg_precise_with_callback
  - verifier_subprog_precision.c:parent_stack_slot_precise_with_callback

Reported-by: Andrew Werner 
Closes: https://lore.kernel.org/bpf/CA+vRuzPChFNXmouzGG+wsy=6eMcfr1mFG0F3g7rbg-sedGKW3w@mail.gmail.com/
Acked-by: Andrii Nakryiko 
Signed-off-by: Eduard Zingerman 
Link: https://lore.kernel.org/r/20231121020701.26440-7-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Greg Kroah-Hartman

bpf: extract setup_func_entry() utility function

2024-02-01T00:18:59+00:00

commit 58124a98cb8eda69d248d7f1de954c8b2767c945 upstream.

Move code for simulated stack frame creation to a separate utility
function. This function would be used in the follow-up change for
callbacks handling.

Acked-by: Andrii Nakryiko 
Signed-off-by: Eduard Zingerman 
Link: https://lore.kernel.org/r/20231121020701.26440-6-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Greg Kroah-Hartman

bpf: extract __check_reg_arg() utility function

2024-02-01T00:18:59+00:00

commit 683b96f9606ab7308ffb23c46ab43cecdef8a241 upstream.

Split check_reg_arg() into two utility functions:
- check_reg_arg() operating on registers from current verifier state;
- __check_reg_arg() operating on a specific set of registers passed as
  a parameter;

The __check_reg_arg() function would be used by a follow-up change for
callbacks handling.

Acked-by: Andrii Nakryiko 
Signed-off-by: Eduard Zingerman 
Link: https://lore.kernel.org/r/20231121020701.26440-5-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Greg Kroah-Hartman

bpf: print full verifier states on infinite loop detection

2024-02-01T00:18:59+00:00

commit b4d8239534fddc036abe4a0fdbf474d9894d4641 upstream.

Additional logging in is_state_visited(): if infinite loop is detected
print full verifier state for both current and equivalent states.

Acked-by: Andrii Nakryiko 
Signed-off-by: Eduard Zingerman 
Link: https://lore.kernel.org/r/20231024000917.12153-8-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Greg Kroah-Hartman

bpf: correct loop detection for iterators convergence

2024-02-01T00:18:58+00:00

commit 2a0992829ea3864939d917a5c7b48be6629c6217 upstream.

It turns out that .branches > 0 in is_state_visited() is not a
sufficient condition to identify if two verifier states form a loop
when iterators convergence is computed. This commit adds logic to
distinguish situations like below:

 (I)            initial       (II)            initial
                  |                             |
                  V                             V
     .---------> hdr                           ..
     |            |                             |
     |            V                             V
     |    .------...                    .------..
     |    |       |                     |       |
     |    V       V                     V       V
     |   ...     ...               .-> hdr     ..
     |    |       |                |    |       |
     |    V       V                |    V       V
     |   succ <- cur               |   succ <- cur
     |    |                        |    |
     |    V                        |    V
     |   ...                       |   ...
     |    |                        |    |
     '----'                        '----'

For both (I) and (II) successor 'succ' of the current state 'cur' was
previously explored and has branches count at 0. However, loop entry
'hdr' corresponding to 'succ' might be a part of current DFS path.
If that is the case 'succ' and 'cur' are members of the same loop
and have to be compared exactly.

Co-developed-by: Andrii Nakryiko 
Co-developed-by: Alexei Starovoitov 
Reviewed-by: Andrii Nakryiko 
Signed-off-by: Eduard Zingerman 
Link: https://lore.kernel.org/r/20231024000917.12153-6-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Greg Kroah-Hartman

bpf: exact states comparison for iterator convergence checks

2024-02-01T00:18:58+00:00

commit 2793a8b015f7f1caadb9bce9c63dc659f7522676 upstream.

Convergence for open coded iterators is computed in is_state_visited()
by examining states with branches count > 1 and using states_equal().
states_equal() computes sub-state relation using read and precision marks.
Read and precision marks are propagated from children states,
thus are not guaranteed to be complete inside a loop when branches
count > 1. This could be demonstrated using the following unsafe program:

     1. r7 = -16
     2. r6 = bpf_get_prandom_u32()
     3. while (bpf_iter_num_next(&fp[-8])) {
     4.   if (r6 != 42) {
     5.     r7 = -32
     6.     r6 = bpf_get_prandom_u32()
     7.     continue
     8.   }
     9.   r0 = r10
    10.   r0 += r7
    11.   r8 = *(u64 *)(r0 + 0)
    12.   r6 = bpf_get_prandom_u32()
    13. }

Here verifier would first visit path 1-3, create a checkpoint at 3
with r7=-16, continue to 4-7,3 with r7=-32.

Because instructions at 9-12 had not been visitied yet existing
checkpoint at 3 does not have read or precision mark for r7.
Thus states_equal() would return true and verifier would discard
current state, thus unsafe memory access at 11 would not be caught.

This commit fixes this loophole by introducing exact state comparisons
for iterator convergence logic:
- registers are compared using regs_exact() regardless of read or
  precision marks;
- stack slots have to have identical type.

Unfortunately, this is too strict even for simple programs like below:

    i = 0;
    while(iter_next(&it))
      i++;

At each iteration step i++ would produce a new distinct state and
eventually instruction processing limit would be reached.

To avoid such behavior speculatively forget (widen) range for
imprecise scalar registers, if those registers were not precise at the
end of the previous iteration and do not match exactly.

This a conservative heuristic that allows to verify wide range of
programs, however it precludes verification of programs that conjure
an imprecise value on the first loop iteration and use it as precise
on the second.

Test case iter_task_vma_for_each() presents one of such cases:

        unsigned int seen = 0;
        ...
        bpf_for_each(task_vma, vma, task, 0) {
                if (seen >= 1000)
                        break;
                ...
                seen++;
        }

Here clang generates the following code:

:
      24:       r8 = r6                          ; stash current value of
                ... body ...                       'seen'
      29:       r1 = r10
      30:       r1 += -0x8
      31:       call bpf_iter_task_vma_next
      32:       r6 += 0x1                        ; seen++;
      33:       if r0 == 0x0 goto +0x2   ; exit on next() == NULL
      34:       r7 += 0x10
      35:       if r8 < 0x3e7 goto -0xc  ; loop on seen < 1000

:
      ... exit ...

Note that counter in r6 is copied to r8 and then incremented,
conditional jump is done using r8. Because of this precision mark for
r6 lags one state behind of precision mark on r8 and widening logic
kicks in.

Adding barrier_var(seen) after conditional is sufficient to force
clang use the same register for both counting and conditional jump.

This issue was discussed in the thread [1] which was started by
Andrew Werner  demonstrating a similar bug
in callback functions handling. The callbacks would be addressed
in a followup patch.

[1] https://lore.kernel.org/bpf/97a90da09404c65c8e810cf83c94ac703705dc0e.camel@gmail.com/

Co-developed-by: Andrii Nakryiko 
Co-developed-by: Alexei Starovoitov 
Signed-off-by: Eduard Zingerman 
Link: https://lore.kernel.org/r/20231024000917.12153-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Greg Kroah-Hartman

bpf: extract same_callsites() as utility function

2024-02-01T00:18:58+00:00

commit 4c97259abc9bc8df7712f76f58ce385581876857 upstream.

Extract same_callsites() from clean_live_states() as a utility function.
This function would be used by the next patch in the set.

Signed-off-by: Eduard Zingerman 
Link: https://lore.kernel.org/r/20231024000917.12153-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Greg Kroah-Hartman

bpf: move explored_state() closer to the beginning of verifier.c

2024-02-01T00:18:58+00:00

commit 3c4e420cb6536026ddd50eaaff5f30e4f144200d upstream.

Subsequent patches would make use of explored_state() function.
Move it up to avoid adding unnecessary prototype.

Signed-off-by: Eduard Zingerman 
Link: https://lore.kernel.org/r/20231024000917.12153-2-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov 
Signed-off-by: Greg Kroah-Hartman