Re: [PATCH] bpf: Annotate struct bpf_cand_cache with __counted_by()

[Eduard Zingerman]

On Tue, 2024-08-13 at 11:57 -0700, Alexei Starovoitov wrote:
> On Tue, Aug 13, 2024 at 10:59 AM Thorsten Blum <thorsten.blum@xxxxxxxxxx> wrote:
> >
> > On 13. Aug 2024, at 18:28, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
> > > On Tue, Aug 13, 2024 at 8:19 AM Thorsten Blum <thorsten.blum@xxxxxxxxxx> wrote:
> > > >
> > > > Add the __counted_by compiler attribute to the flexible array member
> > > > cands to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
> > > > CONFIG_FORTIFY_SOURCE.
> > > >
> > > > Increment cnt before adding a new struct to the cands array.
> > >
> > > why? What happens otherwise?
> >
> > If you try to access cands->cands[cands->cnt] without incrementing
> > cands->cnt first, you're essentially accessing the array out of bounds
> > which will fail during runtime.
>
> What kind of error/warn do you see ?
> Is it runtime or compile time?
>
> Is this the only place?
> what about:
>         new_cands = kmemdup(cands, sizeof_cands(cands->cnt), GFP_KERNEL);
>
> cnt field gets copied with other fields.
> Can compiler/runtime catch that?

I think that generated check is mechanical, sanitizer wraps access to
array with size check using the value of associated counter, e.g:

    12:52:20 tmp$ clang -fsanitize=undefined ./test.c
    12:52:53 tmp$ ./a.out
    test.c:11:3: runtime error: index 0 out of bounds for type 'int[]'
    SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior test.c:11:3
    12:52:55 tmp$ cat test.c
    #include <alloca.h>

    struct arr {
      int cnt;
      int items[] __attribute__((__counted_by__(cnt)));
    };

    int main(int argc, char **argv) {
      struct arr *arr = alloca(sizeof(struct arr) + sizeof(int));
      arr->cnt = 0;
      arr->items[arr->cnt] = 42;
      arr->cnt++;
      asm volatile (""::"r"(arr));
      return 0;
    }
    12:53:07 tmp$ clang -fsanitize=undefined ./test.c
    12:53:10 tmp$ ./a.out
    test.c:11:3: runtime error: index 0 out of bounds for type 'int[]'
    SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior test.c:11:3
    12:53:13 tmp$ cat test.c
    #include <alloca.h>

    struct arr {
      int cnt;
      int items[] __attribute__((__counted_by__(cnt)));
    };

    int main(int argc, char **argv) {
      struct arr *arr = alloca(sizeof(struct arr) + sizeof(int));
      arr->cnt = 1;
      arr->items[arr->cnt - 1] = 42;
      asm volatile (""::"r"(arr));
      return 0;
    }
    12:53:34 tmp$ clang -fsanitize=undefined ./test.c
    12:53:36 tmp$ ./a.out
    12:53:38 tmp$ echo $?
    0

Or here is the IR generated for C program:

    struct arr {
      unsigned int cnt;
      int items[] __attribute__((__counted_by__(cnt)));
    };

    void push(int i, struct arr *arr) {
      arr->items[arr->cnt] = 42;
      arr->cnt++;
    }

Note the 'cnt' passed as a parameter to '@__ubsan_handle_out_of_bounds':

    define dso_local void @push(i32 noundef %0, ptr noundef %1) local_unnamed_addr #0 !func_sanitize !3 {
      ...
      %11 = load i32, ptr %1, align 4
      %12 = zext i32 %11 to i64
      tail call void @__ubsan_handle_out_of_bounds(ptr nonnull @6, i64 %12) #2, !nosanitize !4

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